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SED - AN INTRODUCTION AND TUTORIAL BY BRUCE BARNETT

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Last modified: Tue Jul 25 11:54:15 2023




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TABLE OF CONTENTS

Note - You can click on the table of contents sections to jump to that section.
And if you click on a section header, it returns you to the Table of Contents.
Handy, eh?

 * The Awful Truth about sed
 * The essential command: s for substitution
 * The slash as a delimiter
 * Using & as the matched string
 * Using \1 to keep part of the pattern
 * Extended Regular Expressions
 * Sed Pattern Flags
   * /g - Global replacement
   * Is sed recursive?
   * /1, /2, etc. Specifying which occurrence
   * /p - print
   * Write to a file with /w filename
   * /I - Ignore Case
   * Combining substitution flags
 * Arguments and invocation of sed
   * Multiple commands with -e command
   * Filenames on the command line
   * sed -n: no printing
   * Using 'sed /pattern/'
     * Using 'sed -n /pattern/p' to duplicate the function of grep
   * sed -f scriptname
   * sed in shell scripts
     * Quoting multiple sed lines in the C shell
     * Quoting multiple sed lines in the Bourne shell
   * sed -V
   * sed -h
   * A sed interpreter script
   * Sed Comments
   * Passing arguments into a sed script
   * Using sed in a shell here-is document
   * Multiple commands and order of execution
 * Addresses and Ranges of Text
   * Restricting to a line number
   * Patterns
   * Ranges by line number
   * Ranges by patterns
 * Delete with d
 * Printing with p
 * Reversing the restriction with !
 * Relationships between d, p, and !
 * The q or quit command
 * Grouping with { and }
 * Operating in a pattern range except for the patterns
 * Writing a file with the 'w' command
 * Reading in a file with the 'r' command
 * The # Comment Command
 * Adding, Changing, Inserting new lines
   * Append a line with 'a'
   * Insert a line with 'i'
   * Change a line with 'c'
   * Leading tabs and spaces in a sed script
   * Adding more than one line
   * Adding lines and the pattern space
   * Address ranges and the above commands
 * Multi-Line Patterns
 * Print line number with =
 * Transform with y
 * Displaying control characters with a l
 * Working with Multiple Lines
   * Matching three lines with sed
   * Matching patterns that span multiple lines
   * Using newlines in sed scripts
   * The Hold Buffer
   * Exchange with x
   * Example of Context Grep
   * Hold with h or H
   * Keeping more than one line in the hold buffer
   * Get with g or G
 * Branch (Flow Control)
 * Testing with t
 * Debugging with l
 * An alternate way of adding comments
 * The poorly documented ;
 * Passing regular expressions as arguments
 * Inserting binary characters
 * GNU sed Command Line arguments
   * The -posix argument
   * The --version argument
   * The -h Help argument
   * The -l Line Length Argument
   * The -s Separate argument
   * The -i in-place argument
   * The --follow-symlinks argument
   * The -b Binary argument
   * The -r Extended Regular Expression argument
   * The -u Unbuffered argument
   * The -z Null Data argument
 * FreeBSD Extensions
   * -a or delayed open
   * The -I in-place argument
   * -E or Extended Regular Expressions
 * Using word boundaries
 * Command Summary
 * In Conclusion
 * More References


INTRODUCTION TO SED

How to use sed, a special editor for modifying files automatically. If you want
to write a program to make changes in a file, sed is the tool to use.

There are a few programs that are the real workhorse in the UNIX toolbox. These
programs are simple to use for simple applications, yet have a rich set of
commands for performing complex actions. Don't let the complex potential of a
program keep you from making use of the simpler aspects. I'll start with the
simple concepts and introduce the advanced topics later on.
When I first wrote this (in 1994), most versions of sed did not allow you to
place comments inside the script. Lines starting with the '#' characters are
comments. Newer versions of sed may support comments at the end of the line as
well.

One way to think of this is that the old, "classic" version was the basis of
GNU, FreeBSD and Solaris versions of sed. And to help you understand what I had
to work with, here is the sed(1) manual page from Sun/Oracle.


THE AWFUL TRUTH ABOUT SED

Sed is the ultimate stream editor. If that sounds strange, picture a stream
flowing through a pipe. Okay, you can't see a stream if it's inside a pipe.
That's what I get for attempting a flowing analogy. You want literature, read
James Joyce.

Anyhow, sed is a marvelous utility. Unfortunately, most people never learn its
real power. The language is very simple, but the documentation is terrible. The
Solaris on-line manual pages for sed are five pages long, and two of those pages
describe the 34 different errors you can get. A program that spends as much
space documenting the errors as it does documenting the language has a serious
learning curve.

Do not fret! It is not your fault you don't understand sed. I will cover sed
completely. But I will describe the features in the order that I learned them. I
didn't learn everything at once. You don't need to either.

And remember, clicking on a section title brings you back to the Table Of
Contents. Click on the entry in the Table of contents brings you back to that
section! Try it now!


THE ESSENTIAL COMMAND: S FOR SUBSTITUTION

Sed has several commands, but most people only learn the substitute command: s.
The substitute command changes all occurrences of the regular expression into a
new value. A simple example is changing "day" in the "old" file to "night" in
the "new" file:

sed s/day/night/ <old >new


Or another way (for UNIX beginners),

sed s/day/night/ old >new


and for those who want to test this:

echo day | sed s/day/night/ 


This will output "night".

I didn't put quotes around the argument because this example didn't need them.
If you read my earlier tutorial on quotes, you would understand why it doesn't
need quotes. However, I recommend you do use quotes. If you have meta-characters
in the command, quotes are necessary. And if you aren't sure, it's a good habit,
and I will henceforth quote future examples to emphasize the "best practice."
Using the strong (single quote) character, that would be:

sed 's/day/night/' <old >new


I must emphasize that the sed editor changes exactly what you tell it to. So if
you executed

echo Sunday | sed 's/day/night/'


This would output the word "Sunnight" because sed found the string "day" in the
input.

Another important concept is that sed is line oriented. Suppose you have the
input file:

one two three, one two three
four three two one
one hundred


and you used the command

sed 's/one/ONE/' <file


The output would be

ONE two three, one two three
four three two ONE
ONE hundred


Note that this changed "one" to "ONE" once on each line. The first line had
"one" twice, but only the first occurrence was changed. That is the default
behavior. If you want something different, you will have to use some of the
options that are available. I'll explain them later.

So let's continue.

There are four parts to this substitute command:

s	  Substitute command
/../../	  Delimiter
one	  Regular Expression Pattern Search Pattern
ONE	  Replacement string


The search pattern is on the left hand side and the replacement string is on the
right hand side.

We've covered quoting and regular expressions.. That's 90% of the effort needed
to learn the substitute command. To put it another way, you already know how to
handle 90% of the most frequent uses of sed. There are a ... few fine points
that any future sed expert should know about. (You just finished section 1.
There are only 63 more sections to cover. :-) Oh. And you may want to bookmark
this page, .... just in case you don't finish.


THE SLASH AS A DELIMITER

The character after the s is the delimiter. It is conventionally a slash,
because this is what ed, more, and vi use. It can be anything you want, however.
If you want to change a pathname that contains a slash - say /usr/local/bin to
/common/bin - you could use the backslash to quote the slash:

sed 's/\/usr\/local\/bin/\/common\/bin/' <old >new


Gulp. Some call this a 'Picket Fence' and it's ugly. It is easier to read if you
use an underline instead of a slash as a delimiter:

sed 's_/usr/local/bin_/common/bin_' <old >new


Some people use colons:

sed 's:/usr/local/bin:/common/bin:' <old >new


Others use the "|" character.

sed 's|/usr/local/bin|/common/bin|' <old >new


Pick one you like. As long as it's not in the string you are looking for,
anything goes. And remember that you need three delimiters. If you get a
"Unterminated `s' command" it's because you are missing one of them.


USING & AS THE MATCHED STRING

Sometimes you want to search for a pattern and add some characters, like
parenthesis, around or near the pattern you found. It is easy to do this if you
are looking for a particular string:

sed 's/abc/(abc)/' <old >new


This won't work if you don't know exactly what you will find. How can you put
the string you found in the replacement string if you don't know what it is?

The solution requires the special character "&." It corresponds to the pattern
found.

sed 's/[a-z]*/(&)/' <old >new


You can have any number of "&" in the replacement string. You could also double
a pattern, e.g. the first number of a line:

% echo "123 abc" | sed 's/[0-9]*/& &/'
123 123 abc


Let me slightly amend this example. Sed will match the first string, and make it
as greedy as possible. I'll cover that later. If you don't want it to be so
greedy (i.e. limit the matching), you need to put restrictions on the match.

The first match for '[0-9]*' is the first character on the line, as this matches
zero or more numbers. So if the input was "abc 123" the output would be
unchanged (well, except for a space before the letters). A better way to
duplicate the number is to make sure it matches a number:

% echo "123 abc" | sed 's/[0-9][0-9]*/& &/'
123 123 abc


The string "abc" is unchanged, because it was not matched by the regular
expression. If you wanted to eliminate "abc" from the output, you must expand
the regular expression to match the rest of the line and explicitly exclude part
of the expression using "(", ")" and "\1", which is the next topic.


EXTENDED REGULAR EXPRESSIONS

Let me add a quick comment here because there is another way to write the above
script. "[0-9]*" matches zero or more numbers. "[0-9][0-9]*" matches one or more
numbers. Another way to do this is to use the "+" meta-character and use the
pattern "[0-9]+" as the "+" is a special character when using "extended regular
expressions." Extended regular expressions have more power, but sed scripts that
treated "+" as a normal character would break. Therefore you must explicitly
enable this extension with a command line option.

GNU sed turns this feature on if you use the "-r" command line option. So the
above could also be written using

% echo "123 abc" | sed -r 's/[0-9]+/& &/'
123 123 abc


Mac OS X and FreeBSD uses -E instead of -r. For more information on extended
regular expressions, see Regular Expressions and the description of the -r
command line argument


USING \1 TO KEEP PART OF THE PATTERN

I have already described the use of "(" ")" and "1" in my tutorial on regular
expressions. To review, the escaped parentheses (that is, parentheses with
backslashes before them) remember a substring of the characters matched by the
regular expression. You can use this to exclude part of the characters matched
by the regular expression. The "\1" is the first remembered pattern, and the
"\2" is the second remembered pattern. Sed has up to nine remembered patterns.

If you wanted to keep the first word of a line, and delete the rest of the line,
mark the important part with the parenthesis:

sed 's/\([a-z]*\).*/\1/'


I should elaborate on this. Regular expressions are greedy, and try to match as
much as possible. "[a-z]*" matches zero or more lower case letters, and tries to
match as many characters as possible. The ".*" matches zero or more characters
after the first match. Since the first one grabs all of the contiguous lower
case letters, the second matches anything else. Therefore if you type

echo abcd123 | sed 's/\([a-z]*\).*/\1/'


This will output "abcd" and delete the numbers.

