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docs: mention another issue with variable expansion
In particular, see http://austingroupbugs.net/view.php?id=221 and http://austingroupbugs.net/view.php?id=255. * doc/autoconf.texi (Shell Substitutions) <${var+value}>: New subsection. <${var=literal}>: Tweak wording. Add mention of an ambiguity allowed by POSIX. * tests/torture.at (Substitute and define special characters): Make test more robust; here, the outer "" is in a here-doc, and does not violate the quoting rules of thumb just documented. Signed-off-by: Eric Blake <eblake@redhat.com>
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Eric Blake
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ChangeLog
13
ChangeLog
@ -1,3 +1,16 @@
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2010-08-26 Eric Blake <eblake@redhat.com>
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docs: mention another issue with variable expansion
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In particular, see http://austingroupbugs.net/view.php?id=221
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and http://austingroupbugs.net/view.php?id=255.
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* doc/autoconf.texi (Shell Substitutions) <${var+value}>: New
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subsection.
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<${var=literal}>: Tweak wording. Add mention of an ambiguity
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allowed by POSIX.
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* tests/torture.at (Substitute and define special characters):
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Make test more robust; here, the outer "" is in a here-doc, and
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does not violate the quoting rules of thumb just documented.
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2010-08-25 Eric Blake <eblake@redhat.com>
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m4sh: revert incorrect mix of "${a='b'}"
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@ -15392,52 +15392,157 @@ bad substitution
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@ifnotinfo
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@cindex $@{@var{var}:-@var{value}@}
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@end ifnotinfo
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@cindex $@{@var{var}-@var{value}@}
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Old BSD shells, including the Ultrix @code{sh}, don't accept the
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colon for any shell substitution, and complain and die.
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Similarly for $@{@var{var}:=@var{value}@}, $@{@var{var}:?@var{value}@}, etc.
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@item $@{@var{var}=@var{literal}@}
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@cindex $@{@var{var}=@var{literal}@}
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@item $@{@var{var}+@var{value}@}
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@cindex $@{@var{var}+@var{value}@}
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When using @samp{$@{@var{var}-@var{value}@}} or
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@samp{$@{@var{var}-@var{value}@}} for providing alternate substitutions,
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@var{value} must either be a single shell word or be quoted. Solaris
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@command{/bin/sh} complains otherwise.
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@example
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$ @kbd{/bin/sh -c 'echo $@{a-b c@}'}
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/bin/sh: bad substitution
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$ @kbd{/bin/sh -c 'echo $@{a-'\''b c'\''@}'}
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b c
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$ @kbd{/bin/sh -c 'echo "$@{a-b c@}"'}
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b c
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@end example
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According to Posix, if an expansion occurs inside double quotes, then
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the use of unquoted double quotes within @var{value} is unspecified, and
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any single quotes become literal characters; in that case, escaping must
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be done with backslash.
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@example
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$ @kbd{/bin/sh -c 'echo "$@{a-"b c"@}"'}
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/bin/sh: bad substitution
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$ @kbd{ksh -c 'echo "$@{a-"b c"@}"'}
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b c
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$ @kbd{bash -c 'echo "$@{a-"b c"@}"'}
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b c
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$ @kbd{/bin/sh -c 'a=; echo $@{a+'\''b c'\''@}'}
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b c
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$ @kbd{/bin/sh -c 'a=; echo "$@{a+'\''b c'\''@}"'}
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'b c'
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$ @kbd{/bin/sh -c 'a=; echo "$@{a+\"b c\"@}"'}
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"b c"
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$ @kbd{/bin/sh -c 'a=; echo "$@{a+b c@}"'}
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b c
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@end example
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Perhaps the easiest way to work around quoting issues in a manner
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portable to all shells is to place the results in a temporary variable,
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then use @samp{$tmp} as the @var{value}, rather than trying to inline
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the expression needing quoting.
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@example
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$ @kbd{/bin/sh -c 'tmp="a b\"'\''@}\\"; echo "$@{a-$tmp@}"'}
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b c"'@}\
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$ @kbd{ksh -c 'tmp="a b\"'\''@}\\"; echo "$@{a-$tmp@}"'}
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b c"'@}\
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$ @kbd{bash -c 'tmp="a b\"'\''@}\\"; echo "$@{a-$tmp@}"'}
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b c"'@}\
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@end example
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@item $@{@var{var}=@var{value}@}
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@cindex $@{@var{var}=@var{value}@}
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When using @samp{$@{@var{var}=@var{value}@}} to assign a default value
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to @var{var}, remember that even though the assignment to @var{var} does
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not undergo file name expansion, the result of the variable expansion
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does. In particular, when using @command{:} followed by unquoted
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variable expansion for the side effect of setting a default value, if
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either @samp{value} or the prior contents of @samp{$var} contains
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globbing characters, the shell has to spend time performing file name
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does unless the expansion occurred within double quotes. In particular,
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when using @command{:} followed by unquoted variable expansion for the
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side effect of setting a default value, if the final value of
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@samp{$var} contains any globbing characters (either from @var{value} or
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from prior contents), the shell has to spend time performing file name
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expansion and field splitting even though those results will not be
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used. Therefore, it is a good idea to use double quotes when performing
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default initialization.
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used. Therefore, it is a good idea to consider double quotes when performing
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default initialization; while remembering how this impacts any quoting
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characters appearing in @var{value}.
