This file documents version 4.2.1 of GNU sed, a stream editor.
Copyright © 1998, 1999, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
This document is released under the terms of the GNU Free Documentation License as published by the Free Software Foundation; either version 1.1, or (at your option) any later version.
You should have received a copy of the GNU Free Documentation License along with GNU sed; see the file COPYING.DOC. If not, write to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, MA 02110-1301, USA.
There are no Cover Texts and no Invariant Sections; this text, along with its equivalent in the printed manual, constitutes the Title Page.
--- The detailed node listing ---
sed Programs:
Examples:
sed is a stream editor. A stream editor is used to perform basic text transformations on an input stream (a file or input from a pipeline). While in some ways similar to an editor which permits scripted edits (such as ed), sed works by making only one pass over the input(s), and is consequently more efficient. But it is sed's ability to filter text in a pipeline which particularly distinguishes it from other types of editors.
Normally sed is invoked like this:
sed SCRIPT INPUTFILE...
The full format for invoking sed is:
sed OPTIONS... [SCRIPT] [INPUTFILE...]
If you do not specify INPUTFILE, or if INPUTFILE is -, sed filters the contents of the standard input. The script is actually the first non-option parameter, which sed specially considers a script and not an input file if (and only if) none of the other options specifies a script to be executed, that is if neither of the -e and -f options is specified.
sed may be invoked with the following command-line options:
--version--help-n--quiet--silentsed works).
These options disable this automatic printing,
and sed only produces output when explicitly told to
via the p command.
-e script--expression=script-f script-file--file=script-file-i[SUFFIX]--in-place[=SUFFIX]This option implies -s.
When the end of the file is reached, the temporary file is renamed to the output file's original name. The extension, if supplied, is used to modify the name of the old file before renaming the temporary file, thereby making a backup copy2).
This rule is followed: if the extension doesn't contain a *,
then it is appended to the end of the current filename as a
suffix; if the extension does contain one or more *
characters, then each asterisk is replaced with the
current filename. This allows you to add a prefix to the
backup file, instead of (or in addition to) a suffix, or
even to place backup copies of the original files into another
directory (provided the directory already exists).
If no extension is supplied, the original file is
overwritten without making a backup.
-l N--line-length=Nl command.
A length of 0 (zero) means to never wrap long lines. If
not specified, it is taken to be 70.
--posixPOSIXLY_CORRECT variable
to a non-empty value.
-b--binary--follow-symlinks-r--regexp-extended-s--separate$ refers to the last line of each file,
and files invoked from the R commands are rewound at the
start of each file.
-u--unbufferedIf no -e, -f, --expression, or --file options are given on the command-line, then the first non-option argument on the command line is taken to be the script to be executed.
If any command-line parameters remain after processing the above, these parameters are interpreted as the names of input files to be processed. A file name of ‘-’ refers to the standard input stream. The standard input will be processed if no file names are specified.
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.
Commands within a script or script-file can be
separated by semicolons (;) or newlines (ASCII 10).
Some commands, due to their syntax, cannot be followed by semicolons
working as command separators and thus should be terminated
with newlines or be placed at the end of a script or script-file.
Commands can also be preceded with optional non-significant
whitespace characters.
Each sed command consists of an optional address or
address range, followed by a one-character command name
and any additional command-specific code.
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 line 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).
Addresses in a sed script can be in any of the following forms:
~step1~2;
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.
$/regexp// 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%% 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%II modifier to regular-expression matching is a GNU
extension which causes the regexp to be matched in
a case-insensitive manner.
/regexp/M\%regexp%MM 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/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.
,+N,~NAppending 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.
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.
*\, 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.
\+*, but matches one or more. It is a GNU extension.
\?*, but only matches zero or one. It is a GNU extension.
\{i\}*, but matches exactly i sequences (i is a
decimal integer; for portability, keep it between 0 and 255
inclusive).
\{i,j\}\{i,\}\(regexp\)\(abcd\)*:
this will search for zero or more whole sequences
of ‘abcd’, while abcd* would search
for ‘abc’ followed by zero or more occurrences
of ‘d’. Note that support for \(abcd\)* is
required by POSIX 1003.1-2001, but many non-GNU
implementations do not support it and hence it is not universally
portable.
