* src/autoconf.texi: Modernize description of sed limitations.
Prompted by a bug report by Daniel Locks in:
https://lists.gnu.org/r/bug-autoconf/2025-08/msg00001.html
@caindex path_SED
Set output variable @code{SED} to a Sed implementation that conforms to
POSIX and does not have arbitrary length limits. Report an error if no
@caindex path_SED
Set output variable @code{SED} to a Sed implementation that conforms to
POSIX and does not have arbitrary length limits. Report an error if no
-acceptable Sed is found. @xref{sed, , Limitations of Usual Tools}, for more
-information about portability problems with Sed.
+acceptable Sed is found. @xref{sed, , Limitations of Usual Tools}.
The result of this test can be overridden by setting the @code{SED} variable
and is cached in the @code{ac_cv_path_SED} variable.
The result of this test can be overridden by setting the @code{SED} variable
and is cached in the @code{ac_cv_path_SED} variable.
POSIX also says that @samp{test ! "@var{string}"},
@samp{test -n "@var{string}"} and
POSIX also says that @samp{test ! "@var{string}"},
@samp{test -n "@var{string}"} and
-@samp{test -z "@var{string}"} work with any string, but many
-shells (such as Solaris 10, AIX 3.2, UNICOS 10.0.0.6,
-Digital Unix 4, etc.)@: get confused if
+@samp{test -z "@var{string}"} work with any string, but some
+shells (such as Solaris 10) get confused if
@var{string} looks like an operator:
@example
@var{string} looks like an operator:
@example
It is common to find variations of the following idiom:
@example
It is common to find variations of the following idiom:
@example
-test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" &&
+test -n "`echo $ac_feature | sed 's/[a-zA-Z0-9_-]//g'`" &&
@var{action}
@end example
@var{action}
@end example
It is safe to trap at least the signals 1, 2, 13, and 15. You can also
trap 0, i.e., have the @command{trap} run when the script ends (either via an
explicit @command{exit}, or the end of the script). The trap for 0 should be
It is safe to trap at least the signals 1, 2, 13, and 15. You can also
trap 0, i.e., have the @command{trap} run when the script ends (either via an
explicit @command{exit}, or the end of the script). The trap for 0 should be
-installed outside of a shell function, or AIX 5.3 @command{/bin/sh}
+installed outside of a shell function, or AIX 7.3 @command{/bin/sh}
will invoke the trap at the end of this function.
POSIX says that @samp{trap - 1 2 13 15} resets the traps for the
will invoke the trap at the end of this function.
POSIX says that @samp{trap - 1 2 13 15} resets the traps for the
@item @command{sed}
@c ----------------
@prindex @command{sed}
@item @command{sed}
@c ----------------
@prindex @command{sed}
+The portable options are @option{-e}, @option{-f}, and @option{-n}.
+POSIX standardized @option{-E} in 2024 but some older implementations lack it.
+Although GNU @command{sed} supports other options like @option{-i},
+these can be missing or have different meanings elsewhere.
+
Patterns should not include the separator (unless escaped), even as part
Patterns should not include the separator (unless escaped), even as part
-of a character class. In conformance with POSIX, the Cray
-@command{sed} rejects @samp{s/[^/]*$//}: use @samp{s%[^/]*$%%}.
Even when escaped, patterns should not include separators that are also
used as @command{sed} metacharacters. For example, GNU sed 4.0.9 rejects
@samp{s,x\@{1\,\@},,}, while sed 4.1 strips the backslash before the comma
before evaluating the basic regular expression.
Even when escaped, patterns should not include separators that are also
used as @command{sed} metacharacters. For example, GNU sed 4.0.9 rejects
@samp{s,x\@{1\,\@},,}, while sed 4.1 strips the backslash before the comma
before evaluating the basic regular expression.
-Avoid empty patterns within parentheses (i.e., @samp{\(\)}). POSIX does
-not require support for empty patterns, and Unicos 9 @command{sed} rejects
-them.