If you want to switch two words around, you can remember two patterns and change
the order around:

sed 's/\([a-z]*\) \([a-z]*\)/\2 \1/'


Note the space between the two remembered patterns. This is used to make sure
two words are found. However, this will do nothing if a single word is found, or
any lines with no letters. You may want to insist that words have at least one
letter by using

sed 's/\([a-z][a-z]*\) \([a-z][a-z]*\)/\2 \1/'


or by using extended regular expressions (note that '(' and ')' no longer need
to have a backslash):

sed -r 's/([a-z]+) ([a-z]+)/\2 \1/' # Using GNU sed
sed -E 's/([a-z]+) ([a-z]+)/\2 \1/' # Using Apple Mac OS X


The "\1" doesn't have to be in the replacement string (in the right hand side).
It can be in the pattern you are searching for (in the left hand side). If you
want to eliminate duplicated words, you can try:

sed 's/\([a-z]*\) \1/\1/'


If you want to detect duplicated words, you can use

sed -n '/\([a-z][a-z]*\) \1/p'


or with extended regular expressions

sed -rn '/([a-z]+) \1/p' # GNU sed
sed -En '/([a-z]+) \1/p' # Mac OS X


This, when used as a filter, will print lines with duplicated words.

The numeric value can have up to nine values: "\1" thru "\9." If you wanted to
reverse the first three characters on a line, you can use

sed 's/^\(.\)\(.\)\(.\)/\3\2\1/'



SED PATTERN FLAGS

You can add additional flags after the last delimiter. You might have noticed I
used a 'p' at the end of the previous substitute command. I also added the '-n'
option. Let me first cover the 'p' and other pattern flags. These flags can
specify what happens when a match is found. Let me describe them.


/G - GLOBAL REPLACEMENT

Most UNIX utilities work on files, reading a line at a time. Sed, by default, is
the same way. If you tell it to change a word, it will only change the first
occurrence of the word on a line. You may want to make the change on every word
on the line instead of the first. For an example, let's place parentheses around
words on a line. Instead of using a pattern like "[A-Za-z]*" which won't match
words like "won't," we will use a pattern, "[^ ]*," that matches everything
except a space. Well, this will also match anything because "*" means zero or
more. The current version of Solaris's sed (as I wrote this) can get unhappy
with patterns like this, and generate errors like "Output line too long" or even
run forever. I consider this a bug, and have reported this to Sun. As a
work-around, you must avoid matching the null string when using the "g" flag to
sed. A work-around example is: "[^ ][^ ]*." The following will put parenthesis
around the first word:

sed 's/[^ ]*/(&)/' <old >new


If you want it to make changes for every word, add a "g" after the last
delimiter and use the work-around:

sed 's/[^ ][^ ]*/(&)/g' <old >new



IS SED RECURSIVE?

Sed only operates on patterns found in the in-coming data. That is, the input
line is read, and when a pattern is matched, the modified output is generated,
and the rest of the input line is scanned. The "s" command will not scan the
newly created output. That is, you don't have to worry about expressions like:

sed 's/loop/loop the loop/g' <old >new


This will not cause an infinite loop. If a second "s" command is executed, it
could modify the results of a previous command. I will show you how to execute
multiple commands later.


/1, /2, ETC. SPECIFYING WHICH OCCURRENCE

With no flags, the first matched substitution is changed. With the "g" option,
all matches are changed. If you want to modify a particular pattern that is not
the first one on the line, you could use "\(" and "\)" to mark each pattern, and
use "\1" to put the first pattern back unchanged. This next example keeps the
first word on the line but deletes the second:

sed 's/\([a-zA-Z]*\) \([a-zA-Z]*\) /\1 /' <old >new


Yuck. There is an easier way to do this. You can add a number after the
substitution command to indicate you only want to match that particular pattern.
Example:

sed 's/[a-zA-Z]* //2' <old >new


You can combine a number with the g (global) flag. For instance, if you want to
leave the first word alone, but change the second, third, etc. to be DELETED
instead, use /2g:

sed 's/[a-zA-Z]* /DELETED /2g' <old >new


I've heard that combining the number with the g command does not work on The
MacOS, and perhaps the FreeSBD version of sed as well.

Don't get /2 and \2 confused. The /2 is used at the end. \2 is used in inside
the replacement field.

Note the space after the "*" character. Without the space, sed will run a long,
long time. (Note: this bug is probably fixed by now.) This is because the number
flag and the "g" flag have the same bug. You should also be able to use the
pattern

sed 's/[^ ]*//2' <old >new


but this also eats CPU. If this works on your computer, and it does on some UNIX
systems, you could remove the encrypted password from the password file:

sed 's/[^:]*//2' </etc/passwd >/etc/password.new


But this didn't work for me the time I wrote this. Using "[^:][^:]*" as a
work-around doesn't help because it won't match a non-existent password, and
instead delete the third field, which is the user ID! Instead you have to use
the ugly parenthesis:

sed 's/^\([^:]*\):[^:]:/\1::/'  </etc/passwd >/etc/password.new


You could also add a character to the first pattern so that it no longer matches
the null pattern:

sed 's/[^:]*:/:/2'  </etc/passwd >/etc/password.new


The number flag is not restricted to a single digit. It can be any number from 1
to 512. If you wanted to add a colon after the 80th character in each line, you
could type:

sed 's/./&:/80' <file >new


You can also do it the hard way by using 80 dots:

sed 's/^................................................................................/&:/' <file >new



/P - PRINT

By default, sed prints every line. If it makes a substitution, the new text is
printed instead of the old one. If you use an optional argument to sed, "sed
-n," it will not, by default, print any new lines. I'll cover this and other
options later. When the "-n" option is used, the "p" flag will cause the
modified line to be printed. Here is one way to duplicate the function of grep
with sed:

sed -n 's/pattern/&/p' <file


But a simpler version is described later


WRITE TO A FILE WITH /W FILENAME

There is one more flag that can follow the third delimiter. With it, you can
specify a file that will receive the modified data. An example is the following,
which will write all lines that start with an even number, followed by a space,
to the file even:


sed -n 's/^[0-9]*[02468] /&/w even' <file 


In this example, the output file isn't needed, as the input was not modified.
You must have exactly one space between the w and the filename. You can also
have ten files open with one instance of sed. This allows you to split up a
stream of data into separate files. Using the previous example combined with
multiple substitution commands described later, you could split a file into ten
pieces depending on the last digit of the first number. You could also use this
method to log error or debugging information to a special file.


/I - IGNORE CASE

GNU has added another pattern flags - /I

This flag makes the pattern match case insensitive. This will match abc, aBc,
ABC, AbC, etc.:

sed -n '/abc/I p' <old >new


Note that a space after the '/I' and the 'p' (print) command emphasizes that the
'p' is not a modifier of the pattern matching process, , but a command to
execute after the pattern matching.


COMBINING SUBSTITUTION FLAGS

You can combine flags when it makes sense. Please note that the "w" has to be
the last flag. For example the following command works:

sed -n 's/a/A/2pw /tmp/file' <old >new


Next I will discuss the options to sed, and different ways to invoke sed.


ARGUMENTS AND INVOCATION OF SED

previously, I have only used one substitute command. If you need to make two
changes, and you didn't want to read the manual, you could pipe together
multiple sed commands:

sed 's/BEGIN/begin/' <old | sed 's/END/end/' >new


This used two processes instead of one. A sed guru never uses two processes when
one can do.


MULTIPLE COMMANDS WITH -E COMMAND

One method of combining multiple commands is to use a -e before each command:

sed -e 's/a/A/' -e 's/b/B/' <old >new


A "-e" isn't needed in the earlier examples because sed knows that there must
always be one command. If you give sed one argument, it must be a command, and
sed will edit the data read from standard input.

The long argument version is

sed --expression='s/a/A/' --expression='s/b/B/' <old >new


Also see Quoting multiple sed lines in the Bourne shell


FILENAMES ON THE COMMAND LINE

You can specify files on the command line if you wish. If there is more than one
argument to sed that does not start with an option, it must be a filename. This
next example will count the number of lines in three files that don't begin with
a "#:"

sed 's/^#.*//'  f1 f2 f3 | grep -v '^$' | wc -l


Let's break this down into pieces. The sed substitute command changes every line
that starts with a "#" into a blank line. Grep was used to filter out (delete)
empty lines. Wc counts the number of lines left. Sed has more commands that make
grep unnecessary. And grep -c can replace wc -l. I'll discuss how you can
duplicate some of grep's functionality later.

Of course you could write the last example using the "-e" option:

sed -e 's/^#.*//'  f1 f2 f3 | grep -v '^$' | wc -l


There are two other options to sed.


SED -N: NO PRINTING

The "-n" option will not print anything unless an explicit request to print is
found. I mentioned the "/p" flag to the substitute command as one way to turn
printing back on. Let me clarify this. The command

sed  's/PATTERN/&/p' file


acts like the cat program if PATTERN is not in the file: e.g. nothing is
changed. If PATTERN is in the file, then each line that has this is printed
twice. Add the "-n" option and the example acts like grep:

sed -n 's/PATTERN/&/p' file


Nothing is printed, except those lines with PATTERN included.

The long argument of the -n command is either

sed --quiet 's/PATTERN/&/p' file


or

sed --silent 's/PATTERN/&/p' file



USING 'SED /PATTERN/'

Sed has the ability to specify which lines are to be examined and/or modified,
by specifying addresses before the command. I will just describe the simplest
version for now - the /PATTERN/ address. When used, only lines that match the
pattern are given the command after the address. Briefly, when used with the /p
flag, matching lines are printed twice:

sed '/PATTERN/p' file


And of course PATTERN is any regular expression.

Please note that if you do not include a command, such as the "p" for print, you
will get an error. When I type

echo abc | sed '/a/'


I get the error

sed: -e expression #1, char 3: missing command


Also, you don't need to, but I recommend that you place a space after the
pattern and the command. This will help you distinguish between flags that
modify the pattern matching, and commands to execute after the pattern is
matched. Therefore I recommend this style:

sed '/PATTERN/ p' file



USING 'SED -N /PATTERN/P' TO DUPLICATE THE FUNCTION OF GREP

If you want to duplicate the functionality of grep, combine the -n (noprint)
option with the /p print flag:

sed -n '/PATTERN/p' file



SED -F SCRIPTNAME

If you have a large number of sed commands, you can put them into a file and use

sed -f sedscript <old >new


where sedscript could look like this:

# sed comment - This script changes lower case vowels to upper case
s/a/A/g
s/e/E/g
s/i/I/g
s/o/O/g
s/u/U/g


When there are several commands in one file, each command must be on a separate
line.

The long argument version is

sed --file=sedscript <old >new


Also see here on writing a script that executes sed directly


SED IN SHELL SCRIPTS

If you have many commands and they won't fit neatly on one line, you can break
up the line using a backslash:

sed -e 's/a/A/g' \
    -e 's/e/E/g' \
    -e 's/i/I/g' \
    -e 's/o/O/g' \
    -e 's/u/U/g'  <old >new



QUOTING MULTIPLE SED LINES IN THE C SHELL

You can have a large, multi-line sed script in the C shell, but you must tell
the C shell that the quote is continued across several lines. This is done by
placing a backslash at the end of each line:

#!/bin/csh -f
sed 's/a/A/g  \
s/e/E/g \
s/i/I/g \
s/o/O/g \
s/u/U/g'  <old >new



QUOTING MULTIPLE SED LINES IN THE BOURNE SHELL

The Bourne shell makes this easier as a quote can cover several lines:

#!/bin/sh
sed '
s/a/A/g 
s/e/E/g 
s/i/I/g 
s/o/O/g 
s/u/U/g'  <old >new



SED -V

The -V option will print the version of sed you are using. The long argument of
the command is

sed --version



SED -H

The -h option will print a summary of the sed commands. The long argument of the
command is

sed --help



A SED INTERPRETER SCRIPT

Another way of executing sed is to use an interpreter script. Create a file that
contains:

#!/bin/sed -f
s/a/A/g
s/e/E/g
s/i/I/g
s/o/O/g
s/u/U/g



Click here to get file: CapVowel.sed
If this script was stored in a file with the name "CapVowel" and was executable,
you could use it with the simple command:

CapVowel <old >new



COMMENTS

Sed comments are lines where the first non-white character is a "#." On many
systems, sed can have only one comment, and it must be the first line of the
script. On the Sun (1988 when I wrote this), you can have several comment lines
anywhere in the script. Modern versions of Sed support this. If the first line
contains exactly "#n" then this does the same thing as the "-n" option: turning
off printing by default. This could not done with a sed interpreter script,
because the first line must start with "#!/bin/sed -f" as I think "#!/bin/sed
-nf" generated an error. It worked when I first wrote this (2008). Note that
"#!/bin/sed -fn" does not work because sed thinks the filename of the script is
"n". However,

"#!/bin/sed -nf" 


does work.