.^In most scripts, pattern space is initialized to the content of each
line (see 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 \( or
\|). Portable scripts should avoid ^ at the beginning of
a subexpression, though, as POSIX allows implementations that
treat ^ as an ordinary character in that context.
$^, but refers to end of pattern space.
$ also acts as a special character only at the end
of the regular expression or subexpression (that is, before \)
or \|), and its use at the end of a subexpression is not
portable.
[list][^list][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 Escapes.
\|regexp2\|, ^, and
$, but less tightly than the other regular expression
operators.
\digit\(...\) parenthesized
subexpression in the regular expression. This is called a back
reference. Subexpressions are implicity numbered by counting
occurrences of \( left-to-right.
\n\char$,
*, ., [, \, 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:
If you use sed at all, you will quite likely want to know these commands.
#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]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.
dpn{ commands }{ and } characters.
This is particularly useful when you want a group of commands
to be triggered by a single address (or address-range) match.
s CommandThe 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\U or \E is found,
\l\U\L or \E is found,
\u\E\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:
gNote: 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.
pNote: 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-nameeIiI modifier to regular-expression matching is a GNU
extension which makes sed match regexp in a
case-insensitive manner.
MmM 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.
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// 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\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\Immediately output the lines of text which follow this command
(each but the last ending with a \,
which are removed from the output).
c\\,
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.
=Print out the current input line number (with a trailing newline).
l n\ 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 filenameQueue 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 filenameThe file will be created (or truncated) before the first input line is
read; all w commands (including instances of the w flag
on successful s commands) which refer to the same filename
are output without closing and reopening the file.
Dd command was issued. Otherwise, delete text in the pattern
space up to the first newline, and restart cycle with the resultant
pattern space, without reading a new line of input.
NPhHgGxIn 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.
: labelSpecify the location of label for branch commands.
In all other respects, a no-op.
b labelt labelsubstitution
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.
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]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.
FL nfmt 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 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
$d Quit silently on the last line
N Read another line, silently
g Overwrite pattern space each time to save memory
b eat
R filenameAs with the r command, the special value /dev/stdin
is supported for the file name, which reads a line from the
standard input.
T labelsubstitutions since the last input line was read or
conditional branch was taken. The label may be omitted,
in which case the next cycle is started.
v version4.0.5. The default is 4.0
because that is the first version that implemented this command.
This command enables all GNU extensions even if
POSIXLY_CORRECT is set in the environment.
W filenamew command about
file handling holds here too.
zUntil this chapter, we have only encountered escapes of the form ‘\^’, which tell sed not to interpret the circumflex as a special character, but rather to take it literally. For example, ‘\*’ matches a single asterisk rather than zero or more backslashes.
This chapter introduces another kind of escape6—that is, escapes that are applied to a character or sequence of characters that ordinarily are taken literally, and that sed replaces with a special character. This provides a way of encoding non-printable characters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters in a sed script but when a script is being prepared in the shell or by text editing, it is usually easier to use one of the following escape sequences than the binary character it represents:
The list of these escapes is:
\a\f\n\r\t\v\cx\dxxx\oxxx\xxx‘\b’ (backspace) was omitted because of the conflict with the existing “word boundary” meaning.
Other escapes match a particular character class and are valid only in regular expressions:
\w\W\b\B\`^ in multi-line mode.
\'$ in multi-line mode.
Here are some sed scripts to guide you in the art of mastering sed.
Some exotic examples:
Emulating standard utilities:
This script centers all lines of a file on a 80 columns width.
To change that width, the number in \{...\} must be
replaced, and the number of added spaces also must be changed.
Note how the buffer commands are used to separate parts in the regular expressions to be matched—this is a common technique.
#!/usr/bin/sed -f
# Put 80 spaces in the buffer
1 {
x
s/^$/ /
s/^.*$/&&&&&&&&/
x
}
# del leading and trailing spaces
y/tab/ /
s/^ *//
s/ *$//
# add a newline and 80 spaces to end of line
G
# keep first 81 chars (80 + a newline)
s/^\(.\{81\}\).*$/\1/
# \2 matches half of the spaces, which are moved to the beginning
s/^\(.*\)\n\(.*\)\2/\2\1/
This script is one of a few that demonstrate how to do arithmetic in sed. This is indeed possible,7 but must be done manually.
