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(Shell Substitutions): Document the problem with "$@", the Unix Version 7

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shell, and Zsh.
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Paul Eggert 2002-04-19 19:03:37 +00:00
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@ -8457,13 +8457,16 @@ strings inside double-quoted backquoted expressions (pfew!).
@item $@@
@cindex @samp{"$@@"}
One of the most famous shell-portability issues is related to
@samp{"$@@"}: when there are no positional arguments, it is supposed to
be equivalent to nothing. But some shells, for instance under Digital
Unix 4.0 and 5.0, will then replace it with an empty argument. To be
portable, use @samp{$@{1+"$@@"@}}.
@samp{"$@@"}. When there are no positional arguments, @sc{posix} says
that @samp{"$@@"} is supposed to be equivalent to nothing, but the
original Unix Version 7 Bourne shell treated it as equivalent to
@samp{""} instead, and this behavior survives in later implementations
like Digital Unix 5.0.
But that's not the end of the story. Zsh (3.x and 4.x), when emulating
the Bourne shell, does perform word splitting on @samp{$@{1+"$@@"@}}...
The traditional way to work around this portability problem is to use
@samp{$@{1+"$@@"@}}. Unfortunately this method does not work with
Zsh (3.x and 4.x), which is used on Mac OS X. When emulating
the Bourne shell, Zsh performs word splitting on @samp{$@{1+"$@@"@}}:
@example
zsh $ @kbd{emulate sh}
@ -8477,19 +8480,31 @@ World
@end example
@noindent
It is not clear whether this is a violation of the Bourne shell
standard, nevertheless, in this regard Zsh is different from all the
other shells. Of course Zsh handles @samp{"$@@"} properly, but we can't
use it portably...
Fortunately, there is a workaround which relies on Zsh's ``global
aliases'': let it convert @samp{$@{1+"$@@"@}} into @samp{"$@@"} by
itself:
Zsh handles plain @samp{"$@@"} properly, but we can't use plain
@samp{"$@@"} because of the portability problems mentioned above.
One workaround relies on Zsh's ``global aliases'' to convert
@samp{$@{1+"$@@"@}} into @samp{"$@@"} by itself:
@example
test "$@{ZSH_VERSION+set@}" = set && alias -g '$@{1+"$@@"@}'='"$@@"'
$@{ZSH_VERSION-:@} alias -g '$@{1+"$@@"@}'='"$@@"'
@end example
A more conservative workaround is to avoid @samp{"$@@"} if it is
possible that there may be no positional arguments. For example,
instead of:
@example
cat conftest.c "$@@"
@end example
you can use this instead:
@example
case $# in
0) cat conftest.c;;
*) cat conftest.c "$@@";;
esac
@end example
@item $@{@var{var}:-@var{value}@}
@c Info cannot handle `:' in index entries.