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3. sed Programs

A sed program consists of one or more sed commands, passed in by one or more of the `-e', `-f', `--expression', and `--file' options, or the first non-option argument if zero of these options are used. This document will refer to "the" sed script; this is understood to mean the in-order catenation of all of the scripts and script-files passed in.

Each sed command consists of an optional address or address range, followed by a one-character command name and any additional command-specific code.


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3.1 How sed Works

sed maintains two data buffers: the active pattern space, and the auxiliary hold space. Both are initially empty.

sed operates by performing the following cycle on each lines of input: first, sed reads one line from the input stream, removes any trailing newline, and places it in the pattern space. Then commands are executed; each command can have an address associated to it: addresses are a kind of condition code, and a command is only executed if the condition is verified before the command is to be executed.

When the end of the script is reached, unless the `-n' option is in use, the contents of pattern space are printed out to the output stream, adding back the trailing newline if it was removed.(3) Then the next cycle starts for the next input line.

Unless special commands (like `D') are used, the pattern space is deleted between two cycles. The hold space, on the other hand, keeps its data between cycles (see commands `h', `H', `x', `g', `G' to move data between both buffers).


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3.2 Selecting lines with sed

Addresses in a sed script can be in any of the following forms:

number

Specifying a line number will match only that line in the input. (Note that sed counts lines continuously across all input files unless `-i' or `-s' options are specified.)

first~step

This GNU extension matches every stepth line starting with line first. In particular, lines will be selected when there exists a non-negative n such that the current line-number equals first + (n * step). Thus, to select the odd-numbered lines, one would use 1~2; to pick every third line starting with the second, `2~3' would be used; to pick every fifth line starting with the tenth, use `10~5'; and `50~0' is just an obscure way of saying 50.

$

This address matches the last line of the last file of input, or the last line of each file when the `-i' or `-s' options are specified.

/regexp/

This will select any line which matches the regular expression regexp. If regexp itself includes any / characters, each must be escaped by a backslash (\).

The empty regular expression `//' repeats the last regular expression match (the same holds if the empty regular expression is passed to the s command). Note that modifiers to regular expressions are evaluated when the regular expression is compiled, thus it is invalid to specify them together with the empty regular expression.

\%regexp%

(The % may be replaced by any other single character.)

This also matches the regular expression regexp, but allows one to use a different delimiter than /. This is particularly useful if the regexp itself contains a lot of slashes, since it avoids the tedious escaping of every /. If regexp itself includes any delimiter characters, each must be escaped by a backslash (\).

/regexp/I
\%regexp%I

The I modifier to regular-expression matching is a GNU extension which causes the regexp to be matched in a case-insensitive manner.

/regexp/M
\%regexp%M

The M modifier to regular-expression matching is a GNU sed extension which causes ^ and $ to match respectively (in addition to the normal behavior) the empty string after a newline, and the empty string before a newline. There are special character sequences (\` and \') which always match the beginning or the end of the buffer. M stands for multi-line.

If no addresses are given, then all lines are matched; if one address is given, then only lines matching that address are matched.

An address range can be specified by specifying two addresses separated by a comma (,). An address range matches lines starting from where the first address matches, and continues until the second address matches (inclusively).

If the second address is a regexp, then checking for the ending match will start with the line following the line which matched the first address: a range will always span at least two lines (except of course if the input stream ends).

If the second address is a number less than (or equal to) the line matching the first address, then only the one line is matched.

GNU sed also supports some special two-address forms; all these are GNU extensions:

0,/regexp/

A line number of 0 can be used in an address specification like 0,/regexp/ so that sed will try to match regexp in the first input line too. In other words, 0,/regexp/ is similar to 1,/regexp/, except that if addr2 matches the very first line of input the 0,/regexp/ form will consider it to end the range, whereas the 1,/regexp/ form will match the beginning of its range and hence make the range span up to the second occurrence of the regular expression.

Note that this is the only place where the 0 address makes sense; there is no 0-th line and commands which are given the 0 address in any other way will give an error.

addr1,+N

Matches addr1 and the N lines following addr1.

addr1,~N

Matches addr1 and the lines following addr1 until the next line whose input line number is a multiple of N.

Appending the ! character to the end of an address specification negates the sense of the match. That is, if the ! character follows an address range, then only lines which do not match the address range will be selected. This also works for singleton addresses, and, perhaps perversely, for the null address.