To increment one number you just add 1 to last digit, replacing it by the following digit. There is one exception: when the digit is a nine the previous digits must be also incremented until you don't have a nine.
This solution by Bruno Haible is very clever and smart because
it uses a single buffer; if you don't have this limitation, the
algorithm used in Numbering lines, is faster.
It works by replacing trailing nines with an underscore, then
using multiple s commands to increment the last digit,
and then again substituting underscores with zeros.
#!/usr/bin/sed -f
/[^0-9]/ d
# replace all leading 9s by _ (any other character except digits, could
# be used)
:d
s/9\(_*\)$/_\1/
td
# incr last digit only. The first line adds a most-significant
# digit of 1 if we have to add a digit.
#
# The tn commands are not necessary, but make the thing
# faster
s/^\(_*\)$/1\1/; tn
s/8\(_*\)$/9\1/; tn
s/7\(_*\)$/8\1/; tn
s/6\(_*\)$/7\1/; tn
s/5\(_*\)$/6\1/; tn
s/4\(_*\)$/5\1/; tn
s/3\(_*\)$/4\1/; tn
s/2\(_*\)$/3\1/; tn
s/1\(_*\)$/2\1/; tn
s/0\(_*\)$/1\1/; tn
:n
y/_/0/
This is a pretty strange use of sed. We transform text, and transform it to be shell commands, then just feed them to shell. Don't worry, even worse hacks are done when using sed; I have seen a script converting the output of date into a bc program!
The main body of this is the sed script, which remaps the name from lower to upper (or vice-versa) and even checks out if the remapped name is the same as the original name. Note how the script is parameterized using shell variables and proper quoting.
#! /bin/sh
# rename files to lower/upper case...
#
# usage:
# move-to-lower *
# move-to-upper *
# or
# move-to-lower -R .
# move-to-upper -R .
#
help()
{
cat << eof
Usage: $0 [-n] [-r] [-h] files...
-n do nothing, only see what would be done
-R recursive (use find)
-h this message
files files to remap to lower case
Examples:
$0 -n * (see if everything is ok, then...)
$0 *
$0 -R .
eof
}
apply_cmd='sh'
finder='echo "$@" | tr " " "\n"'
files_only=
while :
do
case "$1" in
-n) apply_cmd='cat' ;;
-R) finder='find "$@" -type f';;
-h) help ; exit 1 ;;
*) break ;;
esac
shift
done
if [ -z "$1" ]; then
echo Usage: $0 [-h] [-n] [-r] files...
exit 1
fi
LOWER='abcdefghijklmnopqrstuvwxyz'
UPPER='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
case `basename $0` in
*upper*) TO=$UPPER; FROM=$LOWER ;;
*) FROM=$UPPER; TO=$LOWER ;;
esac
eval $finder | sed -n '
# remove all trailing slashes
s/\/*$//
# add ./ if there is no path, only a filename
/\//! s/^/.\//
# save path+filename
h
# remove path
s/.*\///
# do conversion only on filename
y/'$FROM'/'$TO'/
# now line contains original path+file, while
# hold space contains the new filename
x
# add converted file name to line, which now contains
# path/file-name\nconverted-file-name
G
# check if converted file name is equal to original file name,
# if it is, do not print nothing
/^.*\/\(.*\)\n\1/b
# now, transform path/fromfile\n, into
# mv path/fromfile path/tofile and print it
s/^\(.*\/\)\(.*\)\n\(.*\)$/mv "\1\2" "\1\3"/p
' | $apply_cmd
This script strips the definition of the shell functions from the output of the set Bourne-shell command.
#!/bin/sh
set | sed -n '
:x
# if no occurrence of ‘=()’ print and load next line
/=()/! { p; b; }
/ () $/! { p; b; }
# possible start of functions section
# save the line in case this is a var like FOO="() "
h
# if the next line has a brace, we quit because
# nothing comes after functions
n
/^{/ q
# print the old line
x; p
# work on the new line now
x; bx
'
This script can be used to reverse the position of characters in lines. The technique moves two characters at a time, hence it is faster than more intuitive implementations.