+Avoid empty patterns, such as the parenthesized empty pattern in @samp{\(\)}
+or the empty pattern followed by an interval expression in @samp{\@{2\@}}.
+POSIX does not require support for empty patterns.
-Unicos 9 @command{sed} loops endlessly on patterns like @samp{.*\n.*}.
+Comments in Sed scripts should not contain @samp{n} immediately after
+the leading @samp{#}. Although POSIX.1-2024 says this is equivalent to the
+@option{-n} option, earlier POSIX editions said that
+the equivalence occurs only if the comment is the first line of the script,
+and many @command{sed} implementations are confused about this.
+It is more portable to use @option{-n}.
-Sed scripts should not use branch labels longer than 7 characters and
-should not contain comments; AIX 5.3 @command{sed} rejects indented comments.
HP-UX sed has a limit of 99 commands (not counting @samp{:} commands) and
48 labels, which cannot be circumvented by using more than one script
file. It can execute up to 19 reads with the @samp{r} command per cycle.
HP-UX sed has a limit of 99 commands (not counting @samp{:} commands) and
48 labels, which cannot be circumvented by using more than one script
file. It can execute up to 19 reads with the @samp{r} command per cycle.
-Solaris @command{/usr/ucb/sed} rejects usages that exceed a limit of
+Solaris 10 @command{/usr/ucb/sed} rejects usages that exceed a limit of
about 6000 bytes for the internal representation of commands.
about 6000 bytes for the internal representation of commands.
-Avoid redundant @samp{;}, as some @command{sed} implementations, such as
-NetBSD 1.4.2's, incorrectly try to interpret the second
-@samp{;} as a command:
-
-@example
-$ @kbd{echo a | sed 's/x/x/;;s/x/x/'}
-sed: 1: "s/x/x/;;s/x/x/": invalid command code ;
-@end example
-
Some @command{sed} implementations have a buffer limited to 4000 bytes,
and this limits the size of input lines, output lines, and internal
buffers that can be processed portably. Likewise,
not all @command{sed} implementations can handle embedded @code{NUL} or
a missing trailing newline.
Some @command{sed} implementations have a buffer limited to 4000 bytes,
and this limits the size of input lines, output lines, and internal
buffers that can be processed portably. Likewise,
not all @command{sed} implementations can handle embedded @code{NUL} or
a missing trailing newline.
-Remember that ranges within a bracket expression of a regular expression
+Ranges within a bracket expression of a regular expression
are only well-defined in the @samp{C} (or @samp{POSIX}) locale.
Meanwhile, support for character classes like @samp{[[:upper:]]} is not
yet universal, so if you cannot guarantee the setting of @env{LC_ALL},
are only well-defined in the @samp{C} (or @samp{POSIX}) locale.
Meanwhile, support for character classes like @samp{[[:upper:]]} is not
yet universal, so if you cannot guarantee the setting of @env{LC_ALL},
Additionally, POSIX states that regular expressions are only
well-defined on characters. Unfortunately, there exist platforms such
as Mac OS X 10.5 where not all 8-bit byte values are valid characters,
Additionally, POSIX states that regular expressions are only
well-defined on characters. Unfortunately, there exist platforms such
as Mac OS X 10.5 where not all 8-bit byte values are valid characters,
-even though that platform has a single-byte @samp{C} locale. And POSIX
-allows the existence of a multi-byte @samp{C} locale, although that does
-not yet appear to be a common implementation. At any rate, it means
-that not all bytes will be matched by the regular expression @samp{.}:
+even though that platform has a single-byte @samp{C} locale. Although
+this practice was disallowed by recent releases of POSIX, it means that
+in the @samp{C} locale not all bytes will be matched by the regular
+expression @samp{.}:
@example
$ @kbd{printf '\200\n' | LC_ALL=C sed -n /./p | wc -l}
@example
$ @kbd{printf '\200\n' | LC_ALL=C sed -n /./p | wc -l}
Anchors (@samp{^} and @samp{$}) inside groups are not portable.
Anchors (@samp{^} and @samp{$}) inside groups are not portable.
-Nested parentheses in patterns (e.g., @samp{\(\(a*\)b*)\)}) are
-quite portable to current hosts, but was not supported by some ancient
-@command{sed} implementations like SVR3.
-
-Some @command{sed} implementations, e.g., Solaris, restrict the special
+Some @command{sed} implementations, e.g., Solaris 11.4, restrict the special
role of the asterisk @samp{*} to one-character regular expressions and
back-references, and the special role of interval expressions
@samp{\@{@var{m}\@}}, @samp{\@{@var{m},\@}}, or @samp{\@{@var{m},@var{n}\@}}
role of the asterisk @samp{*} to one-character regular expressions and
back-references, and the special role of interval expressions
@samp{\@{@var{m}\@}}, @samp{\@{@var{m},\@}}, or @samp{\@{@var{m},@var{n}\@}}
-Portable @command{sed} regular expressions should use @samp{\} only to escape
-characters in the string @samp{$()*.123456789[\^n@{@}}. For example,
-alternation, @samp{\|}, is common but POSIX does not require its
-support, so it should be avoided in portable scripts. Solaris
+In the normal case when @option{-E} is not used,
+portable @command{sed} regular expressions should use @samp{\} only to escape
+characters in the string @samp{$*.123456789[\^n}. For example,
+POSIX.1-2024 says it is implementation-defined
+whether @samp{\|} means alternation or simply matches @samp{|},
+so it should be avoided in portable scripts. Solaris
@command{sed} does not support alternation; e.g., @samp{sed '/a\|b/d'}
deletes only lines that contain the literal string @samp{a|b}.
Similarly, @samp{\+} and @samp{\?} should be avoided.
@command{sed} does not support alternation; e.g., @samp{sed '/a\|b/d'}
deletes only lines that contain the literal string @samp{a|b}.
Similarly, @samp{\+} and @samp{\?} should be avoided.
but POSIX says that this use of a semicolon has undefined effect if
@var{command-1}'s verb is @samp{@{}, @samp{a}, @samp{b}, @samp{c},
but POSIX says that this use of a semicolon has undefined effect if
@var{command-1}'s verb is @samp{@{}, @samp{a}, @samp{b}, @samp{c},
-@samp{i}, @samp{r}, @samp{t}, @samp{w}, @samp{:}, or @samp{#}, so you
+@samp{i}, @samp{r}, @samp{t}, @samp{w} or @samp{:},
+or if @var{command-1} is an @samp{s} with the @samp{w} option, so you
should use semicolon only with simple scripts that do not use these
should use semicolon only with simple scripts that do not use these
+constructs.
+
+Avoid redundant @samp{;}, as some @command{sed} implementations, such as
+NetBSD 1.4.2's, incorrectly try to interpret the second
+@samp{;} as a command:
+
+@example
+$ @kbd{echo a | sed 's/x/x/;;s/x/x/'}
+sed: 1: "s/x/x/;;s/x/x/": invalid command code ;
+@end example
POSIX requires each @option{-e} and @option{-f} option to specify a
syntactically complete script. Although GNU @command{sed} also allows
@option{-e} and @option{-f} options to specify script fragments
that it assembles into a full script, this is not portable. For
POSIX requires each @option{-e} and @option{-f} option to specify a
syntactically complete script. Although GNU @command{sed} also allows
@option{-e} and @option{-f} options to specify script fragments
that it assembles into a full script, this is not portable. For
-example, the @command{sed} programs on Solaris 10, HP-UX 11, and AIX
+example, the @command{sed} programs on Solaris 11, HP-UX 11, and AIX
do not allow script fragments:
@example
do not allow script fragments:
@example
-Commands inside @{ @} brackets are further restricted. POSIX 2008 says that
+Commands should not be followed by white space.
+Although trailing white space often works,
+it can be dicey in some situations and
+it is simpler to avoid it entirely.
+
+Commands inside @{ @} brackets are further restricted. POSIX.1-2004 says that
they cannot be preceded by addresses, @samp{!}, or @samp{;}, and that
each command must be followed immediately by a newline, without any
intervening blanks or semicolons. The closing bracket must be alone on
they cannot be preceded by addresses, @samp{!}, or @samp{;}, and that
each command must be followed immediately by a newline, without any
intervening blanks or semicolons. The closing bracket must be alone on
-a line, other than white space preceding or following it. However, a
-future version of POSIX may standardize the use of addresses within brackets.
+a line, other than white space preceding or following it. Although these
+restrictions were lifted in POSIX.1-2008, it is more portable to
+respect them.
Contrary to yet another urban legend, you may portably use @samp{&} in
the replacement part of the @code{s} command to mean ``what was
Contrary to yet another urban legend, you may portably use @samp{&} in
the replacement part of the @code{s} command to mean ``what was
you use @samp{!}, it is best to put it on a command that is delimited by
newlines rather than @samp{;}.
you use @samp{!}, it is best to put it on a command that is delimited by
newlines rather than @samp{;}.
-Also note that POSIX requires that the @samp{b}, @samp{t}, @samp{r}, and
+POSIX requires that the @samp{b}, @samp{t}, @samp{r}, and
@samp{w} commands be followed by exactly one space before their argument.
On the other hand, no white space is allowed between @samp{:} and the
@samp{w} commands be followed by exactly one space before their argument.
On the other hand, no white space is allowed between @samp{:} and the
+subsequent label. Branch labels should contain at most 8 bytes,
+each of which should be an ASCII graphical character.
+Do not put trailing white space after a branch label.
If a sed script is specified on the command line and ends in an
@samp{a}, @samp{c}, or @samp{i} command, the last line of inserted text
If a sed script is specified on the command line and ends in an
@samp{a}, @samp{c}, or @samp{i} command, the last line of inserted text
-POSIX requires that with an empty regular expression, the last non-empty
+POSIX requires that with a missing regular expression, the last
regular expression from either an address specification or substitution
regular expression from either an address specification or substitution
-command is applied. However, busybox 1.6.1 complains when using a
+command is used. However, busybox 1.6.1 complains when using a
substitution command with a replacement containing a back-reference to
substitution command with a replacement containing a back-reference to
-an empty regular expression; the workaround is repeating the regular
+a missing regular expression; the workaround is repeating the regular
Solaris XPG4 yes no error
NetBSD 5.1 no no yes
FreeBSD 9.1 no no yes
Solaris XPG4 yes no error
NetBSD 5.1 no no yes
FreeBSD 9.1 no no yes
@c LocalWords: Oliva awk Aaaaarg cmd regex xfoo GNV OpenVMS VM url fc
@c LocalWords: sparc Proulx nbar nfoo maxdepth acdilrtu TWG mc ing FP
@c LocalWords: mkdir exe uname OpenBSD Fileutils mktemp umask TMPDIR guid os
@c LocalWords: Oliva awk Aaaaarg cmd regex xfoo GNV OpenVMS VM url fc
@c LocalWords: sparc Proulx nbar nfoo maxdepth acdilrtu TWG mc ing FP
@c LocalWords: mkdir exe uname OpenBSD Fileutils mktemp umask TMPDIR guid os
-@c LocalWords: fooXXXXXX Unicos utimes hpux hppa unescaped SUBST'ed
+@c LocalWords: fooXXXXXX utimes hpux hppa unescaped SUBST'ed
@c LocalWords: pmake DOS's gmake ifoo DESTDIR autoconfiscated pc coff mips gg
@c LocalWords: cpu wildcards rpcc rdtsc powerpc readline
@c LocalWords: withval vxworks gless localcache usr LOFF loff CYGWIN Cygwin
@c LocalWords: pmake DOS's gmake ifoo DESTDIR autoconfiscated pc coff mips gg
@c LocalWords: cpu wildcards rpcc rdtsc powerpc readline
@c LocalWords: withval vxworks gless localcache usr LOFF loff CYGWIN Cygwin
projects / thirdparty / autoconf.git / commitdiff
