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.1‐2017, Section 9.3, Basic Regular
Expressions, with the following additions:
* In a context address, the construction "\cBREc", where c is
any character other than <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.
Editing commands other than {...}, a, b, c, i, r, t, w, :, and #
can be followed by a <semicolon>, optional <blank> characters,
and another editing command. However, when an s editing command
is used with the w flag, following it with another command in
this manner produces undefined results.
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 <blank> characters shall be accepted
before the '!' character. It is unspecified whether <blank>
characters can follow the '!' character, and conforming
applications shall not follow the '!' character with <blank>
characters.
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
...
} 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 <blank> characters. The
<right-brace> shall be preceded by a <newline> or
<semicolon> (before any optional <blank> characters
preceding the <right-brace>).
Each command in the list of commands shall be
terminated by a <newline> character, or by a
<semicolon> character if permitted when the command is
used outside the braces. The editing commands can be
preceded by <blank> characters, but shall not be
followed by <blank> characters.
[1addr]a\
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\
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.1‐2017,
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 <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).
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>.
The meaning of an unescaped <backslash> immediately
followed by any character other than '&', <backslash>,
a digit, <newline>, or the delimiter character used for
this command, is unspecified.
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.