PASSING ARGUMENTS INTO A SED SCRIPT

Passing a word into a shell script that calls sed is easy if you remembered my
tutorial on the UNIX quoting mechanism. To review, you use the single quotes to
turn quoting on and off. A simple shell script that uses sed to emulate grep is:

#!/bin/sh
sed -n 's/'$1'/&/p'

However - there is a subtle problem with this script. If you have a space as an
argument, the script would cause a syntax error, such as


sed: -e expression #1, char 4: unterminated `s' command


A better version would protect from this happening:



#!/bin/sh
sed -n 's/'"$1"'/&/p'



Click here to get file: sedgrep.sed
If this was stored in a file called sedgrep, you could type

sedgrep '[A-Z][A-Z]' <file


This would allow sed to act as the grep command.


USING SED IN A SHELL HERE-IS DOCUMENT

You can use sed to prompt the user for some parameters and then create a file
with those parameters filled in. You could create a file with dummy values
placed inside it, and use sed to change those dummy values. A simpler way is to
use the "here is" document, which uses part of the shell script as if it were
standard input:

#!/bin/sh
echo -n 'what is the value? '
read value
sed  's/XYZ/'$value'/' <<EOF
The value is XYZ
EOF


When executed, the script says:

what is the value?


If you type in "123," the next line will be:

The value is 123


I admit this is a contrived example. "Here is" documents can have values
evaluated without the use of sed. This example does the same thing:

#!/bin/sh
echo -n 'what is the value? '
read value
cat <<EOF
The value is $value
EOF


However, combining "here is" documents with sed can be useful for some complex
cases.
Note that

sed 's/XYZ/'$value'/' <<EOF

will give a syntax error if the user types an answer that contains a space, like
"a b c". Better form would be to put double quotes around the evaluation of the
value:

#!/bin/sh
echo -n 'what is the value? '
read value
sed  's/XYZ/'"$value"'/' <<EOF
The value is XYZ
EOF


I covered this in my tutorial on quotation marks.
Click here to get file: sed_hereis.sed



MULTIPLE COMMANDS AND ORDER OF EXECUTION

As we explore more of the commands of sed, the commands will become complex, and
the actual sequence can be confusing. It's really quite simple. Each line is
read in. Each command, in order specified by the user, has a chance to operate
on the input line. After the substitutions are made, the next command has a
chance to operate on the same line, which may have been modified by earlier
commands. If you ever have a question, the best way to learn what will happen is
to create a small example. If a complex command doesn't work, make it simpler.
If you are having problems getting a complex script working, break it up into
two smaller scripts and pipe the two scripts together.


ADDRESSES AND RANGES OF TEXT

You have only learned one command, and you can see how powerful sed is. However,
all it is doing is a grep and substitute. That is, the substitute command is
treating each line by itself, without caring about nearby lines. What would be
useful is the ability to restrict the operation to certain lines. Some useful
restrictions might be:


 * Specifying a line by its number.
   
 * Specifying a range of lines by number.
   
 * All lines containing a pattern.
   
 * All lines from the beginning of a file to a regular expression
   
 * All lines from a regular expression to the end of the file.
   
 * All lines between two regular expressions.
   

Sed can do all that and more. Every command in sed can be proceeded by an
address, range or restriction like the above examples. The restriction or
address immediately precedes the command:

restriction command



RESTRICTING TO A LINE NUMBER

The simplest restriction is a line number. If you wanted to delete the first
number on line 3, just add a "3" before the command:

sed '3 s/[0-9][0-9]*//' <file >new



PATTERNS

Many UNIX utilities like vi and more use a slash to search for a regular
expression. Sed uses the same convention, provided you terminate the expression
with a slash. To delete the first number on all lines that start with a "#,"
use:

sed '/^#/ s/[0-9][0-9]*//'


I placed a space after the "/expression/" so it is easier to read. It isn't
necessary, but without it the command is harder to fathom. Sed does provide a
few extra options when specifying regular expressions. But I'll discuss those
later. If the expression starts with a backslash, the next character is the
delimiter. To use a comma instead of a slash, use:

sed '\,^#, s/[0-9][0-9]*//'


The main advantage of this feature is searching for slashes. Suppose you wanted
to search for the string "/usr/local/bin" and you wanted to change it for
"/common/all/bin." You could use the backslash to escape the slash:

sed '/\/usr\/local\/bin/ s/\/usr\/local/\/common\/all/'


It would be easier to follow if you used an underline instead of a slash as a
search. This example uses the underline in both the search command and the
substitute command:

sed '\_/usr/local/bin_ s_/usr/local_/common/all_'


This illustrates why sed scripts get the reputation for obscurity. I could be
perverse and show you the example that will search for all lines that start with
a "g," and change each "g" on that line to an "s:"

sed '/^g/s/g/s/g'


Adding a space and using an underscore after the substitute command makes this
much easier to read:

sed '/^g/ s_g_s_g'


Er, I take that back. It's hopeless. There is a lesson here: Use comments
liberally in a sed script. You may have to remove the comments to run the script
under a different (older) operating system, but you now know how to write a sed
script to do that very easily! Comments are a Good Thing. You may have
understood the script perfectly when you wrote it. But six months from now it
could look like modem noise. And if you don't understand that reference, imagine
an 8-month-old child typing on a computer.


RANGES BY LINE NUMBER

You can specify a range on line numbers by inserting a comma between the
numbers. To restrict a substitution to the first 100 lines, you can use:

sed '1,100 s/A/a/'


If you know exactly how many lines are in a file, you can explicitly state that
number to perform the substitution on the rest of the file. In this case, assume
you used wc to find out there are 532 lines in the file:

sed '101,532 s/A/a/'


An easier way is to use the special character "$," which means the last line in
the file.

sed '101,$ s/A/a/'


The "$" is one of those conventions that mean "last" in utilities like cat -e,
vi, and ed. "cat -e" Line numbers are cumulative if several files are edited.
That is,

sed '200,300 s/A/a/' f1 f2 f3 >new


is the same as

cat f1 f2 f3 | sed '200,300 s/A/a/' >new



RANGES BY PATTERNS

You can specify two regular expressions as the range. Assuming a "#" starts a
comment, you can search for a keyword, remove all comments until you see the
second keyword. In this case the two keywords are "start" and "stop:"

sed '/start/,/stop/ s/#.*//'


The first pattern turns on a flag that tells sed to perform the substitute
command on every line. The second pattern turns off the flag. If the "start" and
"stop" pattern occurs twice, the substitution is done both times. If the "stop"
pattern is missing, the flag is never turned off, and the substitution will be
performed on every line until the end of the file.

You should know that if the "start" pattern is found, the substitution occurs on
the same line that contains "start." This turns on a switch, which is line
oriented. That is, the next line is read and the substitute command is checked.
If it contains "stop" the switch is turned off. Switches are line oriented, and
not word oriented.

You can combine line numbers and regular expressions. This example will remove
comments from the beginning of the file until it finds the keyword "start:"

sed -e '1,/start/ s/#.*//'


This example will remove comments everywhere except the lines between the two
keywords:

sed -e '1,/start/ s/#.*//' -e '/stop/,$ s/#.*//'


The last example has a range that overlaps the "/start/,/stop/" range, as both
ranges operate on the lines that contain the keywords. I will show you later how
to restrict a command up to, but not including the line containing the specified
pattern. It is in Operating in a pattern range except for the patterns But I
have to cover some more basic principles.

Before I start discussing the various commands, I should explain that some
commands cannot operate on a range of lines. I will let you know when I mention
the commands. In this next section I will describe three commands, one of which
cannot operate on a range.


DELETE WITH D

Using ranges can be confusing, so you should expect to do some experimentation
when you are trying out a new script. A useful command deletes every line that
matches the restriction: "d." If you want to look at the first 10 lines of a
file, you can use:

sed '11,$ d' <file 


which is similar in function to the head command. If you want to chop off the
header of a mail message, which is everything up to the first blank line, use:

sed '1,/^$/ d' <file


You can duplicate the function of the tail command, assuming you know the length
of a file. Wc can count the lines, and expr can subtract 10 from the number of
lines. A Bourne shell script to look at the last 10 lines of a file might look
like this:

#!/bin/sh
#print last 10 lines of file
# First argument is the filename
lines=$(wc -l "$1" | awk '{print $1}' )
start=$(( lines - 10))
sed "1,$start d" "$1"



Click here to get file: sed_tail.sh
The range for deletions can be regular expressions pairs to mark the begin and
end of the operation. Or it can be a single regular expression. Deleting all
lines that start with a "#" is easy:

sed '/^#/ d'


Removing comments and blank lines takes two commands. The first removes every
character from the "#" to the end of the line, and the second deletes all blank
lines:

sed -e 's/#.*//' -e '/^$/ d'


A third one should be added to remove all blanks and tabs immediately before the
end of line:

sed -e 's/#.*//' -e 's/[ ^I]*$//' -e '/^$/ d' 


The character "^I" is a CTRL-I or tab character. You would have to explicitly
type in the tab. Note the order of operations above, which is in that order for
a very good reason. Comments might start in the middle of a line, with white
space characters before them. Therefore comments are first removed from a line,
potentially leaving white space characters that were before the comment. The
second command removes all trailing blanks, so that lines that are now blank are
converted to empty lines. The last command deletes empty lines. Together, the
three commands remove all lines containing only comments, tabs or spaces.

This demonstrates the pattern space sed uses to operate on a line. The actual
operation sed uses is:


 * Copy the input line into the pattern space.
   
 * Apply the first
   sed command on the pattern space, if the address restriction is true.
   
 * Repeat with the next sed expression, again
   operating on the pattern space.
   
 * When the last operation is performed, write out the pattern space
   and read in the next line from the input file.
   


PRINTING WITH P

Another useful command is the print command: "p." If sed wasn't started with an
"-n" option, the "p" command will duplicate the input. The command

sed 'p'


will duplicate every line. If you wanted to double every empty line, use:

sed '/^$/ p'


Adding the "-n" option turns off printing unless you request it. Another way of
duplicating head's functionality is to print only the lines you want. This
example prints the first 10 lines:

sed -n '1,10 p' <file


Sed can act like grep by combining the print operator to function on all lines
that match a regular expression:

sed -n '/match/ p' 


which is the same as:

grep match



REVERSING THE RESTRICTION WITH !

Sometimes you need to perform an action on every line except those that match a
regular expression, or those outside of a range of addresses. The "!" character,
which often means not in UNIX utilities, inverts the address restriction. You
remember that

sed -n '/match/ p'


acts like the grep command. The "-v" option to grep prints all lines that don't
contain the pattern. Sed can do this with

sed -n '/match/ !p' </tmp/b



RELATIONSHIPS BETWEEN D, P, AND !