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3.3 Overview of Regular Expression Syntax

To know how to use sed, people should understand regular expressions (regexp for short). A regular expression is a pattern that is matched against a subject string from left to right. Most characters are ordinary: they stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern

 
The quick brown fox

matches a portion of a subject string that is identical to itself. The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of special characters, which do not stand for themselves but instead are interpreted in some special way. Here is a brief description of regular expression syntax as used in sed.

char

A single ordinary character matches itself.

*

Matches a sequence of zero or more instances of matches for the preceding regular expression, which must be an ordinary character, a special character preceded by \, a ., a grouped regexp (see below), or a bracket expression. As a GNU extension, a postfixed regular expression can also be followed by *; for example, a** is equivalent to a*. POSIX 1003.1-2001 says that * stands for itself when it appears at the start of a regular expression or subexpression, but many nonGNU implementations do not support this and portable scripts should instead use \* in these contexts.

\+

As *, but matches one or more. It is a GNU extension.

\?

As *, but only matches zero or one. It is a GNU extension.

\{i\}

As *, but matches exactly i sequences (i is a decimal integer; for portability, keep it between 0 and 255 inclusive).

\{i,j\}

Matches between i and j, inclusive, sequences.

\{i,\}

Matches more than or equal to i sequences.

\(regexp\)

Groups the inner regexp as a whole, this is used to:

.

Matches any character, including newline.

^

Matches the null string at beginning of the pattern space, i.e. what appears after the circumflex must appear at the beginning of the pattern space.

In most scripts, pattern space is initialized to the content of each line (see section How sed works). So, it is a useful simplification to think of ^#include as matching only lines where `#include' is the first thing on line--if there are spaces before, for example, the match fails. This simplification is valid as long as the original content of pattern space is not modified, for example with an s command.

^ acts as a special character only at the beginning of the regular expression or subexpression (that is, after \( or \|). Portable scripts should avoid ^ at the beginning of a subexpression, though, as POSIX allows implementations that treat ^ as an ordinary character in that context.

$

It is the same as ^, but refers to end of pattern space. $ also acts as a special character only at the end of the regular expression or subexpression (that is, before \) or \|), and its use at the end of a subexpression is not portable.

[list]
[^list]

Matches any single character in list: for example, [aeiou] matches all vowels. A list may include sequences like char1-char2, which matches any character between (inclusive) char1 and char2.

A leading ^ reverses the meaning of list, so that it matches any single character not in list. To include ] in the list, make it the first character (after the ^ if needed), to include - in the list, make it the first or last; to include ^ put it after the first character.

The characters $, *, ., [, and \ are normally not special within list. For example, [\*] matches either `\' or `*', because the \ is not special here. However, strings like [.ch.], [=a=], and [:space:] are special within list and represent collating symbols, equivalence classes, and character classes, respectively, and [ is therefore special within list when it is followed by ., =, or :. Also, when not in POSIXLY_CORRECT mode, special escapes like \n and \t are recognized within list. See section GNU Extensions for Escapes in Regular Expressions.

regexp1\|regexp2

Matches either regexp1 or regexp2. Use parentheses to use complex alternative regular expressions. The matching process tries each alternative in turn, from left to right, and the first one that succeeds is used. It is a GNU extension.

regexp1regexp2

Matches the concatenation of regexp1 and regexp2. Concatenation binds more tightly than \|, ^, and $, but less tightly than the other regular expression operators.

\digit

Matches the digit-th \(…\) parenthesized subexpression in the regular expression. This is called a back reference. Subexpressions are implicity numbered by counting occurrences of \( left-to-right.

\n

Matches the newline character.

\char

Matches char, where char is one of $, *, ., [, \, or ^. Note that the only C-like backslash sequences that you can portably assume to be interpreted are \n and \\; in particular \t is not portable, and matches a `t' under most implementations of sed, rather than a tab character.

Note that the regular expression matcher is greedy, i.e., matches are attempted from left to right and, if two or more matches are possible starting at the same character, it selects the longest.

Examples:

`abcdef'

Matches `abcdef'.

`a*b'

Matches zero or more `a's followed by a single `b'. For example, `b' or `aaaaab'.

`a\?b'

Matches `b' or `ab'.

`a\+b\+'

Matches one or more `a's followed by one or more `b's: `ab' is the shortest possible match, but other examples are `aaaab' or `abbbbb' or `aaaaaabbbbbbb'.

`.*'
`.\+'

These two both match all the characters in a string; however, the first matches every string (including the empty string), while the second matches only strings containing at least one character.

`^main.*(.*)'

his matches a string starting with `main', followed by an opening and closing parenthesis. The `n', `(' and `)' need not be adjacent.

`^#'

This matches a string beginning with `#'.

`\\$'

This matches a string ending with a single backslash. The regexp contains two backslashes for escaping.

`\$'

Instead, this matches a string consisting of a single dollar sign, because it is escaped.

`[a-zA-Z0-9]'

In the C locale, this matches any ASCII letters or digits.

`[^ tab]\+'

(Here tab stands for a single tab character.) This matches a string of one or more characters, none of which is a space or a tab. Usually this means a word.

`^\(.*\)\n\1$'

This matches a string consisting of two equal substrings separated by a newline.

`.\{9\}A$'

This matches nine characters followed by an `A'.

`^.\{15\}A'

This matches the start of a string that contains 16 characters, the last of which is an `A'.


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3.4 Often-Used Commands

If you use sed at all, you will quite likely want to know these commands.

#

[No addresses allowed.]

The # character begins a comment; the comment continues until the next newline.

If you are concerned about portability, be aware that some implementations of sed (which are not POSIX conformant) may only support a single one-line comment, and then only when the very first character of the script is a #.

Warning: if the first two characters of the sed script are #n, then the `-n' (no-autoprint) option is forced. If you want to put a comment in the first line of your script and that comment begins with the letter `n' and you do not want this behavior, then be sure to either use a capital `N', or place at least one space before the `n'.

q [exit-code]

This command only accepts a single address.

Exit sed without processing any more commands or input. Note that the current pattern space is printed if auto-print is not disabled with the `-n' options. The ability to return an exit code from the sed script is a GNU sed extension.

d

Delete the pattern space; immediately start next cycle.

p

Print out the pattern space (to the standard output). This command is usually only used in conjunction with the `-n' command-line option.

n

If auto-print is not disabled, print the pattern space, then, regardless, replace the pattern space with the next line of input. If there is no more input then sed exits without processing any more commands.

{ commands }

A group of commands may be enclosed between { and } characters. This is particularly useful when you want a group of commands to be triggered by a single address (or address-range) match.


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3.5 The s Command

The syntax of the s (as in substitute) command is `s/regexp/replacement/flags'. The / characters may be uniformly replaced by any other single character within any given s command. The / character (or whatever other character is used in its stead) can appear in the regexp or replacement only if it is preceded by a \ character.

The s command is probably the most important in sed and has a lot of different options. Its basic concept is simple: the s command attempts to match the pattern space against the supplied regexp; if the match is successful, then that portion of the pattern space which was matched is replaced with replacement.

The replacement can contain \n (n being a number from 1 to 9, inclusive) references, which refer to the portion of the match which is contained between the nth \( and its matching \). Also, the replacement can contain unescaped & characters which reference the whole matched portion of the pattern space. Finally, as a GNU sed extension, you can include a special sequence made of a backslash and one of the letters L, l, U, u, or E. The meaning is as follows:

\L

Turn the replacement to lowercase until a \U or \E is found,

\l

Turn the next character to lowercase,

\U

Turn the replacement to uppercase until a \L or \E is found,

\u

Turn the next character to uppercase,

\E

Stop case conversion started by \L or \U.

To include a literal \, &, or newline in the final replacement, be sure to precede the desired \, &, or newline in the replacement with a \.

The s command can be followed by zero or more of the following flags:

g

Apply the replacement to all matches to the regexp, not just the first.

number

Only replace the numberth match of the regexp.

Note: the POSIX standard does not specify what should happen when you mix the g and number modifiers, and currently there is no widely agreed upon meaning across sed implementations. For GNU sed, the interaction is defined to be: ignore matches before the numberth, and then match and replace all matches from the numberth on.

p

If the substitution was made, then print the new pattern space.

Note: when both the p and e options are specified, the relative ordering of the two produces very different results. In general, ep (evaluate then print) is what you want, but operating the other way round can be useful for debugging. For this reason, the current version of GNU sed interprets specially the presence of p options both before and after e, printing the pattern space before and after evaluation, while in general flags for the s command show their effect just once. This behavior, although documented, might change in future versions.

w file-name

If the substitution was made, then write out the result to the named file. As a GNU sed extension, two special values of file-name are supported: `/dev/stderr', which writes the result to the standard error, and `/dev/stdout', which writes to the standard output.(4)

e

This command allows one to pipe input from a shell command into pattern space. If a substitution was made, the command that is found in pattern space is executed and pattern space is replaced with its output. A trailing newline is suppressed; results are undefined if the command to be executed contains a NUL character. This is a GNU sed extension.

I
i

The I modifier to regular-expression matching is a GNU extension which makes sed match regexp in a case-insensitive manner.

M
m

The M modifier to regular-expression matching is a GNU sed extension which causes ^ and $ to match respectively (in addition to the normal behavior) the empty string after a newline, and the empty string before a newline. There are special character sequences (\` and \') which always match the beginning or the end of the buffer. M stands for multi-line.


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3.6 Less Frequently-Used Commands

Though perhaps less frequently used than those in the previous section, some very small yet useful sed scripts can be built with these commands.

y/source-chars/dest-chars/

(The / characters may be uniformly replaced by any other single character within any given y command.)

Transliterate any characters in the pattern space which match any of the source-chars with the corresponding character in dest-chars.

Instances of the / (or whatever other character is used in its stead), \, or newlines can appear in the source-chars or dest-chars lists, provide that each instance is escaped by a \. The source-chars and dest-chars lists must contain the same number of characters (after de-escaping).

a\
text

As a GNU extension, this command accepts two addresses.

Queue the lines of text which follow this command (each but the last ending with a \, which are removed from the output) to be output at the end of the current cycle, or when the next input line is read.

Escape sequences in text are processed, so you should use \\ in text to print a single backslash.

As a GNU extension, if between the a and the newline there is other than a whitespace-\ sequence, then the text of this line, starting at the first non-whitespace character after the a, is taken as the first line of the text block. (This enables a simplification in scripting a one-line add.) This extension also works with the i and c commands.

i\
text

As a GNU extension, this command accepts two addresses.

Immediately output the lines of text which follow this command (each but the last ending with a \, which are removed from the output).

c\
text

Delete the lines matching the address or address-range, and output the lines of text which follow this command (each but the last ending with a \, which are removed from the output) in place of the last line (or in place of each line, if no addresses were specified). A new cycle is started after this command is done, since the pattern space will have been deleted.

=

As a GNU extension, this command accepts two addresses.

Print out the current input line number (with a trailing newline).

l n

Print the pattern space in an unambiguous form: non-printable characters (and the \ character) are printed in C-style escaped form; long lines are split, with a trailing \ character to indicate the split; the end of each line is marked with a $.

n specifies the desired line-wrap length; a length of 0 (zero) means to never wrap long lines. If omitted, the default as specified on the command line is used. The n parameter is a GNU sed extension.

r filename

As a GNU extension, this command accepts two addresses.

Queue the contents of filename to be read and inserted into the output stream at the end of the current cycle, or when the next input line is read. Note that if filename cannot be read, it is treated as if it were an empty file, without any error indication.

As a GNU sed extension, the special value `/dev/stdin' is supported for the file name, which reads the contents of the standard input.

w filename

Write the pattern space to filename. As a GNU sed extension, two special values of file-name are supported: `/dev/stderr', which writes the result to the standard error, and `/dev/stdout', which writes to the standard output.(5)

The file will be created (or truncated) before the first input line is read; all w commands (including instances of w flag on successful s commands) which refer to the same filename are output without closing and reopening the file.

D

Delete text in the pattern space up to the first newline. If any text is left, restart cycle with the resultant pattern space (without reading a new line of input), otherwise start a normal new cycle.

N

Add a newline to the pattern space, then append the next line of input to the pattern space. If there is no more input then sed exits without processing any more commands.

P

Print out the portion of the pattern space up to the first newline.

h

Replace the contents of the hold space with the contents of the pattern space.

H

Append a newline to the contents of the hold space, and then append the contents of the pattern space to that of the hold space.

g

Replace the contents of the pattern space with the contents of the hold space.

G

Append a newline to the contents of the pattern space, and then append the contents of the hold space to that of the pattern space.

x

Exchange the contents of the hold and pattern spaces.


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3.7 Commands for sed gurus

In most cases, use of these commands indicates that you are probably better off programming in something like awk or Perl. But occasionally one is committed to sticking with sed, and these commands can enable one to write quite convoluted scripts.

: label

[No addresses allowed.]

Specify the location of label for branch commands. In all other respects, a no-op.

b label

Unconditionally branch to label. The label may be omitted, in which case the next cycle is started.

t label

Branch to label only if there has been a successful substitution since the last input line was read or conditional branch was taken. The label may be omitted, in which case the next cycle is started.


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3.8 Commands Specific to GNU sed

These commands are specific to GNU sed, so you must use them with care and only when you are sure that hindering portability is not evil. They allow you to check for GNU sed extensions or to do tasks that are required quite often, yet are unsupported by standard seds.

e [command]

This command allows one to pipe input from a shell command into pattern space. Without parameters, the e command executes the command that is found in pattern space and replaces the pattern space with the output; a trailing newline is suppressed.

If a parameter is specified, instead, the e command interprets it as a command and sends its output to the output stream (like r does). The command can run across multiple lines, all but the last ending with a back-slash.

In both cases, the results are undefined if the command to be executed contains a NUL character.

L n

This GNU sed extension fills and joins lines in pattern space to produce output lines of (at most) n characters, like fmt does; if n is omitted, the default as specified on the command line is used. This command is considered a failed experiment and unless there is enough request (which seems unlikely) will be removed in future versions.

Q [exit-code]

This command only accepts a single address.

This command is the same as q, but will not print the contents of pattern space. Like q, it provides the ability to return an exit code to the caller.

This command can be useful because the only alternative ways to accomplish this apparently trivial function are to use the `-n' option (which can unnecessarily complicate your script) or resorting to the following snippet, which wastes time by reading the whole file without any visible effect:

 
:eat
$d       Quit silently on the last line
N        Read another line, silently
g        Overwrite pattern space each time to save memory
b eat
R filename

Queue a line of filename to be read and inserted into the output stream at the end of the current cycle, or when the next input line is read. Note that if filename cannot be read, or if its end is reached, no line is appended, without any error indication.

As with the r command, the special value `/dev/stdin' is supported for the file name, which reads a line from the standard input.

T label

Branch to label only if there have been no successful substitutions since the last input line was read or conditional branch was taken. The label may be omitted, in which case the next cycle is started.

v version

This command does nothing, but makes sed fail if GNU sed extensions are not supported, simply because other versions of sed do not implement it. In addition, you can specify the version of sed that your script requires, such as 4.0.5. The default is 4.0 because that is the first version that implemented this command.

This command enables all GNU extensions even if POSIXLY_CORRECT is set in the environment.

W filename

Write to the given filename the portion of the pattern space up to the first newline. Everything said under the w command about file handling holds here too.

z

This command empties the content of pattern space. It is usually the same as `s/.*//', but is more efficient and works in the presence of invalid multibyte sequences in the input stream. POSIX mandates that such sequences are not matched by `.', so that there is no portable way to clear sed's buffers in the middle of the script in most multibyte locales (including UTF-8 locales).


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3.9 GNU Extensions for Escapes in Regular Expressions

Until this chapter, we have only encountered escapes of the form `\^', which tell sed not to interpret the circumflex as a special character, but rather to take it literally. For example, `\*' matches a single asterisk rather than zero or more backslashes.

This chapter introduces another kind of escape(6)--that is, escapes that are applied to a character or sequence of characters that ordinarily are taken literally, and that sed replaces with a special character. This provides a way of encoding non-printable characters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters in a sed script but when a script is being prepared in the shell or by text editing, it is usually easier to use one of the following escape sequences than the binary character it represents:

The list of these escapes is:

\a

Produces or matches a BEL character, that is an "alert" (ASCII 7).

\f

Produces or matches a form feed (ASCII 12).

\n

Produces or matches a newline (ASCII 10).

\r

Produces or matches a carriage return (ASCII 13).

\t

Produces or matches a horizontal tab (ASCII 9).

\v

Produces or matches a so called "vertical tab" (ASCII 11).

\cx

Produces or matches CONTROL-x, where x is any character. The precise effect of `\cx' is as follows: if x is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus `\cz' becomes hex 1A, but `\c{' becomes hex 3B, while `\c;' becomes hex 7B.

\dxxx

Produces or matches a character whose decimal ASCII value is xxx.

\oxxx

Produces or matches a character whose octal ASCII value is xxx.

\xxx

Produces or matches a character whose hexadecimal ASCII value is xx.

`\b' (backspace) was omitted because of the conflict with the existing "word boundary" meaning.

Other escapes match a particular character class and are valid only in regular expressions:

\w

Matches any "word" character. A "word" character is any letter or digit or the underscore character.

\W

Matches any "non-word" character.

\b

Matches a word boundary; that is it matches if the character to the left is a "word" character and the character to the right is a "non-word" character, or vice-versa.

\B

Matches everywhere but on a word boundary; that is it matches if the character to the left and the character to the right are either both "word" characters or both "non-word" characters.

\`

Matches only at the start of pattern space. This is different from ^ in multi-line mode.

\'

Matches only at the end of pattern space. This is different from $ in multi-line mode.


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