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PROGRAM:

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-argument 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 variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, Section 8.2,
Internationalization Variables 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 regular 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 specified, 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 pattern space shall be copied to standard
output (except when −n is specified) and the pattern space shall be deleted. Whenever the
pattern space is written to standard output or a named file, sed shall immediately 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 cumulatively 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, usually 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 Definitions 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
character, 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 permissible 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 <backslash> 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. Implementations 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 arguments. The following
synopses indicate which arguments shall be separated 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 contents 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 followed by a
<semicolon>, optional <blank> characters, and another command 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
follow 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
preceded or followed by <blank> characters. The editing commands can be preceded
by <blank> characters, but shall not be followed by <blank> characters. The
<right-brace> shall be preceded 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 implementation 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 pattern space and start a
normal new cycle as if the d command was issued. Otherwise, delete the initial
segment of the pattern 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 contents 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
<backslash>) for each byte in the character (most significant byte first).

Long lines shall be folded, with the point of folding indicated 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 starting a new cycle.

[2addr]N Append the next line of input, less its terminating <newline>, 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 starting a new cycle or copying the pattern
space to standard output.

[2addr]p Write the pattern space to standard output.

[2addr]P Write the pattern space, up to the first <newline>, to standard 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 meaning 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-reference 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 considered 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 specified, 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> followed by an 'n' appear in string1 or
string2, the two characters 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 character 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 undefined.

[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 implementations. Implementations are
encouraged to support more, but conforming 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 standard 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 RATIONALE 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 spaces 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
historical 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 preceding 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 implementations. 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-comment 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 historical 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 interaction between the p
flag and the −n option.

* The selected behavior is more correct as there is no technical justification 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 suffix 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 implementations 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 extension.

Earlier versions of this standard allowed for implementations with bytes other than eight
bits, but this has been modified in this version.

FUTURE DIRECTIONS


None.

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