As you may have noticed, there are often several ways to solve the same problem
with sed. This is because print and delete are opposite functions, and it
appears that "!p" is similar to "d," while "!d" is similar to "p." I wanted to
test this, so I created a 20 line test file, and tried every different
combination. The following table, which shows the results of by test,
demonstrates the difference:

Relations between d, p, and ! Sed Range Command Results sed -n 1,10 p Print
first 10 lines sed -n 11,$ !p Print first 10 lines sed 1,10 !d Print first 10
lines sed 11,$ d Print first 10 lines sed -n 1,10 !p Print last 10 lines of my
20-line file sed -n 11,$ p Print last 10 lines of my 20-line file sed 1,10 d
Print last 10 lines of my 20-line file sed 11,$ !d Print last 10 lines of my
20-line file sed -n 1,10 d Nothing printed sed -n 1,10 !d Nothing printed sed -n
11,$ d Nothing printed sed -n 11,$ !d Nothing printed sed 1,10 p Print first 10
lines twice, then next 10 lines once sed 11,$ !p Print first 10 lines twice,
then last 10 lines once sed 1,10 !p Print first 10 lines once, then last 10
lines twice sed 11,$ p Print first 10 lines once, then last 10 lines twice

This table shows that using my 20-line test file, the following commands are
identical:

sed -n '1,10 p'
sed -n '11,$ !p'
sed '1,10 !d'
sed '11,$ d'


It also shows that the "!" command "inverts" the address range, operating on the
other lines.

Of course for files longer than 20 lines, you will get more than 10 lines for
the last two cases.


THE Q OR QUIT COMMAND

There is one more simple command that can restrict the changes to a set of
lines. It is the "q" command: quit. the third way to duplicate the head command
is:

sed '11 q'


which quits when the eleventh line is reached. This command is most useful when
you wish to abort the editing after some condition is reached.

The "q" command is the one command that does not take a range of addresses.
Obviously the command

sed '1,10 q'


cannot quit 10 times. Instead

sed '1 q'


or

sed '10 q'


is correct.


GROUPING WITH { AND }

The curly braces, "{" and "}," are used to group the commands.

Hardly worth the buildup. All that prose and the solution is just matching
squiggles. Well, there is one complication. Since each sed command must start on
its own line, the curly braces and the nested sed commands must be on separate
lines.

Previously, I showed you how to remove comments starting with a "#." If you
wanted to restrict the removal to lines between special "begin" and "end" key
words, you could use:



#!/bin/sh
# This is a Bourne shell script that removes #-type comments
# between 'begin' and 'end' words.
sed -n '
	/begin/,/end/ {
	     s/#.*//
	     s/[ ^I]*$//
	     /^$/ d
	     p
	}
'



Click here to get file: sed_begin_end.sh
These braces can be nested, which allow you to combine address ranges. You could
perform the same action as before, but limit the change to the first 100 lines:



#!/bin/sh
# This is a Bourne shell script that removes #-type comments
# between 'begin' and 'end' words.
sed -n '
	1,100 {
		/begin/,/end/ {
		     s/#.*//
		     s/[ ^I]*$//
		     /^$/ d
		     p
		}
	}
'



Click here to get file: sed_begin_end1.sh
You can place a "!" before a set of curly braces. This inverts the address,
which removes comments from all lines except those between the two reserved
words:



#!/bin/sh
sed '
	/begin/,/end/ !{
	     s/#.*//
	     s/[ ^I]*$//
	     /^$/ d
	     p
	}
'



Click here to get file: sed_begin_end2.sh



OPERATING IN A PATTERN RANGE EXCEPT FOR THE PATTERNS

You may remember that I mentioned you can do a substitute on a pattern range,
like changing "old" to "new" between a begin/end pattern:

#!/bin/sh
sed '
	/begin/,/end/ s/old/new/
'


Another way to write this is to use the curly braces for grouping:

#!/bin/sh
sed '
	/begin/,/end/ {
	    s/old/new/
	}
'


I think this makes the code clearer to understand, and easier to modify, as you
will see below.

If you did not want to make any changes where the word "begin" occurred, you
could simple add a new condition to skip over that line:

#!/bin/sh
sed '
	/begin/,/end/ {
	    /begin/n # skip over the line that has "begin" on it
	    s/old/new/
	}
'


However, skipping over the line that has "end" is trickier. If you use the same
method you used for "begin" then the sed engine will not see the "end" to stop
the range - it skips over that as well. The solution is to do a substitute on
all lines that don't have the "end" by using

#!/bin/sh
sed '
    /begin/,/end/ {
      /begin/n # skip over the line that has "begin" on it
      /end/ !{
        s/old/new/
      }  
    }
'



WRITING A FILE WITH THE 'W' COMMAND

You may remember that the substitute command can write to a file. Here again is
the example that will only write lines that start with an even number (and
followed by a space):


sed -n 's/^[0-9]*[02468] /&/w even' <file 


I used the "&" in the replacement part of the substitution command so that the
line would not be changed. A simpler example is to use the "w" command, which
has the same syntax as the "w" flag in the substitute command:


sed -n '/^[0-9]*[02468]/ w even' <file


Remember - only one space must follow the command. Anything else will be
considered part of the file name. The "w" command also has the same limitation
as the "w" flag: only 10 files can be opened in sed.


READING IN A FILE WITH THE 'R' COMMAND

There is also a command for reading files. The command

sed '$r end' <in>out


will append the file "end" at the end of the file (address "$)." The following
will insert a file after the line with the word "INCLUDE:"

sed '/INCLUDE/ r file' <in >out


You can use the curly braces to delete the line having the "INCLUDE" command on
it:



#!/bin/sh
sed '/INCLUDE/ {
	r file
	d
}'



Click here to get file: sed_include.sh


The order of the delete command "d" and the read file command "r" is important.
Change the order and it will not work. There are two subtle actions that prevent
this from working. The first is the "r" command writes the file to the output
stream. The file is not inserted into the pattern space, and therefore cannot be
modified by any command. Therefore the delete command does not affect the data
read from the file.

The other subtlety is the "d" command deletes the current data in the pattern
space. Once all of the data is deleted, it does make sense that no other action
will be attempted. Therefore a "d" command executed in a curly brace also aborts
all further actions. As an example, the substitute command below is never
executed:



#!/bin/sh
# this example is WRONG
sed -e '1 {
	d
	s/.*//
}'



Click here to get file: sed_bad_example.sh


The earlier example is a crude version of the C preprocessor program. The file
that is included has a predetermined name. It would be nice if sed allowed a
variable (e.g "\1" ) instead of a fixed file name. Alas, sed doesn't have this
ability. You could work around this limitation by creating sed commands on the
fly, or by using shell quotes to pass variables into the sed script. Suppose you
wanted to create a command that would include a file like cpp, but the filename
is an argument to the script. An example of this script is:

% include 'sys/param.h' <file.c >file.c.new


A shell script to do this would be:

#!/bin/sh
# watch out for a '/' in the parameter
# use alternate search delimiter
sed -e '\_#INCLUDE <'"$1"'>_{
	r '"$1"'
	d
}'


Let me elaborate. If you had a file that contains

Test first file
#INCLUDE <file1>
Test second file
#INCLUDE <file2>


you could use the command

sed_include1.sh file1<input|sed_include1.sh file2


to include the specified files.


Click here to get file: sed_include1.sh



THE # COMMENT COMMAND

As we dig deeper into sed, comments will make the commands easier to follow. The
older versions of sed only allow one line as a comment, and it must be the first
line. SunOS (and GNU's sed) allows more than one comment, and these comments
don't have to be first. The last example could be:



#!/bin/sh
# watch out for a '/' in the parameter
# use alternate search delimiter
sed -e '\_#INCLUDE <'"$1"'>_{

	# read the file
	r '"$1"'

	# delete any characters in the pattern space
	# and read the next line in
	d
}'



Click here to get file: sed_include2.sh



ADDING, CHANGING, INSERTING NEW LINES

Sed has three commands used to add new lines to the output stream. Because an
entire line is added, the new line is on a line by itself to emphasize this.
There is no option, an entire line is used, and it must be on its own line. If
you are familiar with many UNIX utilities, you would expect sed to use a similar
convention: lines are continued by ending the previous line with a "\". The
syntax to these commands is finicky, like the "r" and "w" commands.


APPEND A LINE WITH 'A'

The "a" command appends a line after the range or pattern. This example will add
a line after every line with "WORD:"



#!/bin/sh
sed '
/WORD/ a\
Add this line after every line with WORD
'



Click here to get file: sed_add_line_after_word.sh


You could eliminate two lines in the shell script if you wish:



#!/bin/sh
sed '/WORD/ a\
Add this line after every line with WORD'



Click here to get file: sed_add_line_after_word1.sh


I prefer the first form because it's easier to add a new command by adding a new
line and because the intent is clearer. There must not be a space after the "\".


INSERT A LINE WITH 'I'

You can insert a new line before the pattern with the "i" command:



#!/bin/sh
sed '
/WORD/ i\
Add this line before every line with WORD
'



Click here to get file: sed_add_line_before_word.sh



CHANGE A LINE WITH 'C'

You can change the current line with a new line.



#!/bin/sh
sed '
/WORD/ c\
Replace the current line with the line
'



Click here to get file: sed_change_line.sh


A "d" command followed by a "a" command won't work, as I discussed earlier. The
"d" command would terminate the current actions. You can combine all three
actions using curly braces:



#!/bin/sh
sed '
/WORD/ {
i\
Add this line before
a\
Add this line after
c\
Change the line to this one
}'



Click here to get file: sed_insert_append_change.sh



LEADING TABS AND SPACES IN A SED SCRIPT

Sed ignores leading tabs and spaces in all commands. However these white space
characters may or may not be ignored if they start the text following a "a," "c"
or "i" command. In SunOS, both "features" are available. The Berkeley (and
Linux) style sed is in /usr/bin, and the AT&T version (System V) is in
/usr/5bin/.

To elaborate, the /usr/bin/sed command retains white space, while the
/usr/5bin/sed strips off leading spaces. If you want to keep leading spaces, and
not care about which version of sed you are using, put a "\" as the first
character of the line:

#!/bin/sh
sed '
	a\
\	This line starts with a tab
'



ADDING MORE THAN ONE LINE

All three commands will allow you to add more than one line. Just end each line
with a "\:"

#!/bin/sh
sed '
/WORD/ a\
Add this line\
This line\
And this line
'



ADDING LINES AND THE PATTERN SPACE

I have mentioned the pattern space before. Most commands operate on the pattern
space, and subsequent commands may act on the results of the last modification.
The three previous commands, like the read file command, add the new lines to
the output stream, bypassing the pattern space.


ADDRESS RANGES AND THE ABOVE COMMANDS

You may remember that earlier I warned you that some commands can take a range
of lines, and others cannot. To be precise, the commands "a," "i," "r," and "q"
will not take a range like "1,100" or "/begin/,/end/." The documentation states
that the read command can take a range, but I got an error when I tried this.
The "c" or change command allows this, and it will let you change several lines
into one:

#!/bin/sh
sed '
/begin/,/end/ c\
***DELETED***
'



If you need to do this, you can use the curly braces, as that will let you
perform the operation on every line:

#!/bin/sh
# add a blank line after every line
sed '1,$ {
	a\

}'



MULTI-LINE PATTERNS

Most UNIX utilities are line oriented. Regular expressions are line oriented.
Searching for patterns that covers more than one line is not an easy task.
(Hint: It will be very shortly.)

