SED(1P) POSIX Programmer's Manual SED(1P)
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NAME
sed -- stream editor
SYNOPSIS
sed [-n] script [file...]
sed [-n] -e script [-e script]... [-f script_file]... [file...]
sed [-n] [-e script]... -f script_file [-f script_file]... [file...]
DESCRIPTION
The sed utility is a stream editor that shall read one or more text
files, make editing changes according to a script of editing commands,
and write the results to standard output. The script shall be obtained
from either the script operand string or a combination of the option-
arguments from the -e script and -f script_file options.
OPTIONS
The sed utility shall conform to the Base Definitions volume of
POSIX.1-2008, Section 12.2, Utility Syntax Guidelines, except that the
order of presentation of the -e and -f options is significant.
The following options shall be supported:
-e script Add the editing commands specified by the script option-argu-
ment to the end of the script of editing commands.
-f script_file
Add the editing commands in the file script_file to the end
of the script of editing commands.
-n Suppress the default output (in which each line, after it is
examined for editing, is written to standard output). Only
lines explicitly selected for output are written.
If any -e or -f options are specified, the script of editing commands
shall initially be empty. The commands specified by each -e or -f
option shall be added to the script in the order specified. When each
addition is made, if the previous addition (if any) was from a -e
option, a <newline> shall be inserted before the new addition. The
resulting script shall have the same properties as the script operand,
described in the OPERANDS section.
OPERANDS
The following operands shall be supported:
file A pathname of a file whose contents are read and edited. If
multiple file operands are specified, the named files shall
be read in the order specified and the concatenation shall be
edited. If no file operands are specified, the standard input
shall be used.
script A string to be used as the script of editing commands. The
application shall not present a script that violates the
restrictions of a text file except that the final character
need not be a <newline>.
STDIN
The standard input shall be used if no file operands are specified, and
shall be used if a file operand is '-' and the implementation treats
the '-' as meaning standard input. Otherwise, the standard input shall
not be used. See the INPUT FILES section.
INPUT FILES
The input files shall be text files. The script_files named by the -f
option shall consist of editing commands.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of sed:
LANG Provide a default value for the internationalization vari-
ables that are unset or null. (See the Base Definitions vol-
ume of POSIX.1-2008, Section 8.2, Internationalization Vari-
ables for the precedence of internationalization variables
used to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of
all the other internationalization variables.
LC_COLLATE
Determine the locale for the behavior of ranges, equivalence
classes, and multi-character collating elements within regu-
lar expressions.
LC_CTYPE Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as
opposed to multi-byte characters in arguments and input
files), and the behavior of character classes within regular
expressions.
LC_MESSAGES
Determine the locale that should be used to affect the format
and contents of diagnostic messages written to standard
error.
NLSPATH Determine the location of message catalogs for the processing
of LC_MESSAGES.
ASYNCHRONOUS EVENTS
Default.
STDOUT
The input files shall be written to standard output, with the editing
commands specified in the script applied. If the -n option is speci-
fied, only those input lines selected by the script shall be written to
standard output.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
The output files shall be text files whose formats are dependent on the
editing commands given.
EXTENDED DESCRIPTION
The script shall consist of editing commands of the following form:
[address[,address]]function
where function represents a single-character command verb from the list
in Editing Commands in sed, followed by any applicable arguments.
The command can be preceded by <blank> characters and/or <semicolon>
characters. The function can be preceded by <blank> characters. These
optional characters shall have no effect.
In default operation, sed cyclically shall append a line of input, less
its terminating <newline> character, into the pattern space. Reading
from input shall be skipped if a <newline> was in the pattern space
prior to a D command ending the previous cycle. The sed utility shall
then apply in sequence all commands whose addresses select that pattern
space, until a command starts the next cycle or quits. If no commands
explicitly started a new cycle, then at the end of the script the pat-
tern space shall be copied to standard output (except when -n is speci-
fied) and the pattern space shall be deleted. Whenever the pattern
space is written to standard output or a named file, sed shall immedi-
ately follow it with a <newline>.
Some of the editing commands use a hold space to save all or part of
the pattern space for subsequent retrieval. The pattern and hold spaces
shall each be able to hold at least 8192 bytes.
