SED
PROLOG
NAME
SYNOPSIS
DESCRIPTION
OPTIONS
OPERANDS
STDIN
INPUT FILES
ENVIRONMENT VARIABLES
ASYNCHRONOUS EVENTS
STDOUT
STDERR
OUTPUT FILES
EXTENDED DESCRIPTION
EXIT STATUS
CONSEQUENCES OF ERRORS
APPLICATION USAGE
EXAMPLES
RATIONALE
FUTURE DIRECTIONS
SEE ALSO
COPYRIGHT
PROLOG
This manual page is part of the POSIX Programmer’s Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
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.1-2017, 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
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. |
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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 |
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.1-2017, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.) |
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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 and warning 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
In default operation, sed cyclically shall append a line of input, less its terminating
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
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.1-2017, Section 9.3, Basic Regular Expressions, with the following additions:
* |
In a context address, the construction “cBREc”, where c is any character other than |
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* |
The escape sequence ’n’ shall match a |
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* |
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
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
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
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.
Editing commands other than {…}, a, b, c, i, r, t, w, :, and # can be followed by a
A function can be preceded by a ’!’ character, in which case the function shall be applied if the addresses do not select the pattern space. Zero or more
If a label argument (to a b, t, or : command) contains characters outside of the portable filename character set, or if a label is longer than 8 bytes, the behavior is unspecified. The implementation shall support label arguments recognized as unique up to at least 8 bytes; the actual length (greater than or equal to 8) supported by the implementation is unspecified. It is unspecified whether exceeding the maximum supported label length causes an error or a silent truncation.
[2addr] {editing command
editing command
… |
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} |
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. The braces can be preceded or followed by |
Each command in the list of commands shall be terminated by a
[1addr]a |
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text |
Write text to standard output as described previously. |
[2addr]b [label]
Branch to the : command verb bearing the label argument. If label is not specified, branch to the end of the script.
[2addr]c |
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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. |
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[2addr]d |
Delete the pattern space and start the next cycle. |
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[2addr]D |
If the pattern space contains no |
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[2addr]g |
Replace the contents of the pattern space by the contents of the hold space. |
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[2addr]G |
Append to the pattern space a |
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[2addr]h |
Replace the contents of the hold space with the contents of the pattern space. |
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[2addr]H |
Append to the hold space a |
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[1addr]i |
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text |
Write text to standard output. |
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[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.1-2017, 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 |
Long lines shall be folded, with the point of folding indicated by writing 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 |
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 |
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. |
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[2addr]P |
Write the pattern space, up to the first |
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[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
The replacement string shall be scanned from beginning to end. An
A line can be split by substituting a
The meaning of an unescaped
A substitution shall be considered to have been performed even if the replacement string is identical to the string that it replaces. Any
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. |
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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. |
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p |
Write the pattern space to standard output if a replacement was made. |
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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 |
[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
[0addr]:label
Do nothing. This command bears a label to which the b and t commands branch.
[1addr]= |
Write the following to standard output: |
“%dn”, <current line number>
[0addr] |
Ignore this empty command. |
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[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. |
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>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
When using sed to process pathnames, it is recommended that LC_ALL, or at least LC_CTYPE and LC_COLLATE, are set to POSIX or C in the environment, since pathnames can contain byte sequences that do not form valid characters in some locales, in which case the utility’s behavior would be undefined. In the POSIX locale each byte is a valid single-byte character, and therefore this problem is avoided.
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
# 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.1-2017 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.1-2017 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
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
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.1-2017 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.1-2017, 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.1-2017 because:
* |
No known documentation for any historic system describes the interaction between the p flag and the −n option. |
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* |
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. |
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* |
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. |
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* |
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 referenced by a b or t command, jumps to labels that do not exist, and label arguments that are subject to truncation.
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.
SEE ALSO
awk, ed, grep
The Base Definitions volume of POSIX.1-2017, 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 1003.1-2017, Standard for Information Technology — Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/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.kernel.org/doc/man-pages/reporting_bugs.html .