Note the tx command before the definition of the label.
This is often needed to reset the flag that is tested by
the t command.
Imaginative readers will find uses for this script. An example is reversing the output of banner.8
#!/usr/bin/sed -f
/../! b
# Reverse a line. Begin embedding the line between two newlines
s/^.*$/\
&\
/
# Move first character at the end. The regexp matches until
# there are zero or one characters between the markers
tx
:x
s/\(\n.\)\(.*\)\(.\n\)/\3\2\1/
tx
# Remove the newline markers
s/\n//g
This one begins a series of totally useless (yet interesting) scripts emulating various Unix commands. This, in particular, is a tac workalike.
Note that on implementations other than GNU sed this script might easily overflow internal buffers.
#!/usr/bin/sed -nf
# reverse all lines of input, i.e. first line became last, ...
# from the second line, the buffer (which contains all previous lines)
# is *appended* to current line, so, the order will be reversed
1! G
# on the last line we're done -- print everything
$ p
# store everything on the buffer again
h
This script replaces ‘cat -n’; in fact it formats its output exactly like GNU cat does.
Of course this is completely useless and for two reasons: first, because somebody else did it in C, second, because the following Bourne-shell script could be used for the same purpose and would be much faster:
#! /bin/sh
sed -e "=" $@ | sed -e '
s/^/ /
N
s/^ *\(......\)\n/\1 /
'
It uses sed to print the line number, then groups lines two
by two using N. Of course, this script does not teach as much as
the one presented below.
The algorithm used for incrementing uses both buffers, so the line
is printed as soon as possible and then discarded. The number
is split so that changing digits go in a buffer and unchanged ones go
in the other; the changed digits are modified in a single step
(using a y command). The line number for the next line
is then composed and stored in the hold space, to be used in the
next iteration.
#!/usr/bin/sed -nf
# Prime the pump on the first line
x
/^$/ s/^.*$/1/
# Add the correct line number before the pattern
G
h
# Format it and print it
s/^/ /
s/^ *\(......\)\n/\1 /p
# Get the line number from hold space; add a zero
# if we're going to add a digit on the next line
g
s/\n.*$//
/^9*$/ s/^/0/
# separate changing/unchanged digits with an x
s/.9*$/x&/
# keep changing digits in hold space
h
s/^.*x//
y/0123456789/1234567890/
x
# keep unchanged digits in pattern space
s/x.*$//
# compose the new number, remove the newline implicitly added by G
G
s/\n//
h
Emulating ‘cat -b’ is almost the same as ‘cat -n’—we only have to select which lines are to be numbered and which are not.
The part that is common to this script and the previous one is not commented to show how important it is to comment sed scripts properly...
#!/usr/bin/sed -nf
/^$/ {
p
b
}
# Same as cat -n from now
x
/^$/ s/^.*$/1/
G
h
s/^/ /
s/^ *\(......\)\n/\1 /p
x
s/\n.*$//
/^9*$/ s/^/0/
s/.9*$/x&/
h
s/^.*x//
y/0123456789/1234567890/
x
s/x.*$//
G
s/\n//
h
This script shows another way to do arithmetic with sed. In this case we have to add possibly large numbers, so implementing this by successive increments would not be feasible (and possibly even more complicated to contrive than this script).
The approach is to map numbers to letters, kind of an abacus implemented with sed. ‘a’s are units, ‘b’s are tens and so on: we simply add the number of characters on the current line as units, and then propagate the carry to tens, hundreds, and so on.
As usual, running totals are kept in hold space.
On the last line, we convert the abacus form back to decimal.
For the sake of variety, this is done with a loop rather than
with some 80 s commands9: first we
convert units, removing ‘a’s from the number; then we
rotate letters so that tens become ‘a’s, and so on
until no more letters remain.
