path: root/man1p/sed.1p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SED" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" sed 
.SH NAME
sed \- stream editor
.SH SYNOPSIS
.LP
\fBsed\fP \fB[\fP\fB-n\fP\fB]\fP \fIscript\fP\fB[\fP\fIfile\fP\fB...\fP\fB]\fP\fB
.br
.sp
sed\fP \fB[\fP\fB-n\fP\fB][\fP\fB-e\fP \fIscript\fP\fB]\fP\fB...\fP\fB[\fP\fB-f\fP
\fIscript_file\fP\fB]\fP\fB...\fP\fB[\fP\fIfile\fP\fB...\fP\fB]\fP\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIsed\fP 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 \fIscript\fP operand
string or a combination of the option-arguments from the \fB-e\fP
\fIscript\fP and \fB-f\fP \fIscript_file\fP options.
.SH OPTIONS
.LP
The \fIsed\fP utility shall conform to the Base Definitions volume
of IEEE\ Std\ 1003.1-2001, Section 12.2, Utility Syntax Guidelines,
except that the order of presentation of the
\fB-e\fP and \fB-f\fP options is significant.
.LP
The following options shall be supported:
.TP 7
\fB-e\ \fP \fIscript\fP
Add the editing commands specified by the \fIscript\fP option-argument
to the end of the script of editing commands. The
\fIscript\fP option-argument shall have the same properties as the
\fIscript\fP operand, described in the OPERANDS section.
.TP 7
\fB-f\ \fP \fIscript_file\fP
Add the editing commands in the file \fIscript_file\fP to the end
of the script.
.TP 7
\fB-n\fP
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.
.sp
.LP
Multiple \fB-e\fP and \fB-f\fP options may be specified. All commands
shall be added to the script in the order specified,
regardless of their origin.
.SH OPERANDS
.LP
The following operands shall be supported:
.TP 7
\fIfile\fP
A pathname of a file whose contents are read and edited. If multiple
\fIfile\fP operands are specified, the named files shall
be read in the order specified and the concatenation shall be edited.
If no \fIfile\fP operands are specified, the standard input
shall be used.
.TP 7
\fIscript\fP
A string to be used as the script of editing commands. The application
shall not present a \fIscript\fP that violates the
restrictions of a text file except that the final character need not
be a <newline>.
.sp
.SH STDIN
.LP
The standard input shall be used only if no \fIfile\fP operands are
specified. See the INPUT FILES section.
.SH INPUT FILES
.LP
The input files shall be text files. The \fIscript_file\fPs named
by the \fB-f\fP option shall consist of editing
commands.
.SH ENVIRONMENT VARIABLES
.LP
The following environment variables shall affect the execution of
\fIsed\fP:
.TP 7
\fILANG\fP
Provide a default value for the internationalization variables that
are unset or null. (See the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Section 8.2, Internationalization Variables
for
the precedence of internationalization variables used to determine
the values of locale categories.)
.TP 7
\fILC_ALL\fP
If set to a non-empty string value, override the values of all the
other internationalization variables.
.TP 7
\fILC_COLLATE\fP
.sp
Determine the locale for the behavior of ranges, equivalence classes,
and multi-character collating elements within regular
expressions.
.TP 7
\fILC_CTYPE\fP
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.
.TP 7
\fILC_MESSAGES\fP
Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard
error.
.TP 7
\fINLSPATH\fP
Determine the location of message catalogs for the processing of \fILC_MESSAGES
\&.\fP 
.sp
.SH ASYNCHRONOUS EVENTS
.LP
Default.
.SH STDOUT
.LP
The input files shall be written to standard output, with the editing
commands specified in the script applied. If the \fB-n\fP
option is specified, only those input lines selected by the script
shall be written to standard output.
.SH STDERR
.LP
The standard error shall be used only for diagnostic messages.
.SH OUTPUT FILES
.LP
The output files shall be text files whose formats are dependent on
the editing commands given.
.SH EXTENDED DESCRIPTION
.LP
The \fIscript\fP shall consist of editing commands of the following
form:
.sp
.RS
.nf

\fB[\fP\fIaddress\fP\fB[\fP\fB,\fP\fIaddress\fP\fB]]\fP\fIfunction\fP
.fi
.RE
.LP
where \fIfunction\fP represents a single-character command verb from
the list in Editing Commands
in sed , followed by any applicable arguments.
.LP
The command can be preceded by <blank>s and/or semicolons. The function
can be preceded by <blank>s. These optional
characters shall have no effect.