Addresses in sed
An address is either a decimal number that counts input lines cumula-
tively across files, a '$' character that addresses the last line of
input, or a context address (which consists of a BRE, as described in
Regular Expressions in sed, preceded and followed by a delimiter, usu-
ally a <slash>).
An editing command with no addresses shall select every pattern space.
An editing command with one address shall select each pattern space
that matches the address.
An editing command with two addresses shall select the inclusive range
from the first pattern space that matches the first address through the
next pattern space that matches the second. (If the second address is a
number less than or equal to the line number first selected, only one
line shall be selected.) Starting at the first line following the
selected range, sed shall look again for the first address. Thereafter,
the process shall be repeated. Omitting either or both of the address
components in the following form produces undefined results:
[address[,address]]
Regular Expressions in sed
The sed utility shall support the BREs described in the Base Defini-
tions volume of POSIX.1-2008, Section 9.3, Basic Regular Expressions,
with the following additions:
* In a context address, the construction "\cBREc", where c is any
character other than <backslash> or <newline>, shall be identical
to "/BRE/". If the character designated by c appears following a
<backslash>, then it shall be considered to be that literal charac-
ter, which shall not terminate the BRE. For example, in the context
address "\xabc\xdefx", the second x stands for itself, so that the
BRE is "abcxdef".
* The escape sequence '\n' shall match a <newline> embedded in the
pattern space. A literal <newline> shall not be used in the BRE of
a context address or in the substitute function.
* If an RE is empty (that is, no pattern is specified) sed shall
behave as if the last RE used in the last command applied (either
as an address or as part of a substitute command) was specified.
Editing Commands in sed
In the following list of editing commands, the maximum number of per-
missible addresses for each function is indicated by [0addr], [1addr],
or [2addr], representing zero, one, or two addresses.
The argument text shall consist of one or more lines. Each embedded
<newline> in the text shall be preceded by a <backslash>. Other <back-
slash> characters in text shall be removed, and the following character
shall be treated literally.
The r and w command verbs, and the w flag to the s command, take an
rfile (or wfile) parameter, separated from the command verb letter or
flag by one or more <blank> characters; implementations may allow zero
separation as an extension.
The argument rfile or the argument wfile shall terminate the editing
command. Each wfile shall be created before processing begins. Imple-
mentations shall support at least ten wfile arguments in the script;
the actual number (greater than or equal to 10) that is supported by
the implementation is unspecified. The use of the wfile parameter shall
cause that file to be initially created, if it does not exist, or shall
replace the contents of an existing file.
The b, r, s, t, w, y, and : command verbs shall accept additional argu-
ments. The following synopses indicate which arguments shall be sepa-
rated from the command verbs by a single <space>.
The a and r commands schedule text for later output. The text specified
for the a command, and the contents of the file specified for the r
command, shall be written to standard output just before the next
attempt to fetch a line of input when executing the N or n commands, or
when reaching the end of the script. If written when reaching the end
of the script, and the -n option was not specified, the text shall be
written after copying the pattern space to standard output. The con-
tents of the file specified for the r command shall be as of the time
the output is written, not the time the r command is applied. The text
shall be output in the order in which the a and r commands were applied
to the input.
Command verbs other than {, a, b, c, i, r, t, w, :, and # can be fol-
lowed by a <semicolon>, optional <blank> characters, and another com-
mand verb. However, when the s command verb is used with the w flag,
following it with another command in this manner produces undefined
results.
A function can be preceded by one or more '!' characters, in which
case the function shall be applied if the addresses do not select the
pattern space. Zero or more <blank> characters shall be accepted before
the first '!' character. It is unspecified whether <blank> characters
can follow a '!' character, and conforming applications shall not fol-
low a '!' character with <blank> characters.
[2addr] {editing command
editing command
...
} Execute a list of sed editing commands only when the pattern
space is selected. The list of sed editing commands shall be
surrounded by braces and separated by <newline> characters,
and conform to the following rules. The braces can be pre-
ceded or followed by <blank> characters. The editing commands
can be preceded by <blank> characters, but shall not be fol-
lowed by <blank> characters. The <right-brace> shall be pre-
ceded by a <newline> and can be preceded or followed by
<blank> characters.
[1addr]a\
text Write text to standard output as described previously.
[2addr]b [label]
Branch to the : function bearing the label. If label is not
specified, branch to the end of the script. The implementa-
tion shall support labels recognized as unique up to at least
8 characters; the actual length (greater than or equal to 8)
that shall be supported by the implementation is unspecified.
It is unspecified whether exceeding a label length causes an
error or a silent truncation.
[2addr]c\
text Delete the pattern space. With a 0 or 1 address or at the end
of a 2-address range, place text on the output and start the
next cycle.
[2addr]d Delete the pattern space and start the next cycle.
[2addr]D If the pattern space contains no <newline>, delete the pat-
tern space and start a normal new cycle as if the d command
was issued. Otherwise, delete the initial segment of the pat-
tern space through the first <newline>, and start the next
cycle with the resultant pattern space and without reading
any new input.
[2addr]g Replace the contents of the pattern space by the contents of
the hold space.
[2addr]G Append to the pattern space a <newline> followed by the con-
tents of the hold space.
[2addr]h Replace the contents of the hold space with the contents of
the pattern space.
[2addr]H Append to the hold space a <newline> followed by the contents
of the pattern space.
[1addr]i\
text Write text to standard output.
[2addr]l (The letter ell.) Write the pattern space to standard output
in a visually unambiguous form. The characters listed in the
Base Definitions volume of POSIX.1-2008, Table 5-1, Escape
Sequences and Associated Actions ('\\', '\a', '\b', '\f',
'\r', '\t', '\v') shall be written as the corresponding
escape sequence; the '\n' in that table is not applicable.
Non-printable characters not in that table shall be written
as one three-digit octal number (with a preceding <back-
slash>) for each byte in the character (most significant byte
first).
Long lines shall be folded, with the point of folding indi-
cated by writing a <backslash> followed by a <newline>; the
length at which folding occurs is unspecified, but should be
appropriate for the output device. The end of each line shall
be marked with a '$'.
[2addr]n Write the pattern space to standard output if the default
output has not been suppressed, and replace the pattern space
with the next line of input, less its terminating <newline>.
If no next line of input is available, the n command verb
shall branch to the end of the script and quit without start-
ing a new cycle.
[2addr]N Append the next line of input, less its terminating <new-
line>, to the pattern space, using an embedded <newline> to
separate the appended material from the original material.
Note that the current line number changes.
If no next line of input is available, the N command verb
shall branch to the end of the script and quit without start-
ing a new cycle or copying the pattern space to standard out-
put.
[2addr]p Write the pattern space to standard output.
[2addr]P Write the pattern space, up to the first <newline>, to stan-
dard output.
[1addr]q Branch to the end of the script and quit without starting a
new cycle.
[1addr]r rfile
Copy the contents of rfile to standard output as described
previously. If rfile does not exist or cannot be read, it
shall be treated as if it were an empty file, causing no
error condition.
[2addr]s/BRE/replacement/flags
Substitute the replacement string for instances of the BRE in
the pattern space. Any character other than <backslash> or
<newline> can be used instead of a <slash> to delimit the BRE
and the replacement. Within the BRE and the replacement, the
BRE delimiter itself can be used as a literal character if it
is preceded by a <backslash>.
The replacement string shall be scanned from beginning to
end. An <ampersand> ('&') appearing in the replacement shall
be replaced by the string matching the BRE. The special mean-
ing of '&' in this context can be suppressed by preceding it
by a <backslash>. The characters "\n", where n is a digit,
shall be replaced by the text matched by the corresponding
back-reference expression. If the corresponding back-refer-
ence expression does not match, then the characters "\n"
shall be replaced by the empty string. The special meaning of
"\n" where n is a digit in this context, can be suppressed by
preceding it by a <backslash>. For each other <backslash>
encountered, the following character shall lose its special
meaning (if any). The meaning of a <backslash> immediately
followed by any character other than '&', <backslash>, a
digit, or the delimiter character used for this command, is
unspecified.
A line can be split by substituting a <newline> into it. The
application shall escape the <newline> in the replacement by
preceding it by a <backslash>. A substitution shall be con-
sidered to have been performed even if the replacement string
is identical to the string that it replaces. Any <backslash>
used to alter the default meaning of a subsequent character
shall be discarded from the BRE or the replacement before
evaluating the BRE or using the replacement.
The value of flags shall be zero or more of:
n Substitute for the nth occurrence only of the BRE
found within the pattern space.
g Globally substitute for all non-overlapping
instances of the BRE rather than just the first
one. If both g and n are specified, the results are
unspecified.
p Write the pattern space to standard output if a
replacement was made.
w wfile Write. Append the pattern space to wfile if a
replacement was made. A conforming application
shall precede the wfile argument with one or more
<blank> characters. If the w flag is not the last
flag value given in a concatenation of multiple
flag values, the results are undefined.
[2addr]t [label]
Test. Branch to the : command verb bearing the label if any
substitutions have been made since the most recent reading of
an input line or execution of a t. If label is not speci-
fied, branch to the end of the script.
[2addr]w wfile
Append (write) the pattern space to wfile.
[2addr]x Exchange the contents of the pattern and hold spaces.
[2addr]y/string1/string2/
Replace all occurrences of characters in string1 with the
corresponding characters in string2. If a <backslash> fol-
lowed by an 'n' appear in string1 or string2, the two charac-
ters shall be handled as a single <newline>. If the number
of characters in string1 and string2 are not equal, or if any
of the characters in string1 appear more than once, the
results are undefined. Any character other than <backslash>
or <newline> can be used instead of <slash> to delimit the
strings. If the delimiter is not 'n', within string1 and
string2, the delimiter itself can be used as a literal char-
acter if it is preceded by a <backslash>. If a <backslash>
character is immediately followed by a <backslash> character
in string1 or string2, the two <backslash> characters shall
be counted as a single literal <backslash> character. The
meaning of a <backslash> followed by any character that is
not 'n', a <backslash>, or the delimiter character is unde-
fined.
[0addr]:label
Do nothing. This command bears a label to which the b and t
commands branch.
[1addr]= Write the following to standard output:
"%d\n", <current line number>
[0addr] Ignore this empty command.
[0addr]# Ignore the '#' and the remainder of the line (treat them as a
comment), with the single exception that if the first two
characters in the script are "#n", the default output shall
be suppressed; this shall be the equivalent of specifying -n
on the command line.
EXIT STATUS
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
Regular expressions match entire strings, not just individual lines,
but a <newline> is matched by '\n' in a sed RE; a <newline> is not
allowed by the general definition of regular expression in
POSIX.1-2008. Also note that '\n' cannot be used to match a <newline>
at the end of an arbitrary input line; <newline> characters appear in
the pattern space as a result of the N editing command.
EXAMPLES
This sed script simulates the BSD cat -s command, squeezing excess
empty lines from standard input.
sed -n '
# Write non-empty lines.
/./ {
p
d
}
# Write a single empty line, then look for more empty lines.
/^$/ p
# Get next line, discard the held <newline> (empty line),
# and look for more empty lines.
:Empty
/^$/ {
N
s/.//
b Empty
}
# Write the non-empty line before going back to search
# for the first in a set of empty lines.
p
'
The following sed command is a much simpler method of squeezing empty
lines, although it is not quite the same as cat -s since it removes any
initial empty lines:
sed -n '/./,/^$/p'
RATIONALE
This volume of POSIX.1-2008 requires implementations to support at
least ten distinct wfiles, matching historical practice on many imple-
mentations. Implementations are encouraged to support more, but con-
forming applications should not exceed this limit.
The exit status codes specified here are different from those in System
V. System V returns 2 for garbled sed commands, but returns zero with
its usage message or if the input file could not be opened. The stan-
dard developers considered this to be a bug.
The manner in which the l command writes non-printable characters was
changed to avoid the historical backspace-overstrike method, and other
requirements to achieve unambiguous output were added. See the RATIO-
NALE for ed for details of the format chosen, which is the same as that
chosen for sed.
This volume of POSIX.1-2008 requires implementations to provide pattern
and hold spaces of at least 8192 bytes, larger than the 4000 bytes spa-
ces used by some historical implementations, but less than the 20480
bytes limit used in an early proposal. Implementations are encouraged
to allocate dynamically larger pattern and hold spaces as needed.
The requirements for acceptance of <blank> and <space> characters in
command lines has been made more explicit than in early proposals to
describe clearly the historical practice and to remove confusion about
the phrase ``protect initial blanks [sic] and tabs from the stripping
that is done on every script line'' that appears in much of the histor-
ical documentation of the sed utility description of text. (Not all
implementations are known to have stripped <blank> characters from text
lines, although they all have allowed leading <blank> characters pre-
ceding the address on a command line.)
The treatment of '#' comments differs from the SVID which only allows a
comment as the first line of the script, but matches BSD-derived imple-
mentations. The comment character is treated as a command, and it has
the same properties in terms of being accepted with leading <blank>
characters; the BSD implementation has historically supported this.
Early proposals required that a script_file have at least one non-com-
ment line. Some historical implementations have behaved in unexpected
ways if this were not the case. The standard developers considered that
this was incorrect behavior and that application developers should not
have to avoid this feature. A correct implementation of this volume of
POSIX.1-2008 shall permit script_files that consist only of comment
lines.
Early proposals indicated that if -e and -f options were intermixed,
all -e options were processed before any -f options. This has been
changed to process them in the order presented because it matches his-
torical practice and is more intuitive.
The treatment of the p flag to the s command differs between System V
and BSD-based systems when the default output is suppressed. In the two
examples:
echo a | sed 's/a/A/p'
echo a | sed -n 's/a/A/p'
this volume of POSIX.1-2008, BSD, System V documentation, and the SVID
indicate that the first example should write two lines with A, whereas
the second should write one. Some System V systems write the A only
once in both examples because the p flag is ignored if the -n option is
not specified.
This is a case of a diametrical difference between systems that could
not be reconciled through the compromise of declaring the behavior to
be unspecified. The SVID/BSD/System V documentation behavior was
adopted for this volume of POSIX.1-2008 because:
* No known documentation for any historic system describes the inter-
action between the p flag and the -n option.
* The selected behavior is more correct as there is no technical jus-
tification for any interaction between the p flag and the -n
option. A relationship between -n and the p flag might imply that
they are only used together, but this ignores valid scripts that
interrupt the cyclical nature of the processing through the use of
the D, d, q, or branching commands. Such scripts rely on the p suf-
fix to write the pattern space because they do not make use of the
default output at the ``bottom'' of the script.
* Because the -n option makes the p flag unnecessary, any interaction
would only be useful if sed scripts were written to run both with
and without the -n option. This is believed to be unlikely. It is
even more unlikely that programmers have coded the p flag expecting
it to be unnecessary. Because the interaction was not documented,
the likelihood of a programmer discovering the interaction and
depending on it is further decreased.
* Finally, scripts that break under the specified behavior produce
too much output instead of too little, which is easier to diagnose
and correct.
The form of the substitute command that uses the n suffix was limited
to the first 512 matches in an early proposal. This limit has been
removed because there is no reason an editor processing lines of
{LINE_MAX} length should have this restriction. The command s/a/A/2047
should be able to substitute the 2047th occurrence of a on a line.
The b, t, and : commands are documented to ignore leading white space,
but no mention is made of trailing white space. Historical implementa-
tions of sed assigned different locations to the labels 'x' and "x ".
This is not useful, and leads to subtle programming errors, but it is
historical practice, and changing it could theoretically break working
scripts. Implementors are encouraged to provide warning messages about
labels that are never used or jumps to labels that do not exist.
Historically, the sed ! and } editing commands did not permit multiple
commands on a single line using a <semicolon> as a command delimiter.
Implementations are permitted, but not required, to support this exten-
sion.
Earlier versions of this standard allowed for implementations with
bytes other than eight bits, but this has been modified in this ver-
sion.
FUTURE DIRECTIONS
None.
SEE ALSO
awk, ed, grep
The Base Definitions volume of POSIX.1-2008, Table 5-1, Escape
Sequences and Associated Actions, Chapter 8, Environment Variables,
Section 9.3, Basic Regular Expressions, Section 12.2, Utility Syntax
Guidelines
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SED(1P)
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