#!/usr/bin/sed -nf
# Add n+1 a's to hold space (+1 is for the newline)
s/./a/g
H
x
s/\n/a/
# Do the carry. The t's and b's are not necessary,
# but they do speed up the thing
t a
: a; s/aaaaaaaaaa/b/g; t b; b done
: b; s/bbbbbbbbbb/c/g; t c; b done
: c; s/cccccccccc/d/g; t d; b done
: d; s/dddddddddd/e/g; t e; b done
: e; s/eeeeeeeeee/f/g; t f; b done
: f; s/ffffffffff/g/g; t g; b done
: g; s/gggggggggg/h/g; t h; b done
: h; s/hhhhhhhhhh//g
: done
$! {
h
b
}
# On the last line, convert back to decimal
: loop
/a/! s/[b-h]*/&0/
s/aaaaaaaaa/9/
s/aaaaaaaa/8/
s/aaaaaaa/7/
s/aaaaaa/6/
s/aaaaa/5/
s/aaaa/4/
s/aaa/3/
s/aa/2/
s/a/1/
: next
y/bcdefgh/abcdefg/
/[a-h]/ b loop
p
This script is almost the same as the previous one, once each of the words on the line is converted to a single ‘a’ (in the previous script each letter was changed to an ‘a’).
It is interesting that real wc programs have optimized loops for ‘wc -c’, so they are much slower at counting words rather than characters. This script's bottleneck, instead, is arithmetic, and hence the word-counting one is faster (it has to manage smaller numbers).
Again, the common parts are not commented to show the importance of commenting sed scripts.
#!/usr/bin/sed -nf
# Convert words to a's
s/[ tab][ tab]*/ /g
s/^/ /
s/ [^ ][^ ]*/a /g
s/ //g
# Append them to hold space
H
x
s/\n//
# From here on it is the same as in wc -c.
/aaaaaaaaaa/! bx; s/aaaaaaaaaa/b/g
/bbbbbbbbbb/! bx; s/bbbbbbbbbb/c/g
/cccccccccc/! bx; s/cccccccccc/d/g
/dddddddddd/! bx; s/dddddddddd/e/g
/eeeeeeeeee/! bx; s/eeeeeeeeee/f/g
/ffffffffff/! bx; s/ffffffffff/g/g
/gggggggggg/! bx; s/gggggggggg/h/g
s/hhhhhhhhhh//g
:x
$! { h; b; }
:y
/a/! s/[b-h]*/&0/
s/aaaaaaaaa/9/
s/aaaaaaaa/8/
s/aaaaaaa/7/
s/aaaaaa/6/
s/aaaaa/5/
s/aaaa/4/
s/aaa/3/
s/aa/2/
s/a/1/
y/bcdefgh/abcdefg/
/[a-h]/ by
p
No strange things are done now, because sed gives us ‘wc -l’ functionality for free!!! Look:
#!/usr/bin/sed -nf
$=
This script is probably the simplest useful sed script.
It displays the first 10 lines of input; the number of displayed
lines is right before the q command.
#!/usr/bin/sed -f
10q
Printing the last n lines rather than the first is more complex but indeed possible. n is encoded in the second line, before the bang character.
This script is similar to the tac script in that it keeps the final output in the hold space and prints it at the end:
#!/usr/bin/sed -nf
1! {; H; g; }
1,10 !s/[^\n]*\n//
$p
h
Mainly, the scripts keeps a window of 10 lines and slides it
by adding a line and deleting the oldest (the substitution command
on the second line works like a D command but does not
restart the loop).
The “sliding window” technique is a very powerful way to write
efficient and complex sed scripts, because commands like
P would require a lot of work if implemented manually.
To introduce the technique, which is fully demonstrated in the
rest of this chapter and is based on the N, P
and D commands, here is an implementation of tail
using a simple “sliding window.”
This looks complicated but in fact the working is the same as
the last script: after we have kicked in the appropriate number
of lines, however, we stop using the hold space to keep inter-line
state, and instead use N and D to slide pattern
space by one line:
#!/usr/bin/sed -f
1h
2,10 {; H; g; }
$q
1,9d
N
D
Note how the first, second and fourth line are inactive after the first ten lines of input. After that, all the script does is: exiting on the last line of input, appending the next input line to pattern space, and removing the first line.
This is an example of the art of using the N, P
and D commands, probably the most difficult to master.