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@example
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$ time bash -c ': "$@{a=/usr/bin/*@}"; echo "$a"'
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$ @kbd{time bash -c ': "$@{a=/usr/bin/*@}"; echo "$a"'}
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/usr/bin/*
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real 0m0.005s
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user 0m0.002s
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sys 0m0.003s
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$ time bash -c ': $@{a=/usr/bin/*@}; echo "$a"'
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$ @kbd{time bash -c ': $@{a=/usr/bin/*@}; echo "$a"'}
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/usr/bin/*
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real 0m0.039s
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user 0m0.026s
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sys 0m0.009s
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$ @kbd{time bash -c 'a=/usr/bin/*; : $@{a=noglob@}; echo "$a"'}
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/usr/bin/*
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real 0m0.031s
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user 0m0.020s
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sys 0m0.010s
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$ @kbd{time bash -c 'a=/usr/bin/*; : "$@{a=noglob@}"; echo "$a"'}
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/usr/bin/*
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real 0m0.006s
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user 0m0.002s
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sys 0m0.003s
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@end example
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Use quotes if @var{literal} contains more than one shell word:
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As with @samp{+} and @samp{-}, you must use quotes when using @samp{=}
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if the @var{value} contains more than one shell word; either single
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quotes for just the @var{value}, or double quotes around the entire
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expansion:
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@example
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: "$@{var='Some words'@}"
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$ @kbd{: $@{var1='Some words'@}}
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$ @kbd{: "$@{var2=like this@}"}
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$ @kbd{echo $var1 $var2}
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Some words like this
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@end example
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@noindent
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otherwise some shells, such as on Digital Unix V 5.0, die because
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of a ``bad substitution''.
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otherwise some shells, such as Solaris @command{/bin/sh} or on Digital
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Unix V 5.0, die because of a ``bad substitution''. Meanwhile, Posix
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requires that with @samp{=}, quote removal happens prior to the
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assignment, and the expansion be the final contents of @var{var} without
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quoting (and thus subject to field splitting), in contrast to the
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behavior with @samp{-} passing the quoting through to the final
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expansion. However, @command{bash} 4.1 does not obey this rule.
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@sp 1
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@example
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$ @kbd{ksh -c 'echo $@{var-a\ \ b@}'}
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a b
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$ @kbd{ksh -c 'echo $@{var=a\ \ b@}'}
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a b
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$ @kbd{bash -c 'echo $@{var=a\ \ b@}'}
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a b
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@end example
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Solaris @command{/bin/sh} has a frightening bug in its interpretation
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of this. Imagine you need set a variable to a string containing
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Finally, Posix states that when mixing @samp{$@{a=b@}} with regular
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commands, it is unspecified whether the assignments affect the parent
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shell environment. It is best to perform assignments independently from
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commands, to avoid the problems demonstrated in this example:
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@example
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$ @kbd{bash -c 'x= y=$@{x:=b@} sh -c "echo +\$x+\$y+";echo -$x-'}
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+b+b+
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-b-
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$ @kbd{/bin/sh -c 'x= y=$@{x:=b@} sh -c "echo +\$x+\$y+";echo -$x-'}
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++b+
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--
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$ @kbd{ksh -c 'x= y=$@{x:=b@} sh -c "echo +\$x+\$y+";echo -$x-'}
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+b+b+
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--
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@end example
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@item $@{@var{var}=@var{value}@}
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@cindex $@{@var{var}=@var{literal}@}
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Solaris @command{/bin/sh} has a frightening bug in its handling of
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literal assignments. Imagine you need set a variable to a string containing
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@samp{@}}. This @samp{@}} character confuses Solaris @command{/bin/sh}
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when the affected variable was already set. This bug can be exercised
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by running:
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@ -15458,7 +15563,8 @@ $ @kbd{echo $foo}
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It seems that @samp{@}} is interpreted as matching @samp{$@{}, even
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though it is enclosed in single quotes. The problem doesn't happen
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using double quotes.
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using double quotes, or when using a temporary variable holding the
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problematic string.
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@item $@{@var{var}=@var{expanded-value}@}
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@cindex $@{@var{var}=@var{expanded-value}@}
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@ -15467,7 +15573,7 @@ running
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@example
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default="yu,yaa"
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: "$@{var="$default"@}"
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: $@{var="$default"@}
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@end example
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@noindent
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@ -15493,7 +15599,7 @@ One classic incarnation of this bug is:
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@example
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default="a b c"
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: "$@{list="$default"@}"
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: $@{list="$default"@}
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for c in $list; do
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echo $c
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done
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@ -15755,7 +15861,7 @@ the variable being initialized is not intended to be IFS-split
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(i.e., it's not a list), then use:
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@example
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: "$@{var="$default"@}"
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: $@{var="$default"@}
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@end example
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@item
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@ -909,7 +909,7 @@ AC_DEFINE_UNQUOTED([unq1], [$baz], [unquoted, test 1])
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AC_DEFINE_UNQUOTED([unq2], [\$baz], [unquoted, test 2])
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AC_DEFINE_UNQUOTED([unq3], ["$baz"], [unquoted, test 3])
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AC_DEFINE_UNQUOTED([unq4], [${baz+set}], [unquoted, test 4])
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AC_DEFINE_UNQUOTED([unq5], ["${baz+`echo "a b"`}"], [unquoted, test 5])
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AC_DEFINE_UNQUOTED([unq5], ["${baz+`echo "a "' b'`}"], [unquoted, test 5])
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AC_DEFINE_UNQUOTED([unq6], [`echo hi`], [unquoted, test 6])
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AC_DEFINE_UNQUOTED([unq7], ['\\"'], [unquoted, test 7])
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AC_PROG_AWK
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@ -943,7 +943,7 @@ X@file@
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#define unq2 $baz
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#define unq3 "bla"
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#define unq4 set
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#define unq5 "a b"
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#define unq5 "a b"
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#define unq6 hi
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#define unq7 '\"'
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]])
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