.LP
In default operation, \fIsed\fP cyclically shall append a line of
input, less its terminating <newline>, into the pattern
space. Normally the pattern space will be empty, unless a \fBD\fP
command terminated the last cycle. The \fIsed\fP utility shall
then apply in sequence all commands whose addresses select that pattern
space, and at the end of the script copy the pattern space
to standard output (except when \fB-n\fP is specified) and delete
the pattern space. Whenever the pattern space is written to
standard output or a named file, \fIsed\fP shall immediately follow
it with a <newline>.
.LP
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.
.SS Addresses in sed
.LP
An address is either a decimal number that counts input lines cumulatively
across files, a \fB'$'\fP 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).
.LP
An editing command with no addresses shall select every pattern space.
.LP
An editing command with one address shall select each pattern space
that matches the address.
.LP
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, \fIsed\fP 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:
.sp
.RS
.nf

\fB[\fP\fIaddress\fP\fB[\fP\fB,\fP\fIaddress\fP\fB]]\fP
.fi
.RE
.SS Regular Expressions in sed
.LP
The \fIsed\fP utility shall support the BREs described in the Base
Definitions volume of IEEE\ Std\ 1003.1-2001, Section 9.3, Basic Regular
Expressions, with the following additions:
.IP " *" 3
In a context address, the construction \fB"\\cBREc"\fP , where \fIc\fP
is any character other than backslash or
<newline>, shall be identical to \fB"/BRE/"\fP . If the character
designated by \fIc\fP 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
\fB"\\xabc\\xdefx"\fP , the second \fIx\fP stands for itself, so that
the BRE is \fB"abcxdef"\fP .
.LP
.IP " *" 3
The escape sequence \fB'\\n'\fP 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.
.LP
.IP " *" 3
If an RE is empty (that is, no pattern is specified) \fIsed\fP 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.
.LP
.SS Editing Commands in sed
.LP
In the following list of editing commands, the maximum number of permissible
addresses for each function is indicated by [
\fI0addr\fP], [ \fI1addr\fP], or [ \fI2addr\fP], representing zero,
one, or two addresses.
.LP
The argument \fItext\fP shall consist of one or more lines. Each embedded
<newline> in the text shall be preceded by a
backslash. Other backslashes in text shall be removed, and the following
character shall be treated literally.
.LP
The \fBr\fP and \fBw\fP command verbs, and the \fIw\fP flag to the
\fBs\fP command, take an optional \fIrfile\fP (or
\fIwfile\fP) parameter, separated from the command verb letter or
flag by one or more <blank>s; implementations may allow
zero separation as an extension.
.LP
The argument \fIrfile\fP or the argument \fIwfile\fP shall terminate
the editing command. Each \fIwfile\fP shall be created
before processing begins. Implementations shall support at least ten
\fIwfile\fP 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 \fIwfile\fP parameter shall
cause that file to be initially created, if it does not exist, or
shall replace the contents of an existing file.
.LP
The \fBb\fP, \fBr\fP, \fBs\fP, \fBt\fP, \fBw\fP, \fBy\fP, and \fB:\fP
command verbs shall accept additional arguments.
The following synopses indicate which arguments shall be separated
from the command verbs by a single <space>.
.LP
The \fBa\fP and \fBr\fP commands schedule text for later output. The
text specified for the \fBa\fP command, and the contents
of the file specified for the \fBr\fP command, shall be written to
standard output just before the next attempt to fetch a line of
input when executing the \fBN\fP or \fBn\fP commands, or when reaching
the end of the script. If written when reaching the end of
the script, and the \fB-n\fP 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 \fBr\fP command
shall be as of the time the output is written, not the time the
\fBr\fP command is applied. The text shall be output in the order
in which the \fBa\fP and \fBr\fP commands were applied to the
input.
.LP
Command verbs other than \fB{\fP, \fBa\fP, \fBb\fP, \fBc\fP, \fBi\fP,
\fBr\fP, \fBt\fP, \fBw\fP, \fB:\fP, and \fB#\fP
can be followed by a semicolon, optional <blank>s, and another command
verb. However, when the \fBs\fP command verb is used
with the \fIw\fP flag, following it with another command in this manner
produces undefined results.
.LP
A function can be preceded by one or more \fB'!'\fP characters, in
which case the function shall be applied if the addresses
do not select the pattern space. Zero or more <blank>s shall be accepted
before the first \fB'!'\fP character. It is
unspecified whether <blank>s can follow a \fB'!'\fP character, and
conforming applications shall not follow a \fB'!'\fP
character with <blank>s.