#!/usr/bin/sed -f
h
:b
# On the last line, print and exit
$b
N
/^\(.*\)\n\1$/ {
# The two lines are identical. Undo the effect of
# the n command.
g
bb
}
# If the N command had added the last line, print and exit
$b
# The lines are different; print the first and go
# back working on the second.
P
D
As you can see, we mantain a 2-line window using P and D.
This technique is often used in advanced sed scripts.
This script prints only duplicated lines, like ‘uniq -d’.
#!/usr/bin/sed -nf
$b
N
/^\(.*\)\n\1$/ {
# Print the first of the duplicated lines
s/.*\n//
p
# Loop until we get a different line
:b
$b
N
/^\(.*\)\n\1$/ {
s/.*\n//
bb
}
}
# The last line cannot be followed by duplicates
$b
# Found a different one. Leave it alone in the pattern space
# and go back to the top, hunting its duplicates
D
This script prints only unique lines, like ‘uniq -u’.
#!/usr/bin/sed -f
# Search for a duplicate line --- until that, print what you find.
$b
N
/^\(.*\)\n\1$/ ! {
P
D
}
:c
# Got two equal lines in pattern space. At the
# end of the file we simply exit
$d
# Else, we keep reading lines with N until we
# find a different one
s/.*\n//
N
/^\(.*\)\n\1$/ {
bc
}
# Remove the last instance of the duplicate line
# and go back to the top
D
As a final example, here are three scripts, of increasing complexity and speed, that implement the same function as ‘cat -s’, that is squeezing blank lines.
The first leaves a blank line at the beginning and end if there are some already.
#!/usr/bin/sed -f
# on empty lines, join with next
# Note there is a star in the regexp
:x
/^\n*$/ {
N
bx
}
# now, squeeze all '\n', this can be also done by:
# s/^\(\n\)*/\1/
s/\n*/\
/
This one is a bit more complex and removes all empty lines at the beginning. It does leave a single blank line at end if one was there.
#!/usr/bin/sed -f
# delete all leading empty lines
1,/^./{
/./!d
}
# on an empty line we remove it and all the following
# empty lines, but one
:x
/./!{
N
s/^\n$//
tx
}
This removes leading and trailing blank lines. It is also the
fastest. Note that loops are completely done with n and
b, without relying on sed to restart the
the script automatically at the end of a line.
#!/usr/bin/sed -nf
# delete all (leading) blanks
/./!d
# get here: so there is a non empty
:x
# print it
p
# get next
n
# got chars? print it again, etc...
/./bx
# no, don't have chars: got an empty line
:z
# get next, if last line we finish here so no trailing
# empty lines are written
n
# also empty? then ignore it, and get next... this will
# remove ALL empty lines
/./!bz
# all empty lines were deleted/ignored, but we have a non empty. As
# what we want to do is to squeeze, insert a blank line artificially
i\
bx
For those who want to write portable sed scripts,
be aware that some implementations have been known to
limit line lengths (for the pattern and hold spaces)
to be no more than 4000 bytes.
The posix standard specifies that conforming sed
implementations shall support at least 8192 byte line lengths.
GNU sed has no built-in limit on line length;
as long as it can malloc() more (virtual) memory,
you can feed or construct lines as long as you like.
However, recursion is used to handle subpatterns and indefinite repetition. This means that the available stack space may limit the size of the buffer that can be processed by certain patterns.
In addition to several books that have been written about sed
(either specifically or as chapters in books which discuss
shell programming), one can find out more about sed
(including suggestions of a few books) from the FAQ
for the sed-users mailing list, available from:
http://sed.sourceforge.net/sedfaq.html
Also of interest are http://www.student.northpark.edu/pemente/sed/index.htm and http://sed.sf.net/grabbag, which include sed tutorials and other sed-related goodies.
The sed-users mailing list itself maintained by Sven Guckes.
To subscribe, visit http://groups.yahoo.com and search
for the sed-users mailing list.
Email bug reports to bonzini@gnu.org.
Be sure to include the word “sed” somewhere in the Subject: field.
Also, please include the output of ‘sed --version’ in the body
of your report if at all possible.
Please do not send a bug report like this:
while building frobme-1.3.4
$ configure
error--> sed: file sedscr line 1: Unknown option to 's'
If GNU sed doesn't configure your favorite package, take a few extra minutes to identify the specific problem and make a stand-alone test case. Unlike other programs such as C compilers, making such test cases for sed is quite simple.
A stand-alone test case includes all the data necessary to perform the test, and the specific invocation of sed that causes the problem. The smaller a stand-alone test case is, the better. A test case should not involve something as far removed from sed as “try to configure frobme-1.3.4”. Yes, that is in principle enough information to look for the bug, but that is not a very practical prospect.
Here are a few commonly reported bugs that are not bugs.
N command on the last lineFor example, the behavior of
sed N foo bar
would depend on whether foo has an even or an odd number of
lines10. Or, when writing a script to read the
next few lines following a pattern match, traditional
implementations of sed would force you to write
something like
/foo/{ $!N; $!N; $!N; $!N; $!N; $!N; $!N; $!N; $!N }
instead of just
/foo/{ N;N;N;N;N;N;N;N;N; }
In any case, the simplest workaround is to use $d;N in
scripts that rely on the traditional behavior, or to set
the POSIXLY_CORRECT variable to a non-empty value.
\|,
\+, \?, \`, \', \<,
\>, \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’.
This syntax may cause problems when running scripts written for other
seds. Some sed programs have been written with the
assumption that \| and \+ match the literal characters
| and +. Such scripts must be modified by removing the
spurious backslashes if they are to be used with modern implementations
of sed, like
GNU sed.
On the other hand, some scripts use s|abc\|def||g to remove occurrences
of either abc or def. While this worked until
sed 4.0.x, newer versions interpret this as removing the
string abc|def. This is again undefined behavior according to
POSIX, and this interpretation is arguably more robust: older
seds, for example, required that the regex matcher parsed
\/ as / in the common case of escaping a slash, which is
again undefined behavior; the new behavior avoids this, and this is good
because the regex matcher is only partially under our control.
In addition, this version of sed supports several escape characters
(some of which are multi-character) to insert non-printable characters
in scripts (\a, \c, \d, \o, \r,
\t, \v, \x). These can cause similar problems
with scripts written for other seds.
The permissions on a file say what can happen to the data
in that file, while the permissions on a directory say what can
happen to the list of files in that directory. ‘sed -i’
will not ever open for writing a file that is already on disk.
Rather, it will work on a temporary file that is finally renamed
to the original name: if you rename or delete files, you're actually
modifying the contents of the directory, so the operation depends on
the permissions of the directory, not of the file. For this same
reason, sed does not let you use -i on a writeable file
in a read-only directory, and will break hard or symbolic links when
-i is used on such a file.
0a does not work (gives an error)0,/RE/ as active when the script starts: if
you write 1,/abc/d and the first line includes the word ‘abc’,
then that match would be ignored because address ranges must span at least
two lines (barring the end of the file); but what you probably wanted is
to delete every line up to the first one including ‘abc’, and this
is obtained with 0,/abc/d.
[a-z] is case insensitive[a-z]
uses the current locale's collation order – in C parlance, that means using
strcoll(3) instead of strcmp(3). Some locales have a
case-insensitive collation order, others don't.
Another problem is that [a-z] tries to use collation symbols.
This only happens if you are on the GNU system, using
GNU libc's regular expression matcher instead of compiling the
one supplied with GNU sed. In a Danish locale, for example,
the regular expression ^[a-z]$ matches the string ‘aa’,
because this is a single collating symbol that comes after ‘a’
and before ‘b’; ‘ll’ behaves similarly in Spanish
locales, or ‘ij’ in Dutch locales.
To work around these problems, which may cause bugs in shell scripts, set
the LC_COLLATE and LC_CTYPE environment variables to ‘C’.
s/.*// does not clear pattern spaceTo work around these problems, which may cause bugs in shell scripts, set the LC_COLLATE and LC_CTYPE environment variables to ‘C’.
The only difference between basic and extended regular expressions is in the behavior of a few characters: ‘?’, ‘+’, parentheses, and braces (‘{}’). While basic regular expressions require these to be escaped if you want them to behave as special characters, when using extended regular expressions you must escape them if you want them to match a literal character.
Examples:
abc?c\+a\{3,\}\(abc\)\{2,3\}\(abc*\)\1This is a general index of all issues discussed in this manual, with the exception of the sed commands and command-line options.
0 address: Reporting Bugss/// failed: Extended Commandss/// succeeded: Programming Commands0 address: Reporting Bugs0 address: Addressess/// failed: Extended Commandss commands: The "s" Commandg and number modifier interaction in s command: The "s" CommandI modifier: The "s" CommandI modifier: AddressesL command: Extended CommandsM modifier: The "s" CommandR command: Extended Commandsg and number modifiers in the s command: The "s" Command0 address: Reporting BugsN command on the last line: Reporting BugsN command on the last line: Reporting BugsPOSIXLY_CORRECT behavior, bracket expressions: Regular ExpressionsPOSIXLY_CORRECT behavior, enabling: Invoking sedPOSIXLY_CORRECT behavior, escapes: EscapesPOSIXLY_CORRECT behavior, N command: Reporting BugsThis is an alphabetical list of all sed commands and command-line options.
# (comments): Common Commands--binary: Invoking sed--expression: Invoking sed--file: Invoking sed--follow-symlinks: Invoking sed--help: Invoking sed--in-place: Invoking sed--line-length: Invoking sed--posix: Invoking sed--quiet: Invoking sed--regexp-extended: Invoking sed--separate: Invoking sed--silent: Invoking sed--unbuffered: Invoking sed--version: Invoking sed-b: Invoking sed-e: Invoking sed-f: Invoking sed-i: Invoking sed-l: Invoking sed-n: Invoking sed-n, forcing from within a script: Common Commands-r: Invoking sed-s: Invoking sed-u: Invoking sed: (label) command: Programming Commands= (print line number) command: Other Commandsa (append text lines) command: Other Commandsb (branch) command: Programming Commandsc (change to text lines) command: Other CommandsD (delete first line) command: Other Commandsd (delete) command: Common Commandse (evaluate) command: Extended CommandsF (File name) command: Extended CommandsG (appending Get) command: Other Commandsg (get) command: Other CommandsH (append Hold) command: Other Commandsh (hold) command: Other Commandsi (insert text lines) command: Other CommandsL (fLow paragraphs) command: Extended Commandsl (list unambiguously) command: Other CommandsN (append Next line) command: Other Commandsn (next-line) command: Common CommandsP (print first line) command: Other Commandsp (print) command: Common Commandsq (quit) command: Common CommandsQ (silent Quit) command: Extended Commandsr (read file) command: Other CommandsR (read line) command: Extended Commandss command, option flags: The "s" CommandT (test and branch if failed) command: Extended Commandst (test and branch if successful) command: Programming Commandsv (version) command: Extended Commandsw (write file) command: Other CommandsW (write first line) command: Extended Commandsx (eXchange) command: Other Commandsy (transliterate) command: Other Commandsz (Zap) command: Extended Commands{} command grouping: Common Commands[1] This applies to commands such as =,
a, c, i, l, p. You can
still write to the standard output by using the w
or W commands together with the /dev/stdout
special file
[2] Note that GNU sed creates the backup file whether or not any output is actually changed.
[3] Actually, if sed prints a line without the terminating newline, it will nevertheless print the missing newline as soon as more text is sent to the same output stream, which gives the “least expected surprise” even though it does not make commands like ‘sed -n p’ exactly identical to cat.
[4] This is equivalent to p unless the -i
option is being used.
[5] This is equivalent to p unless the -i
option is being used.
[6] All
the escapes introduced here are GNU
extensions, with the exception of \n. In basic regular
expression mode, setting POSIXLY_CORRECT disables them inside
bracket expressions.
[7] sed guru Greg Ubben wrote an implementation of the dc rpn calculator! It is distributed together with sed.
[8] This requires another script to pad the output of banner; for example
#! /bin/sh
banner -w $1 $2 $3 $4 |
sed -e :a -e '/^.\{0,'$1'\}$/ { s/$/ /; ba; }' |
~/sedscripts/reverseline.sed
[9] Some implementations have a limit of 199 commands per script
[10] which is the actual “bug” that prompted the change in behavior