Sed reads in a line of text, performs commands which may modify the line, and
outputs modification if desired. The main loop of a sed script looks like this:

 1. The next line is read from the input file and places it in the pattern
    space. If the end of file is found, and if there are additional files to
    read, the current file is closed, the next file is opened, and the first
    line of the new file is placed into the pattern space.
 2. The line count is incremented by one. Opening a new file does not reset this
    number.
 3. Each sed command is examined. If there is a restriction placed on the
    command, and the current line in the pattern space meets that restriction,
    the command is executed. Some commands, like "n" or "d" cause sed to go to
    the top of the loop. The "q" command causes sed to stop. Otherwise the next
    command is examined.
 4. After all of the commands are examined, the pattern space is output unless
    sed has the optional "-n" argument.

The restriction before the command determines if the command is executed. If the
restriction is a pattern, and the operation is the delete command, then the
following will delete all lines that have the pattern:

/PATTERN/ d


If the restriction is a pair of numbers, then the deletion will happen if the
line number is equal to the first number or greater than the first number and
less than or equal to the last number:

10,20 d


If the restriction is a pair of patterns, there is a variable that is kept for
each of these pairs. If the variable is false and the first pattern is found,
the variable is made true. If the variable is true, the command is executed. If
the variable is true, and the last pattern is on the line, after the command is
executed the variable is turned off:

/begin/,/end/ d


Whew! That was a mouthful. If you have read carefully up to here, you should
have breezed through this. You may want to refer back, because I covered several
subtle points. My choice of words was deliberate. It covers some unusual cases,
like:

# what happens if the second number
# is less than the first number?
sed -n '20,1 p' file


and

# generate a 10 line file with line numbers
# and see what happens when two patterns overlap
yes | head -10 | cat -n | \
sed -n -e '/1/,/7/ p' -e '/5/,/9/ p'


Enough mental punishment. Here is another review, this time in a table format.
Assume the input file contains the following lines:

AB
CD
EF
GH
IJ


When sed starts up, the first line is placed in the pattern space. The next line
is "CD." The operations of the "n," "d," and "p" commands can be summarized as:

Pattern Space Next Input Command Output New Pattern Space New Text Input AB CD n
<default> CD EF AB CD d - CD EF AB CD p AB CD EF

The "n" command may or may not generate output depending upon the existence of
the "-n" flag.

That review is a little easier to follow, isn't it? Before I jump into
multi-line patterns, I wanted to cover three more commands:


PRINT LINE NUMBER WITH =

The "=" command prints the current line number to standard output. One way to
find out the line numbers that contain a pattern is to use:

# add line numbers first,
# then use grep, 
# then just print the number
cat -n file | grep 'PATTERN' | awk '{print $1}'


The sed solution is:

sed -n '/PATTERN/ =' file


Earlier I used the following to find the number of lines in a file

#!/bin/sh
lines=$(wc -l file | awk '{print $1}' )


Using the "=" command can simplify this:

#!/bin/sh
lines=$(sed -n '$=' file )


The "=" command only accepts one address, so if you want to print the number for
a range of lines, you must use the curly braces:

#!/bin/sh
# Just print the line numbers 
sed -n '/begin/,/end/ {
=
d
}' file



Since the "=" command only prints to standard output, you cannot print the line
number on the same line as the pattern. You need to edit multi-line patterns to
do this.


TRANSFORM WITH Y

If you wanted to change a word from lower case to upper case, you could write 26
character substitutions, converting "a" to "A," etc. Sed has a command that
operates like the tr program. It is called the "y" command. For instance, to
change the letters "a" through "f" into their upper case form, use:

sed 'y/abcdef/ABCDEF/' file


Here's a sed example that converts all uppercase letters to lowercase letters,
like the tr command:

sed 'y/ABCDEFGHIJKLMNOPQRSTUVWXYZ/abcdefghijklmnopqrstuvwxyz/' <uppercase >lowercase


If you wanted to convert a line that contained a hexadecimal number (e.g.
0x1aff) to upper case (0x1AFF), you could use:

sed '/0x[0-9a-zA-Z]*/ y/abcdef/ABCDEF' file


This works fine if there are only numbers in the file. If you wanted to change
the second word in a line to upper case, and you are using classic sed, you are
out of luck - unless you use multi-line editing. (Hey - I think there is some
sort of theme here!)

However, GNU sed has a uppercase and lowercase extension.


DISPLAYING CONTROL CHARACTERS WITH A L

The "l" command prints the current pattern space. It is therefore useful in
debugging sed scripts. It also converts unprintable characters into printing
characters by outputting the value in octal preceded by a "\" character. I found
it useful to print out the current pattern space, while probing the subtleties
of sed.


WORKING WITH MULTIPLE LINES

There are three new commands used in multiple-line patterns: "N," "D," and "P."
I will explain their relation to the matching "n," "d," and "p" single-line
commands.

The "n" command will print out the current pattern space (unless the "-n" flag
is used), empty the current pattern space, and read in the next line of input.
The "N" command does not print out the current pattern space and does not empty
the pattern space. It reads in the next line, but appends a new line character
along with the input line itself to the pattern space.

The "d" command deletes the current pattern space, reads in the next line, puts
the new line into the pattern space, and aborts the current command, and starts
execution at the first sed command. This is called starting a new "cycle." The
"D" command deletes the first portion of the pattern space, up to the new line
character, leaving the rest of the pattern alone. Like "d," it stops the current
command and starts the command cycle over again. However, it will not print the
current pattern space. You must print it yourself, a step earlier. If the "D"
command is executed with a group of other commands in a curly brace, commands
after the "D" command are ignored. The next group of sed commands is executed,
unless the pattern space is emptied. If this happens, the cycle is started from
the top and a new line is read.

The "p" command prints the entire pattern space. The "P" command only prints the
first part of the pattern space, up to the NEWLINE character. Neither the "p"
nor the "P" command changes the patterns space.

Some examples might demonstrate "N" by itself isn't very useful. the filter

sed -e 'N'


doesn't modify the input stream. Instead, it combines the first and second line,
then prints them, combines the third and fourth line, and prints them, etc. It
does allow you to use a new "anchor" character: "\n." This matches the new line
character that separates multiple lines in the pattern space. If you wanted to
search for a line that ended with the character "#," and append the next line to
it, you could use

#!/bin/sh
sed '
# look for a "#" at the end of the line
/#$/ {
# Found one - now read in the next line
	N
# delete the "#" and the new line character, 
	s/#\n//
}' file


You could search for two lines containing "ONE" and "TWO" and only print out the
two consecutive lines:

#!/bin/sh
sed -n '
/ONE/ {
# found "ONE" - read in next line
	N
# look for "TWO" on the second line
# and print if there.
	/\n.*TWO/ p
}' file


The next example would delete everything between "ONE" and "TWO:"

#!/bin/sh
sed '
/ONE/ {
# append a line
	N
# search for TWO on the second line	
	/\n.*TWO/ {
# found it - now edit making one line
		s/ONE.*\n.*TWO/ONE TWO/
	}
}' file



MATCHING THREE LINES WITH SED

You can match multiple lines in searches.

Here is a way to look for the string "skip3", and if found, delete that line and
the next two lines.

#!/bin/sh
sed '/skip3/ {
           N
           N
           s/skip3\n.*\n.*/# 3 lines deleted/
}'  


Note that it doesn't matter what the next two lines are. If you wanted to match
3 particular lines, it's a little more work.

This script looks for three lines, where the first line contains "one", the
second contained "two" and the third contains "three", and if found, replace
them with the string "1+2+3":

#!/bin/sh
sed '
/one/ {
      N
      /two/ {
            N
            /three/ {
                    N
                    s/one\ntwo\nthree/1+2+3/
                    }
             }
       }
'



MATCHING PATTERNS THAT SPAN MULTIPLE LINES

You can either search for a particular pattern on two consecutive lines, or you
can search for two consecutive words that may be split on a line boundary. The
next example will look for two words which are either on the same line or one is
on the end of a line and the second is on the beginning of the next line. If
found, the first word is deleted:

#!/bin/sh
sed '
/ONE/ {
# append a line
	N
# "ONE TWO" on same line
	s/ONE TWO/TWO/
# "ONE
# TWO" on two consecutive lines
	s/ONE\nTWO/TWO/
}' file


Let's use the
"D" command, and if we find a line containing
"TWO" immediately after a line containing
"ONE," then delete the first line:




#!/bin/sh
sed '
/ONE/ {
# append a line
	N
# if TWO found, delete the first line
	/\n.*TWO/ D
}' file



Click here to get file: sed_delete_line_after_word.sh


If we wanted to print the first line instead of deleting it, and not print every
other line, change the "D" to a "P" and add a "-n" as an argument to sed:



#!/bin/sh
sed -n '
# by default - do not print anything
/ONE/ {
# append a line
	N
# if TWO found, print the first line
	/\n.*TWO/ P
}' file



Click here to get file: sed_print_line_after_word.sh


It is very common to combine all three multi-line commands. The typical order is
"N," "P" and lastly "D." This one will delete everything between "ONE" and "TWO"
if they are on one or two consecutive lines:



#!/bin/sh
sed '
/ONE/ {
# append the next line
	N
# look for "ONE" followed by "TWO"
	/ONE.*TWO/ {
#	delete everything between
		s/ONE.*TWO/ONE TWO/
#	print
		P
#	then delete the first line
		D
	}
}' file



Click here to get file: sed_delete_between_two_words.sh


Earlier I talked about the "=" command, and using it to add line numbers to a
file. You can use two invocations of sed to do this (although it is possible to
do it with one, but that must wait until next section). The first sed command
will output a line number on one line, and then print the line on the next line.
The second invocation of sed will merge the two lines together:



#!/bin/sh
sed '=' file | \
sed '{
	N
	s/\n/ /
}'



Click here to get file: sed_merge_two_lines.sh


If you find it necessary, you can break one line into two lines, edit them, and
merge them together again. As an example, if you had a file that had a
hexadecimal number followed by a word, and you wanted to convert the first word
to all upper case, you can use the "y" command, but you must first split the
line into two lines, change one of the two, and merge them together. That is, a
line containing

0x1fff table2


will be changed into two lines:

0x1fff
table2


and the first line will be converted into upper case. I will use tr to convert
the space into a new line, and then use sed to do the rest. The command would be

./sed_split <file


and sed_split would be:

#!/bin/sh
tr ' ' '\012' | 
sed ' {
	y/abcdef/ABCDEF/
	N
	s/\n/ /
}'



Click here to get file: sed_split.sh


It isn't obvious, but sed could be used instead of tr. You can embed a new line
in a substitute command, but you must escape it with a backslash. It is
unfortunate that you must use "\n" in the left side of a substitute command, and
an embedded new line in the right hand side. Heavy sigh. Here is the example:

#!/bin/sh
sed '
s/ /\
/' | \
sed ' {
	y/abcdef/ABCDEF/
	N
	s/\n/ /
}'



Click here to get file: sed_split_merge.sh


Sometimes I add a special character as a marker, and look for that character in
the input stream. When found, it indicates the place a blank used to be. A
backslash is a good character, except it must be escaped with a backslash, and
makes the sed script obscure. Save it for that guy who keeps asking dumb
questions. The sed script to change a blank into a "\" following by a new line
would be:

#!/bin/sh
sed 's/ /\\\
/' file



Click here to get file: sed_addslash_before_blank.sh


Yeah. That's the ticket. Or use the C shell and really confuse him!