.TP 7
\fB[\fP\fI2addr\fP\fB]\ {\fP\fIfunction\fP
.TP 7
\fIfunction\fP
.TP 7
\&...
.TP 7
\fB}\fP
Execute a list of \fIsed\fP functions only when the pattern space
is selected. The list of \fIsed\fP functions shall be
surrounded by braces and separated by <newline>s, and conform to the
following rules. The braces can be preceded or followed
by <blank>s. The functions can be preceded by <blank>s, but shall
not be followed by <blank>s. The
<right-brace> shall be preceded by a <newline> and can be preceded
or followed by <blank>s.
.TP 7
\fB[\fP\fI1addr\fP\fB]a\\\fP
.TP 7
\fItext\fP
Write text to standard output as described previously.
.TP 7
\fB[\fP\fI2addr\fP\fB]b\ [\fP\fIlabel\fP\fB]\fP
.sp
Branch to the \fB:\fP function bearing the \fIlabel\fP. If \fIlabel\fP
is not specified, branch to the end of the script. The
implementation shall support \fIlabel\fPs 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.
.TP 7
\fB[\fP\fI2addr\fP\fB]c\\\fP
.TP 7
\fItext\fP
Delete the pattern space. With a 0 or 1 address or at the end of a
2-address range, place \fItext\fP on the output and start
the next cycle.
.TP 7
\fB[\fP\fI2addr\fP\fB]d\fP
Delete the pattern space and start the next cycle.
.TP 7
\fB[\fP\fI2addr\fP\fB]D\fP
Delete the initial segment of the pattern space through the first
<newline> and start the next cycle.
.TP 7
\fB[\fP\fI2addr\fP\fB]g\fP
Replace the contents of the pattern space by the contents of the hold
space.
.TP 7
\fB[\fP\fI2addr\fP\fB]G\fP
Append to the pattern space a <newline> followed by the contents of
the hold space.
.TP 7
\fB[\fP\fI2addr\fP\fB]h\fP
Replace the contents of the hold space with the contents of the pattern
space.
.TP 7
\fB[\fP\fI2addr\fP\fB]H\fP
Append to the hold space a <newline> followed by the contents of the
pattern space.
.TP 7
\fB[\fP\fI1addr\fP\fB]i\\\fP
.TP 7
\fItext\fP
Write \fItext\fP to standard output.
.TP 7
\fB[\fP\fI2addr\fP\fB]l\fP
(The letter ell.) Write the pattern space to standard output in a
visually unambiguous form. The characters listed in the Base
Definitions volume of IEEE\ Std\ 1003.1-2001, Table 5-1, Escape Sequences
and Associated Actions ( \fB'\\\\'\fP ,
\fB'\\a'\fP , \fB'\\b'\fP , \fB'\\f'\fP , \fB'\\r'\fP , \fB'\\t'\fP
, \fB'\\v'\fP ) shall be written as the
corresponding escape sequence; the \fB'\\n'\fP 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). If the size of a byte on the system is greater than 9 bits,
the format used for non-printable characters is
implementation-defined. 
.LP
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 \fB'$'\fP .
.TP 7
\fB[\fP\fI2addr\fP\fB]n\fP
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>. 
.LP
If no next line of input is available, the \fBn\fP command verb shall
branch to the end of the script and quit without starting
a new cycle.
.TP 7
\fB[\fP\fI2addr\fP\fB]N\fP
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. 
.LP
If no next line of input is available, the \fBN\fP 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.
.TP 7
\fB[\fP\fI2addr\fP\fB]p\fP
Write the pattern space to standard output.
.TP 7
\fB[\fP\fI2addr\fP\fB]P\fP
Write the pattern space, up to the first <newline>, to standard output.
.TP 7
\fB[\fP\fI1addr\fP\fB]q\fP
Branch to the end of the script and quit without starting a new cycle.
.TP 7
\fB[\fP\fI1addr\fP\fB]r\ \fP \fIrfile\fP
Copy the contents of \fIrfile\fP to standard output as described previously.
If \fIrfile\fP does not exist or cannot be read,
it shall be treated as if it were an empty file, causing no error
condition.
.TP 7
\fB[\fP\fI2addr\fP\fB]s/\fP\fIBRE\fP\fB/\fP\fIreplacement\fP\fB/\fP\fIflags\fP
.sp
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. 
.LP
The replacement string shall be scanned from beginning to end. An
ampersand ( \fB'&'\fP ) appearing in the replacement
shall be replaced by the string matching the BRE. The special meaning
of \fB'&'\fP in this context can be suppressed by
preceding it by a backslash. The characters \fB"\\\fP\fIn"\fP, where
\fIn\fP is a digit, shall be replaced by the text matched
by the corresponding backreference expression. The special meaning
of \fB"\\\fP\fIn"\fP where \fIn\fP is a digit in this
context, can be suppressed by preceding it by a backslash. For each
other backslash ( \fB'\\'\fP ) encountered, the following
character shall lose its special meaning (if any). The meaning of
a \fB'\\'\fP immediately followed by any character other than
\fB'&'\fP , \fB'\\'\fP , a digit, or the delimiter character used
for this command, is unspecified.
.LP
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.
.LP
The value of \fIflags\fP shall be zero or more of:
.TP 7
\fIn\fP
.RS
Substitute for the \fIn\fPth occurrence only of the BRE found within
the pattern space.
.RE
.TP 7
\fBg\fP
.RS
Globally substitute for all non-overlapping instances of the BRE rather
than just the first one. If both \fBg\fP and \fIn\fP
are specified, the results are unspecified.
.RE
.TP 7
\fBp\fP
.RS
Write the pattern space to standard output if a replacement was made.
.RE
.TP 7
\fBw\ \fP \fIwfile\fP
.RS
Write. Append the pattern space to \fIwfile\fP if a replacement was
made. A conforming application shall precede the
\fIwfile\fP argument with one or more <blank>s. If the \fBw\fP flag
is not the last flag value given in a concatenation of
multiple flag values, the results are undefined.
.RE
.sp
.TP 7
\fB[\fP\fI2addr\fP\fB]t\ [\fP\fIlabel\fP\fB]\fP
.sp
Test. Branch to the \fB:\fP command verb bearing the \fIlabel\fP if
any substitutions have been made since the most recent
reading of an input line or execution of a \fBt\fP. If \fIlabel\fP
is not specified, branch to the end of the script.
.TP 7
\fB[\fP\fI2addr\fP\fB]w\ \fP \fIwfile\fP
.sp
Append (write) the pattern space to \fIwfile\fP.
.TP 7
\fB[\fP\fI2addr\fP\fB]x\fP
Exchange the contents of the pattern and hold spaces.
.TP 7
\fB[\fP\fI2addr\fP\fB]y/\fP\fIstring1\fP\fB/\fP\fIstring2\fP\fB/\fP
.sp
Replace all occurrences of characters in \fIstring1\fP with the corresponding
characters in \fIstring2\fP. If a backslash
followed by an \fB'n'\fP appear in \fIstring1\fP or \fIstring2\fP,
the two characters shall be handled as a single
<newline>. If the number of characters in \fIstring1\fP and \fIstring2\fP
are not equal, or if any of the characters in
\fIstring1\fP 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 \fIn\fP,
within \fIstring1\fP and \fIstring2\fP, 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 \fIstring1\fP or \fIstring2\fP, 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 \fB'n'\fP , a backslash, or the delimiter
character is undefined.
.TP 7
\fB[\fP\fI0addr\fP\fB]:\fP\fIlabel\fP
Do nothing. This command bears a \fIlabel\fP to which the \fBb\fP
and \fBt\fP commands branch.
.TP 7
\fB[\fP\fI1addr\fP\fB]=\fP
Write the following to standard output: 
.sp
.RS
.nf

\fB"%d\\n", <\fP\fIcurrent line number\fP\fB>
\fP
.fi
.RE
.TP 7
\fB[\fP\fI0addr\fP\fB]\fP
Ignore this empty command.
.TP 7
\fB[\fP\fI0addr\fP\fB]#\fP
Ignore the \fB'#'\fP 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 \fB"#n"\fP , the default output shall
be suppressed; this shall be the equivalent of specifying
\fB-n\fP on the command line.
.sp
.SH EXIT STATUS
.LP
The following exit values shall be returned:
.TP 7
\ 0
Successful completion.
.TP 7
>0
An error occurred.
.sp
.SH CONSEQUENCES OF ERRORS
.LP
Default.
.LP
\fIThe following sections are informative.\fP
.SH APPLICATION USAGE
.LP
Regular expressions match entire strings, not just individual lines,
but a <newline> is matched by \fB'\\n'\fP in a
\fIsed\fP RE; a <newline> is not allowed by the general definition
of regular expression in IEEE\ Std\ 1003.1-2001.
Also note that \fB'\\n'\fP cannot be used to match a <newline> at
the end of an arbitrary input line; <newline>s
appear in the pattern space as a result of the \fBN\fP editing command.
.SH EXAMPLES
.LP
This \fIsed\fP script simulates the BSD \fIcat\fP \fB-s\fP command,
squeezing excess
blank lines from standard input.
.sp
.RS
.nf

\fBsed -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
'
\fP
.fi
.RE
.SH RATIONALE
.LP
This volume of IEEE\ Std\ 1003.1-2001 requires implementations to
support at least ten distinct \fIwfile\fPs, matching
historical practice on many implementations. Implementations are encouraged
to support more, but conforming applications should not
exceed this limit.
.LP
The exit status codes specified here are different from those in System
V. System V returns 2 for garbled \fIsed\fP 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.
.LP
The manner in which the \fBl\fP 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 \fIed\fP for details of the
format chosen, which is the same as that chosen for \fIsed\fP.
.LP
This volume of IEEE\ Std\ 1003.1-2001 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.
.LP
The requirements for acceptance of <blank>s and <space>s 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
[\fIsic\fP] and tabs from the stripping that is done on every script
line" that appears in much of the historical documentation
of the \fIsed\fP utility description of text. (Not all implementations
are known to have stripped <blank>s from text lines,
although they all have allowed leading <blank>s preceding the address
on a command line.)
.LP
The treatment of \fB'#'\fP 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>s; the BSD implementation has historically
supported this.
.LP
Early proposals required that a \fIscript_file\fP 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
IEEE\ Std\ 1003.1-2001 shall permit \fIscript_file\fPs that consist
only of comment lines.
.LP
Early proposals indicated that if \fB-e\fP and \fB-f\fP options were
intermixed, all \fB-e\fP options were processed before
any \fB-f\fP options. This has been changed to process them in the
order presented because it matches historical practice and is
more intuitive.
.LP
The treatment of the \fBp\fP flag to the \fBs\fP command differs between
System V and BSD-based systems when the default
output is suppressed. In the two examples:
.sp
.RS
.nf

\fBecho a | sed    's/a/A/p'
echo a | sed -n 's/a/A/p'
\fP
.fi
.RE
.LP
this volume of IEEE\ Std\ 1003.1-2001, BSD, System V documentation,
and the SVID indicate that the first example should
write two lines with \fBA\fP, whereas the second should write one.
Some System V systems write the \fBA\fP only once in both
examples because the \fBp\fP flag is ignored if the \fB-n\fP option
is not specified.
.LP
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
IEEE\ Std\ 1003.1-2001 because:
.IP " *" 3
No known documentation for any historic system describes the interaction
between the \fBp\fP flag and the \fB-n\fP option.
.LP
.IP " *" 3
The selected behavior is more correct as there is no technical justification
for any interaction between the \fBp\fP flag and
the \fB-n\fP option. A relationship between \fB-n\fP and the \fBp\fP
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 \fBD\fP, \fBd\fP, \fBq\fP, or
branching commands. Such scripts rely on the \fBp\fP suffix to write
the pattern space because they do not make use of the default
output at the "bottom" of the script.
.LP
.IP " *" 3
Because the \fB-n\fP option makes the \fBp\fP flag unnecessary, any
interaction would only be useful if \fIsed\fP scripts
were written to run both with and without the \fB-n\fP option. This
is believed to be unlikely. It is even more unlikely that
programmers have coded the \fBp\fP 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.
.LP
.IP " *" 3
Finally, scripts that break under the specified behavior produce too
much output instead of too little, which is easier to
diagnose and correct.
.LP
.LP
The form of the substitute command that uses the \fBn\fP 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 \fBs/a/A/2047\fP should be able to substitute the 2047th occurrence
of \fBa\fP on a line.
.LP
The \fBb\fP, \fBt\fP, and \fB:\fP commands are documented to ignore
leading white space, but no mention is made of trailing
white space. Historical implementations of \fIsed\fP assigned different
locations to the labels \fB'x'\fP and
\fB"x\ "\fP . 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.
.LP
Historically, the \fIsed\fP \fB!\fP and \fB}\fP 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.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIawk\fP , \fIed\fP , \fIgrep\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .