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* doc/autoconf.texi (Testing Values and Files): Delete, move its
content into... (Limitations of Usual Tools::test): here. Document more limitations.
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Akim Demaille
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@ -1,3 +1,10 @@
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2000-05-19 Akim Demaille <akim@epita.fr>
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* doc/autoconf.texi (Testing Values and Files): Delete, move its
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content into...
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(Limitations of Usual Tools::test): here.
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Document more limitations.
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2000-05-19 Akim Demaille <akim@epita.fr>
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* acspecific.m4 (_AC_LIBOBJ_GETLOADAVG): New macro, extracted from
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@ -252,8 +252,7 @@ Checking Run Time Behavior
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Portable Shell Programming
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* Special Shell Variables::
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* Testing Values and Files:: Checking strings and files
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* Special Shell Variables:: Variables you should not change
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* Shell Substitutions:: Test and assign
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* Limitations of Usual Tools:: Portable use of portable tools
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* Exiting from Shell Scripts:: How to exit from an autoconf shell script
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@ -4528,13 +4527,12 @@ well; for example, don't rely on @code{ln} having a @samp{-f} option or
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@menu
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* Special Shell Variables:: Variables you should not change
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* Testing Values and Files:: Checking strings and files
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* Shell Substitutions:: Test and assign
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* Limitations of Usual Tools:: Portable use of portable tools
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* Exiting from Shell Scripts:: How to exit from an autoconf shell script
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@end menu
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@node Special Shell Variables, Testing Values and Files, Portable Shell, Portable Shell
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@node Special Shell Variables, Shell Substitutions, Portable Shell, Portable Shell
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@subsection Special Shell Variables
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Some shell variables shall not be used. Since currently Autoconf does
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@ -4554,18 +4552,27 @@ Setting @code{CDPATH} to the empty value is not enough for most shells.
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A simple colon is enough but for @code{zsh}, which prefers a leading dot:
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@example
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~ % mkdir foo && CDPATH=: cd foo
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~/foo
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~ % CDPATH=:. cd foo
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~/foo
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~ % CDPATH=.: cd foo
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~ %
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zsh-3.1.6 % mkdir foo && (CDPATH=: cd foo)
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/tmp/foo
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zsh-3.1.6 % (CDPATH=:. cd foo)
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/tmp/foo
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zsh-3.1.6 % (CDPATH=.: cd foo)
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zsh-3.1.6 %
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@end example
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@noindent
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(of course we could just @code{unset} @code{CDPATH}, it also behaves
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properly if set to the empty string).
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Life wouldn't be so much fun if @command{bash} abd @command{zsh} had the
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same behavior:
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@example
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bash-2.02 % (CDPATH=:. cd foo)
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bash-2.02 % (CDPATH=.: cd foo)
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/tmp/foo
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@end example
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Therefore a portable solution to neutralize @samp{CDPATH} is
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@samp{CDPATH=$@{ZSH_VERSION+.@}:}.
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@ -4579,7 +4586,7 @@ Therefore a portable solution to neutralize @samp{CDPATH} is
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@evindex LC_CTYPE
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These must not be set unconditionally because not all systems understand
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e.g. @strong{LANG=C} (notably SCO). Fixing @code{LC_MESSAGES} prevents
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e.g. @samp{LANG=C} (notably SCO). Fixing @code{LC_MESSAGES} prevents
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Solaris @command{sh} from translating var values in @code{set}! Non-C
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@code{LC_CTYPE} values break the ctype check. Therefore, run:
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@ -4597,35 +4604,7 @@ hence read-only. Do not use it.
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@end table
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@node Testing Values and Files, Shell Substitutions, Special Shell Variables, Portable Shell
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@subsection Testing Values and Files
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@code{configure} scripts need to test properties of many files and
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strings. Here are some portability problems to watch out for when doing
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those tests.
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The @code{test} program is the way to perform many file and string
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tests. It is often invoked by the alternate name @samp{[}, but using
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that name in Autoconf code is asking for trouble since it is an
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@code{m4} quote character.
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If you need to make multiple checks using @code{test}, combine them with
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the shell operators @samp{&&} and @samp{||} instead of using the
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@code{test} operators @samp{-a} and @samp{-o}. On System V, the
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precedence of @samp{-a} and @samp{-o} is wrong relative to the unary
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operators; consequently, @sc{posix} does not specify them, so using them
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is nonportable. If you combine @samp{&&} and @samp{||} in the same
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statement, keep in mind that they have equal precedence.
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@c FIXME: Hm, I'd say the `target' here should be `build'.
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To enable @code{configure} scripts to support cross-compilation, they
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shouldn't do anything that tests features of the host system instead of
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the target system. But occasionally you may find it necessary to check
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whether some arbitrary file exists. To do so, use @samp{test -f} or
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@samp{test -r}. Do not use @samp{test -x}, because 4.3@sc{bsd} does not
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have it.
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@node Shell Substitutions, Limitations of Usual Tools, Testing Values and Files, Portable Shell
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@node Shell Substitutions, Limitations of Usual Tools, Special Shell Variables, Portable Shell
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@subsection Shell Substitutions
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A nonportable shell programming construct is
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@ -4655,6 +4634,35 @@ If the value is a literal string, it should be quoted:
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otherwise some shells, such as on Digital Unix V 5.0, will die because
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of a @emph{bad substitution}.
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Contrary to a persistent urban legend, the Bourne shell does not
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systematically split variables and backquoted expressions, in
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particular, the following code:
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@example
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case "$given_srcdir" in
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.) top_srcdir="`echo "$dots" | sed 's,/$,,'`"
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*) top_srcdir="$dots$given_srcdir" ;;
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esac
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@end example
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@noindent
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is more readable with the right-hand side of the assignments, and the
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argument of @code{case} left without quotes:
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@example
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case $given_srcdir in
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.) top_srcdir=`echo "$dots" | sed 's,/$,,'`
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*) top_srcdir=$dots$given_srcdir ;;
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esac
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@end example
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@noindent
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and in fact it is even @emph{more} portable: in the first case of the
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first attempt, the computation of @code{top_srcdir} is not portable,
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since not all the shells understand properly @samp{"`... "foo"... `"}.
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Worse yet, not all the shells understand @samp{"`... \"foo\"... `"} the
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same way: there is just no portable way to use double-quoted strings
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inside double-quoted backquoted expressions (Pfew!).
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@node Limitations of Usual Tools, Exiting from Shell Scripts, Shell Substitutions, Portable Shell
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@subsection Limitations of Usual Tools
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@ -4662,22 +4670,26 @@ of a @emph{bad substitution}.
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The small set of tools you can expect to find on any machine can still
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find some limitations you should be aware of.
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@table @code
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@item egrep
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@table @asis
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@item @command{egrep}
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@cindex @command{egrep}
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The empty alternative is not portable, use @samp{?} instead. For
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instance with Digital Unix v5.0:
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@example
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> printf "foo\n|foo" | egrep '^(|foo|bar)$'
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> printf "foo\n|foo\n" | egrep '^(|foo|bar)$'
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|foo
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> printf "bar\nbar|" | egrep '^(foo|bar|)$'
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> printf "bar\nbar|\n" | egrep '^(foo|bar|)$'
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bar|
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> printf "foo\nfoo|\n|bar\nbar" | egrep '^(foo||bar)$'
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> printf "foo\nfoo|\n|bar\nbar\n" | egrep '^(foo||bar)$'
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foo
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|bar
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@end example
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@item grep
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@command{egrep} also suffers the limitations of @command{grep}.
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@item @command{grep}
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@cindex @command{grep}
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Don't use @samp{grep -s} to suppress output, because @samp{grep -s} on
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System V does not suppress output, only error messages. Instead,
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redirect the standard output and standard error (in case the file
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@ -4689,14 +4701,87 @@ honor the last pattern (eg., IRIX 6.5 and Solaris 2.5.1). Anyway,
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Stardent Vistra SVR4 @code{grep} lacks @samp{-e}... Instead, use
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alternation and @code{egrep}.
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@item sed
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@item @command{sed}
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@cindex @command{sed}
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@code{sed} scripts should not use branch labels longer than 8 characters
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and should not contain comments.
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@code{sed} input should have reasonably long lines, since some
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@code{sed} have an input buffer limited to 4000 bytes.
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Alternation, @samp{\|}, is not portable.
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Alternation, @samp{\|}, is common but not portable.
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@item @command{test}
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@cindex @command{test}
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The @code{test} program is the way to perform many file and string
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tests. It is often invoked by the alternate name @samp{[}, but using
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that name in Autoconf code is asking for trouble since it is an
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@code{m4} quote character.
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If you need to make multiple checks using @code{test}, combine them with
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the shell operators @samp{&&} and @samp{||} instead of using the
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@code{test} operators @samp{-a} and @samp{-o}. On System V, the
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precedence of @samp{-a} and @samp{-o} is wrong relative to the unary
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operators; consequently, @sc{posix} does not specify them, so using them
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is nonportable. If you combine @samp{&&} and @samp{||} in the same
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statement, keep in mind that they have equal precedence.
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@item @command{test} (files)
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@c FIXME: Hm, I'd say the sentence should be
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@c To enable @code{configure} scripts to support cross-compilation, they
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@c shouldn't do anything that tests features of the build system instead of
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@c the host system.
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To enable @code{configure} scripts to support cross-compilation, they
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shouldn't do anything that tests features of the host system instead of
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the target system. But occasionally you may find it necessary to check
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whether some arbitrary file exists. To do so, use @samp{test -f} or
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@samp{test -r}. Do not use @samp{test -x}, because @sc{4.3bsd} does not
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have it.
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@item @command{test} (strings)
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Avoid @samp{test "@var{string}"}, in particular if @var{string} might
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start with a dash, since @code{test} might interpret its argument as an
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option (e.g., @samp{@var{string} = "-n"}).
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Contrary to a common belief, @samp{test -n @var{string}} and @samp{test
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-z @var{string}} @strong{are} portable, nevertheless many shells (e.g.,
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FIXME: AIX IIRC) have bizarre precedence and may be confused if
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@var{string} looks like an operator:
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@example
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@c FIXME: Hm, IIRC:
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$ test -n "="
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error: missing argument
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@end example
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If there are risks, use @samp{test "x@var{string}" = x} or @samp{test
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"x@var{string}" != x} instead.
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It is frequent to find variations of the following idiom:
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@example
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test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" &&
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@var{action}
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@end example
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@noindent
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to take an action when a token matches a given pattern. Such constructs
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are always avoidable, and should always be. Rather, use:
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@example
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echo "$ac_feature" | grep '[^-a-zA-Z0-9_]' >/dev/null 2>&1 &&
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@var{action}
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@end example
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@noindent
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Where possible, use @code{case} since, being a shell builtin, it is
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faster:
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@example
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case $ac_feature in
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*[^-a-zA-Z0-9_]*) @var{action};;
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esac
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@end example
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@end table
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