#!/bin/csh -f
sed '\
s/ /\\\\
/' file



Click here to get file: sed_addslash_before_blank.csh


A few more examples of that, and he'll never ask you a question again! I think
I'm getting carried away. I'll summarize with a chart that covers the features
we've talked about:

Pattern Space Next Input Command Output New Pattern Space New Text Input AB CD n
<default> CD EF AB CD N - AB\nCD EF AB CD d - - EF AB CD D - - EF AB CD p AB AB
CD AB CD P AB AB CD AB\nCD EF n <default> EF GH AB\nCD EF N - AB\nCD\nEF GH
AB\nCD EF d - EF GH AB\nCD EF D - CD EF AB\nCD EF p AB\nCD AB\nCD EF AB\nCD EF P
AB AB\nCD EF


USING NEWLINES IN SED SCRIPTS

Occasionally one wishes to use a new line character in a sed script. Well, this
has some subtle issues here. If one wants to search for a new line, one has to
use "\n." Here is an example where you search for a phrase, and delete the new
line character after that phrase - joining two lines together.

(echo a;echo x;echo y) | sed '/x$/ {
N
s:x\n:x:
}'


which generates

a
xy


However, if you are inserting a new line, don't use "\n" - instead insert a
literal new line character:

(echo a;echo x;echo y) | sed 's:x:X\
:'


generates

a
X

y



THE HOLD BUFFER

So far we have talked about three concepts of sed: (1) The input stream or data
before it is modified, (2) the output stream or data after it has been modified,
and (3) the pattern space, or buffer containing characters that can be modified
and send to the output stream.

There is one more "location" to be covered: the hold buffer or hold space. Think
of it as a spare pattern buffer. It can be used to "copy" or "remember" the data
in the pattern space for later. There are five commands that use the hold
buffer.


EXCHANGE WITH X

The "x" command eXchanges the pattern space with the hold buffer. By itself, the
command isn't useful. Executing the sed command

sed 'x'


as a filter adds a blank line in the front, and deletes the last line. It looks
like it didn't change the input stream significantly, but the sed command is
modifying every line.

The hold buffer starts out containing a blank line. When the "x" command
modifies the first line, line 1 is saved in the hold buffer, and the blank line
takes the place of the first line. The second "x" command exchanges the second
line with the hold buffer, which contains the first line. Each subsequent line
is exchanged with the preceding line. The last line is placed in the hold
buffer, and is not exchanged a second time, so it remains in the hold buffer
when the program terminates, and never gets printed. This illustrates that care
must be taken when storing data in the hold buffer, because it won't be output
unless you explicitly request it.


EXAMPLE OF CONTEXT GREP

One use of the hold buffer is to remember previous lines. An example of this is
a utility that acts like grep as it shows you the lines that match a pattern. In
addition, it shows you the line before and after the pattern. That is, if line 8
contains the pattern, this utility would print lines 7, 8 and 9.

One way to do this is to see if the line has the pattern. If it does not have
the pattern, put the current line in the hold buffer. If it does, print the line
in the hold buffer, then the current line, and then the next line. After each
set, three dashes are printed. The script checks for the existence of an
argument, and if missing, prints an error. Passing the argument into the sed
script is done by turning off the single quote mechanism, inserting the "$1"
into the script, and starting up the single quote again:





#!/bin/sh
# grep3 - prints out three lines around pattern
# if there is only one argument, exit

case $# in 
	1);;
	*) echo "Usage: $0 pattern";exit;;
esac;
# I hope the argument doesn't contain a /
# if it does, sed will complain

# use sed -n to disable printing 
# unless we ask for it
sed -n '
'/"$1"/' !{
	#no match - put the current line in the hold buffer
	x
	# delete the old one, which is 
	# now in the pattern buffer
	d
}
'/"$1"/' {
	# a match - get last line
	x
	# print it
	p
	# get the original line back
	x
	# print it
	p
	# get the next line 
	n
	# print it
	p
	# now add three dashes as a marker
	a\
---
	# now put this line into the hold buffer
	x
}'



Click here to get file: grep3.sh


You could use this to show the three lines around a keyword, i.e.:

grep3 vt100 </etc/termcap



HOLD WITH H OR H

The "x" command exchanges the hold buffer and the pattern buffer. Both are
changed. The "h" command copies the pattern buffer into the hold buffer. The
pattern buffer is unchanged. An identical script to the above uses the hold
commands:





#!/bin/sh
# grep3 version b - another version using the hold commands
# if there is only one argument, exit

case $# in 
	1);;
	*) echo "Usage: $0 pattern";exit;;
esac;

# again - I hope the argument doesn't contain a /

# use sed -n to disable printing 

sed -n '
'/"$1"/' !{
	# put the non-matching line in the hold buffer
	h
}
'/"$1"/' {
	# found a line that matches
	# append it to the hold buffer
	H
	# the hold buffer contains 2 lines
	# get the next line
	n
	# and add it to the hold buffer
	H
	# now print it back to the pattern space
	x
	# and print it.
	p
	# add the three hyphens as a marker
	a\
---
}'



Click here to get file: grep3a.sh



KEEPING MORE THAN ONE LINE IN THE HOLD BUFFER

The "H" command allows you to combine several lines in the hold buffer. It acts
like the "N" command as lines are appended to the buffer, with a "\n" between
the lines. You can save several lines in the hold buffer, and print them only if
a particular pattern is found later.

As an example, take a file that uses spaces as the first character of a line as
a continuation character. The files /etc/termcap, /etc/printcap, makefile and
mail messages use spaces or tabs to indicate a continuing of an entry. If you
wanted to print the entry before a word, you could use this script. I use a "^I"
to indicate an actual tab character:





#!/bin/sh 
# print previous entry
sed -n '
/^[ ^I]/!{
	# line does not start with a space or tab,
	# does it have the pattern we are interested in?
	'/"$1"/' {
		# yes it does. print three dashes
		i\
---
		# get hold buffer, save current line
		x
		# now print what was in the hold buffer
		p
		# get the original line back
		x
	}
	# store it in the hold buffer
	h
}
# what about lines that start
# with a space or tab?
/^[ ^I]/ {
	# append it to the hold buffer
	H
}'



Click here to get file: grep_previous.sh


You can also use the "H" to extend the context grep. In this example, the
program prints out the two lines before the pattern, instead of a single line.
The method to limit this to two lines is to use the "s" command to keep one new
line, and deleting extra lines. I call it grep4:





#!/bin/sh

# grep4: prints out 4 lines around pattern
# if there is only one argument, exit

case $# in 
	1);;
	*) echo "Usage: $0 pattern";exit;;
esac;

sed -n '
'/"$1"/' !{
	# does not match - add this line to the hold space
	H
	# bring it back into the pattern space
	x
	# Two lines would look like .*\n.*
	# Three lines look like .*\n.*\n.*
	# Delete extra lines - keep two
	s/^.*\n\(.*\n.*\)$/\1/
	# now put the two lines (at most) into 
	# the hold buffer again
	x
}
'/"$1"/' {
	# matches - append the current line
	H
	# get the next line
	n
	# append that one also
	H
	# bring it back, but keep the current line in
	# the hold buffer. This is the line after the pattern,
	# and we want to place it in hold in case the next line
	# has the desired pattern
	x
	# print the 4 lines
	p
	# add the mark
	a\
---
}'



Click here to get file: grep4.sh
You can modify this to print any number of lines around a pattern. As you can
see, you must remember what is in the hold space, and what is in the pattern
space. There are other ways to write the same routine.


GET WITH G OR G

Instead of exchanging the hold space with the pattern space, you can copy the
hold space to the pattern space with the "g" command. This deletes the pattern
space. If you want to append to the pattern space, use the "G" command. This
adds a new line to the pattern space, and copies the hold space after the new
line.

Here is another version of the "grep3" command. It works just like the previous
one, but is implemented differently. This illustrates that sed has more than one
way to solve many problems. What is important is you understand your problem,
and document your solution:





#!/bin/sh
# grep3 version c: use 'G'  instead of H

# if there is only one argument, exit

case $# in 
	1);;
	*) echo "Usage: $0 pattern";exit;;
esac;

# again - I hope the argument doesn't contain a /

sed -n '
'/"$1"/' !{
	# put the non-matching line in the hold buffer
	h
}
'/"$1"/' {
	# found a line that matches
	# add the next line to the pattern space
	N
	# exchange the previous line with the 
	# 2 in pattern space
	x
	# now add the two lines back
	G
	# and print it.
	p
	# add the three hyphens as a marker
	a\
---
	# remove first 2 lines
	s/.*\n.*\n\(.*\)$/\1/
	# and place in the hold buffer for next time
	h
}'




Click here to get file: grep3c.sh


The "G" command makes it easy to have two copies of a line. Suppose you wanted
to the convert the first hexadecimal number to uppercase, and don't want to use
the sed_split.sh
script I described earlier. That script only works when there are exactly 2
words per line. If you wanted to allow more than one word on a line and only
convert the first hex word to upperxase, then this is a better approach:



#!/bin/sh
# change the first hex number to upper case format, leave the rest of the line alone
# uses sed twice
# used as a filter
# convert2uc <in >out
sed '
s/ /\
/' | \
sed ' {
	y/abcdef/ABCDEF/
	N
	s/\n/ /
}'



Click here to get file: convert2uc.sh


Here is a solution that does not require two invocations of sed because it uses
the "h" and "G" command:



#!/bin/sh
# convert2uc version b
# change the first hex number to upper case format, leave the rest of the line alone
# uses sed once
# used as a filter
# convert2uc <in >out
sed '
{
	# remember the line
	h
	#change the current line to upper case
	y/abcdef/ABCDEF/
	# add the old line back
	G
	# Keep the first word of the first line, 
	# and second word of the second line
	# with one humongous regular expression
	s/^\([^ ]*\) .*\n[^ ]* \(.*\)/\1 \2/
}'



Click here to get file: convert2uc1.sh
Carl Henrik Lunde suggested a way to make this simpler, but not as general
purpose. The two previous verion converted works with lines with multiple words.
This one only converts two words - the first and the last. However, it deletes
any word in-between.



#!/bin/sh
# convert2uc version b
# change the first hex number to upper case format, and keeps the last word
# Note that it deletes the words in-between
# uses sed once
# used as a filter
# convert2uc <in >out
sed '
{
	# remember the line
	h
	#change the current line to upper case
	y/abcdef/ABCDEF/
	# add the old line back
	G
	# Keep the first word of the first line, 
	# and last word of the second line
	# with one humongous regular expression
	s/ .* / / # delete all but the first and last word
}'



Click here to get file: convert2uc2.sh
This example only converts the letters "a" through "f" to upper case. This was
chosen to make the script easier to print in these narrow columns. You can
easily modify the script to convert all letters to uppercase, or to change the
first letter, second word, etc.


BRANCH (FLOW CONTROL)

As you learn about sed you realize that it has its own programming language. It
is true that it's a very specialized and simple language. What language would be
complete without a method of changing the flow control? There are three commands
sed uses for this. You can specify a label with a text string preceded by a
colon. The "b" command branches to the label. The label follows the command. If
no label is there, branch to the end of the script. The "t" command is used to
test conditions. Before I discuss the "t" command, I will show you an example
using the "b" command.

This example remembers paragraphs, and if it contains the pattern (specified by
an argument), the script prints out the entire paragraph.



#!/bin/sh
sed -n '
# if an empty line, check the paragraph
/^$/ b para
# else add it to the hold buffer
H
# at end of file, check paragraph
$ b para
# now branch to end of script
b
# this is where a paragraph is checked for the pattern
:para
# return the entire paragraph
# into the pattern space
x
# look for the pattern, if there - print
/'"$1"'/ p
'



Click here to get file: grep_paragraph.sh



TESTING WITH T

You can execute a branch if a pattern is found. You may want to execute a branch
only if a substitution is made. The command "t label" will branch to the label
if the last substitute command modified the pattern space.

One use for this is recursive patterns. Suppose you wanted to remove white space
inside parenthesis. These parentheses might be nested. That is, you would want
to delete a string that looked like "( ( ( ())) )." The sed expressions

sed 's/([ ^I]*)/g'


would only remove the innermost set. You would have to pipe the data through the
script four times to remove each set or parenthesis. You could use the regular
expression

sed 's/([ ^I()]*)/g'


but that would delete non-matching sets of parenthesis. The "t" command would
solve this:

#!/bin/sh
sed '
:again
	s/([ ^I]*)//
	t again
'


An earlier version had a 'g' after the 's' expression. This is not needed.


Click here to get file: delete_nested_parens.sh



DEBUGGING WITH L

The 'l' command will print the pattern space in an unambiguous form.
Non-printing characters are printed in a C-style escaped format.

This can be useful when debugging a complex multi-line sed script.


AN ALTERNATE WAY OF ADDING COMMENTS

There is one way to add comments in a sed script if you don't have a version
that supports it. Use the "a" command with the line number of zero:



#!/bin/sh
sed '
/begin/ {
0i\
	This is a comment\
	It can cover several lines\
	It will work with any version of sed
}'



Click here to get file: sed_add_comments.sh



THE POORLY DOCUMENTED ;

There is one more sed command that isn't well documented. It is the ";" command.
This can be used to combined several sed commands on one line. Here is the grep4
script I described earlier, but without the comments or error checking and with
semicolons between commands:

#!/bin/sh
sed -n '
'/"$1"/' !{;H;x;s/^.*\n\(.*\n.*\)$/\1/;x;}
'/"$1"/' {;H;n;H;x;p;a\
---
}'



Click here to get file: grep4a.sh


Yessireebob! Definitely character building. I think I have made my point. As far
as I am concerned, the only time the semicolon is useful is when you want to
type the sed script on the command line. If you are going to place it in a
script, format it so it is readable. I have mentioned earlier that many versions
of sed do not support comments except on the first line. You may want to write
your scripts with comments in them, and install them in "binary" form without
comments. This should not be difficult. After all, you have become a sed guru by
now. I won't even tell you how to write a script to strip out comments. That
would be insulting your intelligence. Also - some operating systems do NOT let
you use semicolons. So if you see a script with semicolons, and it does not work
on a non-Linux system, replace the semicolon with a new line character. (As long
as you are not using csh/tcsh, but that's another topic.


PASSING REGULAR EXPRESSIONS AS ARGUMENTS

In the earlier scripts, I mentioned that you would have problems if you passed
an argument to the script that had a slash in it. In fact, regular expression
might cause you problems. A script like the following is asking to be broken
some day:

#!/bin/sh
sed 's/'"$1"'//g'


If the argument contains any of these characters in it, you may get a broken
script: "/\.*[]^$" For instance, if someone types a "/" then the substitute
command will see four delimiters instead of three. You will also get syntax
errors if you provide a "]" without a "]". One solution is to have the user put
a backslash before any of these characters when they pass it as an argument.
However, the user has to know which characters are special.
Here's another solution - add a backslash before each of those special
characters in the script.

#!/bin/sh
# put two backslashes before each of these characters: ][^$.*/
# Note that the first ']' doesn't need a backslash
arg=$(echo "$1" | sed 's:[]\[\^\$\.\*\/]:\\\\&:g')
# We need two backslashes because the shell converts each double backslash in quotes to a single backslash
sed 's/'"$arg"'//g'



Click here to get file: sed_with_regular_expressions1.sh
If you were searching for the pattern "^../," the script would convert this into
"\^\.\.\/" before passing it to sed.


INSERTING BINARY CHARACTERS

Dealing with binary characters can be trick, especially when writing scripts for
people to read. I can insert a binary character using an editor like EMACS but
if I show the binary character, the terminal may change it to show it to you.

The easiest way I have found to do this in a script in a portable fashion is to
use the tr(1) command. It understands octal notations, and it can be output into
a variable which can be used.

Here's a script that will replace the string "ding" with the ASCII bell
character:

#!/bin/sh
BELL=$(echo x | tr 'x' '\007')
sed "s/ding/$BELL/"


Please note that I used double quotes. Since special characters are interpreted,
you have to be careful when you use this mechanism.


GNU SED COMMAND LINE ARGUMENTS

One of the conventions UNIX systems have is to use single letters are command
line arguments. This makes typing faster, and shorted, which is an advantage if
you are in a contest. Normal people often find sed's terseness cryptic. You can
improve the readability of sed scripts by using the long word equivalent
options. That is, instead of typing

sed -n 20p


You can type the long word version of the -n argument

sed --quiet 20p


Or

sed --silent 20p


The long form of sed's command line arguments always have 2 hyphens before their
names. GNU sed has the following long-form command line arguments:

GNU Command Line Arguments

Short Form

Long Form

-n

--quiet

--silent

-e script

--expression=SCRIPT

-f SCRIPTFILE

--file=SCRIPTFILE

-i[SUFFIX]

--in-place[=SUFFIX]

-l N

--line-length=N




--posix

-b

--binary




--follow-symlinks

-r

--regular-extended

-s

--separate

-u

--unbuffered




--help




--version

Let's define each of these.


THE -POSIX ARGUMENT

The GNU version of sed has many features that are not available in other
versions. When portability is important, test your script with the -posix
option. If you had an example that used a feature of GNU sed, such as the 'v'
command to test the version number, such as

#this is a sed command file
v 4.0.1
# print the number of lines
$=


And you executed it with the command

sed -nf sedfile --posix <file


then the GNU version of sed program would give you a warning that your sed
script is not compatible. It would report:

sed: -e expression #1, char 2: unknown command: `v'



THE --VERSION ARGUMENT

You can determine which version of sed you are using with the GNU sed --version
command. This is what it outputs on my computer

# sed --version
GNU sed version 4.2.1
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE,
to the extent permitted by law.

GNU sed home page: <https://www.gnu.org/software/sed/>.
General help using GNU software: <https://www.gnu.org/gethelp/>.
E-mail bug reports to: <bug-gnu-utils@gnu.org>.
Be sure to include the word ``sed'' somewhere in the ``Subject:'' field.



THE -H HELP ARGUMENT

The -h option will print a summary of the sed commands. The long argument of the
command is

sed --help


It provides a nice summary of the command line arguments.


THE -L LINE LENGTH ARGUMENT

I've already described the 'l' command. The default line width for the 'l'
command is 70 characters. This default value can be changed by adding the '-l N'
option and specifying the maximum line length as the number after the '-l'.

sed -n -l 80 'l' <file


The long form version of the command line is

 sed -n --line-length=80 'l' <file



THE -S SEPARATE ARGUMENT

Normally, when you specify several files on the command line, sed concatenates
the files into one stream, and then operates on that single stream. If you had
three files, each with 100 lines, then the command

sed -n '1,10 p' file1 file2 file3


would only print the first 10 lines of file file1. The '-s' command tells GNU
sed to treat the files are independent files, and to print out the first 10
lines of each file, which is similar to the head command. Here's another
example: If you wanted to print the number of lines of each file, you could use
'wc -l' which prints the number of lines, and the filename, for each file, and
at the end print the total number of lines. Here is a simple shell script that
does something similar, just using sed:

#!/bin/sh
FILES=$*
sed -s -n '$=' $FILES # print the number of lines for each file
sed -n '$=' $FILES # print the total number of lines.


The 'wc -l' command does print out the filenames, unlike the above script. A
better emulation of the 'wc -l' command would execute the command in a loop, and
print the filenames. Here is a more advanced script that does this, but it
doesn't use the '-s' command:

#!/bin/sh
for F in "$@"
do
 NL=$(sed -n '$=' < "$F" ) &&  printf "  %d %s\n" $NL "$F"
done
TOTAL=$(sed -n '$=' "$@")
printf "  %d total\n" $TOTAL



THE -I IN-PLACE ARGUMENT

I've already described in Editing multiple files the way I like to do this. For
those who want a simpler method, GNU Sed allows you to do this with a command
line option - "-i". Let's assume that we are going to make the same simple
change - adding a tab before each line. This is a way to do this for all files
in a directory with the ".txt" extension in the current directory:

sed -i 's/^/\t/' *.txt


The long argument name version is

sed --in-place 's/^/\t/' *.txt


This version deletes the original file. If you are as cautious as I am, you may
prefer to specify an extension, which is used to keep a copy of the original:

sed -i.tmp 's/^/\t/' *.txt


And the long argument name version is

sed --in-place=.tmp 's/^/\t/'  *.txt


In the last two versions, the original version of the "a.txt" file would have
the name "a.txt.tmp". You can then delete the original files after you make sure
all worked as you expected. Please consider the backup option, and heed my
warning. You can easily delete the backed-up original file, as long as the
extension is unique.

The GNU version of sed allows you to use "-i" without an argument. The
FreeBSD/Mac OS X does not. You must provide an extension for the FreeBSD/Mac OS
X version. If you want to do in-place editing without creating a backup, you can
use

sed -i ''  's/^/\t/'  *.txt



THE --FOLLOW-SYMLINKS ARGUMENT

The in-place editing feature is handy to have. But what happens if the file you
are editing is a symbolic link to another file? Let's assume you have a file
named "b" in a directory called "tmp", with a symbolic link to this file:

$ ls -l b
lrwxrwxrwx 1 barnett adm 6 Mar 16 16:03 b.txt -> tmp/b.txt


If you executed the above command to do in place editing, there will be a new
file called "b.txt" in the current directory, and "tmp/b.txt" will be unchanged.
Now you have two versions of the file, one is changed (in the current
directory), and one is not (in the "tmp" directory). And where you had a
symbolic link, it has been replaced with a modified version of the original
file. If you want to edit the real file, and keep the symbolic link in place,
use the "--follow-symlinks" command line option:

sed -i --follow-symlinks 's/^/\t/' *.txt


This follows the symlink to the original location, and modifies the file in the
"tmp" directory, If you specify an extension, the original file will be found
with that extension in the same directory as the real source. Without the
--follow-symlinks command line option, the "backup" file "b.tmp" will be in the
same directory that held the symbolic link, and will still be a symbolic link -
just renamed to give it a new extension.


THE -B BINARY ARGUMENT

Unix and Linux systems consider the new line character "\n" to be the end of the
line. However, MS-DOS, Windows, and Cygwin systems end each line with "\r\n" -
Carriage return and line-feed. If you are using any of these operating systems,
the "-b" or --binary" command line option will treat the carriage return/new
line combination as the end of the line. Otherwise the carriage return is
treated as an unprintable character immediately before the end-of-line. I think.
(Note to self - verify this).


THE -R EXTENDED REGULAR EXPRESSION ARGUMENT

When I mention patterns, such as "s/pattern/", the pattern is a regular
expression. There are two common classes of regular expressions, the original
"basic" expressions, and the "extended" regular expressions. For more on the
differences see My tutorial on regular expressions and the the section on
extended regular expressions. Because the meaning of certain characters are
different between the regular and extended expressions, you need a command line
argument to enable sed to use the extension. To enable this extension, use the
"-r" command, as mentioned in the example on finding duplicated words on a line

sed -r -n '/\([a-z]+\) \1/p'


or

sed --regular-extended -quiet '/\([a-z]+\) \1/p'


I already mentioned that Mac OS X and FreeBSD uses -E instead of -r.


THE -U UNBUFFERED ARGUMENT

Normally - Unix and Linux systems apply some intelligence to handling standard
output. It's assumed that if you are sending results to a terminal, you want the
output as soon as it becomes available. However, if you are sending the output
to a file, then it's assumed you want better performance, so it buffers the
output until the buffer is full, and then the contents of the buffer is written
to the file. Let me elaborate on this. Let's assume for this example you have a
very large file, and you are using sed to search for a string, and to print it
when it is found:

sed -n '/MATCH/p' <file


Since the output is the terminal, as soon as a match is found, it is printed.
However, if sed pipes its output to another program, it will buffer the results.
But there are times when you want immediate results. This is especially true
when you are dealing with large files, or files that occasionally generate data.
To summarize, you have lots of input data, and you want sed to process it, and
then send this to another program that processes the results, but you want the
results when it happens, and not delayed. Let me make up a simple example. It's
contrived, but it does explain how this works. Here's a program called SlowText
that prints numbers from 1 to 60, once a second:

#!/bin/sh
for i in $(seq 1 60)
do
  echo $i
  sleep 1
done


Let's use sed to search for lines that have the character '1', and have it send
results to awk, which will calculate the square of that number. This would be
the admittedly contrived script:

SlowText | sed -n '/1/p' | awk '{print $1*$1}'


This works, but because sed is buffering the results, we have to wait until the
buffer fills up, or until the SlowText program exists, before we the results.
You can eliminate the buffering, and see the results as soon as SlowText outputs
them, by using the "-u" option. With this option, you will see the squares
printed as soon as possible:

SlowText | sed -un '/1/p' | awk '{print $1*$1}'


The long form of the argument is "--unbuffered".

Mac OS X and FreeBSD use the argument "-l".

GNU Sed 4.2.2 and later will also be unbuffered while reading files, not just
writing them.


THE -Z NULL DATA ARGUMENT

Normally, sed reads a line by reading a string of characters up to the
end-of-line character (new line or carriage return). See the -b Binary command
line argument The GNU version of sed added a feature in version 4.2.2 to use the
"NULL" character instead. This can be useful if you have files that use the NULL
as a record separator. Some GNU utilities can generate output that uses a NULL
instead a new line, such as "find . -print0" or "grep -lZ". This feature is
useful if you are operating on filenames that might contain spaces or binary
characters.

For instance, if you wanted to use "find" to search for files and you used the
"-print0" option to print a NULL at the end of each filename, you could use sed
to delete the directory pathname:

find . -type f -print0  | sed -z 's:^.*/::' | xargs -0 echo 


The above example is not terribly useful as the "xargs" use of echo does not
retain the ability to retain spaces as part of the filename. But is does show
how to use the sed "-z" command.

GNU grep also has a -Z option to search for strings in files, placing a "NULL"
at the end of each filename instead of a new line. And with the -l command, grep
will print the filename that contains the string, retaining non-printing and
binary characters:

grep -lZ STRING */*/* | sed -z 's:^.*/::' | xargs -0 echo


This feature is very useful when users have the ability to create their own
filenames.


FREEBSD EXTENSIONS

Apple uses the FreeBSD version of sed for Mac OS X instead of the GNU sed.
However, the FreeBSD version has a couple of additions.


THE -A OR DELAYED OPEN ARGUMENT

Normally, as soon as sed starts up, it opens all files that are referred to by
the "w" command. The FreeBSD version of sed has an option to delay this action
until the "w" command is executed.


THE -I IN-PLACE ARGUMENT

FreeBSD added a "-I" option that is similar to the -i option. The "-i" option
treats the editing each file as a separate instance of sed. If the "-I" option
is used, then line numbers do not get reset at the beginning of each line, and
ranges of addresses continue from one file to the next. That is, if you used the
range '/BEGIN/,/END/' and you used the "-I" option, you can have the "BEGIN" in
the first file, and "END" in the second file, and the commands executed within
the range would span both files. If you used "-i", then the commands would not.

And like the -i option, the extension used to store the backup file must be
specified.


-E OR EXTENDED REGULAR EXPRESSIONS

I mentioned extended regular expressions earlier. FreeBSD (and Mac OS X) uses
"-E" to enable this. However, FreeBSD later added the -r command to be
compatible with GNU sed.


USING WORD BOUNDARIES

Someone once asked me to help them solve a tricky sed problem involving word
boundaries. Let's suppose you have the following input

/usr/bin /usr/local/bin /usr/local /usr/local/project/bin


and you wanted to delete '/usr/local' but leave the other 3 paths alone. You
could use the simple (and incorrect) command:

sed 's@/usr/local@@'


which would output

/usr/bin /bin /usr/local /usr/local/project/bin


That is, it would mistakenly change '/usr/local/bin' to '/bin' and not delete
'/usr/local' which was the intention of the programmer. The better method is to
include spaces around the search:

sed 's@ /usr/local @ @'


However, this won't work if '/usr/local' is at the beginning, or at the end of
the line. It also won't work if '/usr/local' is the only path on the line. To
handle these edge cases, you can simply describe all of these conditions as
separate cases:

#!/bin/sh
sed '
s@ /usr/local @ @g
s@^/usr/local @@
s@ /usr/local$@@
s@^/usr/local$@@
'


This works fine if the string you are searching for is surrounded by a space.
But what happens if the string is surrounded by other characters, which may be
one of several possible characters? You can always make up your own class of
characters that define the 'end of a word'; For instance, if your string
consists of alphanumeric characters and the slash, the class of characters can
be defined by '[a-zA-Z0-9/]' or the more flexible '[[:alnum:]/]'. We can define
the class of characters to be all but these, by using the caret, i.e.
'[^[:alnum:]/]'. And unlike the space before, if you are going to use character
classes, you may have to remember what these characters are and not delete them.
So we can replace the space with '[^[:alnum:]/]' and then change the command to
be

#!/bin/sh
sed '
s@\([^[:alnum:]/]\)/usr/local\([^[:alnum:]/]\)@\1\2@g
s@^/usr/local\([^[:alnum:]/]\)@\1@
s@\([^[:alnum:]/]\)/usr/local$@\1@
s@^/usr/local$@@
'


The first version would replace ' /usr/local ' with a single space. This method
would replace ':/usr/local:' with '::' - because the redundant delimiters are
not deleted. Be sure to fix this if you need to.

This method always works, but it is inelegant and error prone. There are other
methods, but they may not be portable. Solaris's version of sed used the special
characters ‘\<’ and ‘\>’ as anchors that indicated a word boundary. So you could
use

s@\</usr/local\>@@


However, the GNU version of sed says the usage of these special characters are
undefined. According to the manual page:

Regex syntax clashes (problems with backslashes)
     `sed' uses the POSIX basic regular expression syntax.  According to
     the standard, the meaning of some escape sequences is undefined in
     this syntax;  notable in the case of `sed' are `\|', `\+', `\?',
     `\`', `\'', `\<', `\>', `\b', `\B', `\w', and `\W'.

     As in all GNU programs that use POSIX basic regular expressions,
     `sed' interprets these escape sequences as special characters.
     So, `x\+' matches one or more occurrences of `x'.  `abc\|def'
     matches either `abc' or `def'.


When in doubt, experiment.


COMMAND SUMMARY

As I promised earlier, here is a table that summarizes the different commands.
The second column specifies if the command can have a range or pair of addresses
or a single address or pattern. The next four columns specifies which of the
four buffers or streams are modified by the command. Some commands only affect
the output stream, others only affect the hold buffer. If you remember that the
pattern space is output (unless a "-n" was given to sed), this table should help
you keep track of the various commands.

Command Address or Range Modification to
Input Stream Modification to
Output Stream Modification to
Pattern Space Modification to
Hold Buffer = - - Y - - a Address - Y - - b Range - - - - c Range - Y - - d
Range Y - Y - D Range Y - Y - g Range - - Y - G Range - - Y - h Range - - - Y H
Range - - - Y i Address - Y - - l Address - Y - - n Range Y * - - N Range Y - Y
- p Range - Y - - P Range - Y - - q Address - - - - r Address - Y - - s Range -
- Y - t Range - - - - w Range - Y - - x Range - - Y Y y Range - - Y -

The "n" command may or may not generate output, depending on the "-n" option.
The "r" command can only have one address, despite the documentation.

Check out my Sed Reference Chart


IN CONCLUSION

This concludes my tutorial on sed. It is possible to find shorter forms of some
of my scripts. However, I chose these examples to illustrate some basic
constructs. I wanted clarity, not obscurity. I hope you enjoyed it.






MORE REFERENCES

This concludes my tutorial on sed. Other of my UNIX shell tutorials can be found
here. Other shell tutorials and references can be found at

 * FreeBSD Sed Man Page
 * Apple/Mac OS X Sed Man Page
 * GNU Sed Manual
 * GNU Sed 4.2.2 Release Notes
 * sed(1) Seventh Edition Unix
 * sed(1) manual page from Sun/Oracle
 * Heiner's SHELLdorado
 * Chris F. A. Johnson's UNIX Shell Page
 * The Wikipedia Entry on SED
 * SED one-liners

 * And don't forget The SED FAQ


This document was originally converted from NROFF to TEXT to HTML.
Please forgive errors in the translation.
If you are confused, grab the actual script if possible. No translations
occurred in the scripts.



THANKS FOR THE FEEDBACK, GANG


Thanks to Keelan Evans, Fredrik Nilsson, and Kurt McKee for spotting some typos.
Thanks to Wim Stolker and Jose' Sebrosa as well.
Thanks to Olivier Mengue.
Thanks to Andrew M. Goth.
Thanks to David P. Brown.
Thanks to Axel Schulze for some corrections
Thanks to Martin Jan for the corrections in sed format (grin)
Thanks to David Ward for some corrections
A big thanks for Fazl Rahman for spotting dozens of errors.
Thanks to Carl Henrik Lunde who suggested an improvement to convert2uc1.sh
A big thanks to Bryan Hyun Huh who spotted an error in the table and reference
chart
Thanks for input from

	Marten Jan
	Gordon Wilson
	Tom Konantz
	Peter Bratton
	Grant Root
	Keith Briggs
	Zoltan Miklos
	Peggy Russell
	Lorens Kockkum.net
	John Poulin
	Rihards
	Corey Richardson
	Eric Mathison
	Ildar Mulyukov 
	Tom Zhu
	Abhijeet Rastogi @shadyabhi
	Steve LeBlanc   @sleveo
	dontforget yourtowel @whatissixbynine
	Yiming
	Fei Wang
	Kenneth R. Beesley
	Duncan Sung W. Kim @DuncanSungWKim
	Juan Eugenio Abadie
	Zander Hill @_ZPH
    Cornelius Roemer @CorneliusRoemer 
    Rob Smith
    Peter Moore




Copyright 1994, 1995 Bruce Barnett and General Electric Company
Copyright 2001,2005,2007,2011,2013 Bruce Barnett
All rights reserved
You are allowed to print copies of this tutorial for your personal use, and link
to this page, but you are not allowed to make electronic copies, or redistribute
this tutorial in any form without permission.
Original version written in 1994 and published in the Sun Observer

This document was translated by troff2html v0.21 on September 22, 2001 and then
manually edited to make it compliant with: