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* doc/autoconf.texi (Shellology): Rework treatment of the 'test'

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command and case statements to make it a bit clearer and describe
more pitfalls.
This commit is contained in:
Paul Eggert 2007-03-26 20:19:23 +00:00
parent 83c9a63e19
commit b0fd59a2a7
2 changed files with 75 additions and 55 deletions
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@ -1,3 +1,9 @@
2007-03-26 Paul Eggert <eggert@cs.ucla.edu>
* doc/autoconf.texi (Shellology): Rework treatment of the 'test'
command and case statements to make it a bit clearer and describe
more pitfalls.
2007-03-23 Paul Eggert <eggert@cs.ucla.edu>
* doc/autoconf.texi (C Compiler): Mention that AC_PROG_CC_C99 also

124
doc/autoconf.texi
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@ -473,6 +473,7 @@ Portable Shell Programming
* Here-Documents:: Quirks and tricks
* File Descriptors:: FDs and redirections
* File System Conventions:: File names
* Shell Pattern Matching:: Pattern matching
* Shell Substitutions:: Variable and command expansions
* Assignments:: Varying side effects of assignments
* Parentheses:: Parentheses in shell scripts
@ -10966,6 +10967,7 @@ subset described above, is fairly portable nowadays. Also please see
* Here-Documents:: Quirks and tricks
* File Descriptors:: FDs and redirections
* File System Conventions:: File names
* Shell Pattern Matching:: Pattern matching
* Shell Substitutions:: Variable and command expansions
* Assignments:: Varying side effects of assignments
* Parentheses:: Parentheses in shell scripts
@ -11503,6 +11505,18 @@ File names are case insensitive, so even names like
@end table
@node Shell Pattern Matching
@section Shell Pattern Matching
@cindex Shell pattern matching
Nowadays portable patterns can use negated character classes like
@samp{[!-aeiou]}. The older syntax @samp{[^-aeiou]} is supported by
some shells but not others; hence portable scripts should never use
@samp{^} as the first character of a bracket pattern.
Outside the C locale, patterns like @samp{[a-z]} are problematic since
they may match characters that are not lower-case letters.
@node Shell Substitutions
@section Shell Substitutions
@cindex Shell substitutions
@ -11931,19 +11945,6 @@ To work around this problem, insert a space between the two opening
parentheses. There is a similar problem and workaround with
@samp{$((}; see @ref{Shell Substitutions}.
Posix requires support for @code{case} patterns with opening
parentheses like this:
@example
case $file_name in
(*.c) echo "C source code";;
esac
@end example
@noindent
but the @code{(} in this example is not portable to many older Bourne
shell implementations. It can be omitted safely.
@node Slashes
@section Slashes in Shell Scripts
@cindex Shell slashes
@ -12331,6 +12332,19 @@ You don't need to quote the argument; no splitting is performed.
You don't need the final @samp{;;}, but you should use it.
Posix requires support for @code{case} patterns with opening
parentheses like this:
@example
case $file_name in
(*.c) echo "C source code";;
esac
@end example
@noindent
but the @code{(} in this example is not portable to many older Bourne
shell implementations. It can be omitted safely.
Because of a bug in its @code{fnmatch}, Bash fails to properly
handle backslashes in character classes:
@ -12809,13 +12823,27 @@ tests. It is often invoked by the alternate name @samp{[}, but using
that name in Autoconf code is asking for trouble since it is an M4 quote
character.
If you need to make multiple checks using @code{test}, combine them with
the shell operators @samp{&&} and @samp{||} instead of using the
@code{test} operators @option{-a} and @option{-o}. On System V, the
precedence of @option{-a} and @option{-o} is wrong relative to the unary
operators; consequently, Posix does not specify them, so using them
is nonportable. If you combine @samp{&&} and @samp{||} in the same
statement, keep in mind that they have equal precedence.
The @option{-a}, @option{-o}, @samp{(}, and @samp{)} operands are not
portable and should be avoided. Thus, portable uses of @command{test}
should never have more than four arguments, and scripts should use shell
constructs like @samp{&&} and @samp{||} instead. If you combine
@samp{&&} and @samp{||} in the same statement, keep in mind that they
have equal precedence, so it is often better to parenthesize even when
this is redundant. For example:
@smallexample
# Not portable:
test "X$a" = "X$b" -a \
'(' "X$c" != "X$d" -o "X$e" = "X$f" ')'
# Portable:
test "X$a" = "X$b" &&
@{ test "X$c" != "X$d" || test "X$e" = "X$f"; @}
@end smallexample
@command{test} does not process options like most other commands do; for
example, it does not recognize the @option{--} argument as marking the
end of options.
It is safe to use @samp{!} as a @command{test} operator. For example,
@samp{if test ! -d foo; @dots{}} is portable even though @samp{if ! test
@ -12837,23 +12865,32 @@ Posix 1003.1-2001, but older shells like Solaris 8
@item @command{test} (strings)
@c ---------------------------
Avoid @samp{test "@var{string}"}, in particular if @var{string} might
start with a dash, since @code{test} might interpret its argument as an
option (e.g., @samp{@var{string} = "-n"}).
Posix says that @samp{test "@var{string}"} succeeds if @var{string} is
not null, but this usage is not portable to traditional platforms like
Solaris 10 @command{/bin/sh}, which mishandle strings like @samp{!} and
@samp{-n}.
Contrary to a common belief, @samp{test -n @var{string}} and
@samp{test -z @var{string}} @strong{are} portable. Nevertheless many
Posix 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, @acronym{AIX} 3.2, @sc{unicos} 10.0.0.6,
Digital Unix 4, etc.)@: have bizarre precedence and may be confused if
Digital Unix 4, etc.)@: get confused if
@var{string} looks like an operator:
@example
$ @kbd{test -n =}
test: argument expected
$ @kbd{test ! -n}
test: argument expected
@end example
If there are risks, use @samp{test "x@var{string}" = x} or @samp{test
"x@var{string}" != x} instead.
Similarly, Posix says that @samp{test "@var{string1}" = "@var{string2"}}
and @samp{test "@var{string1}" != "@var{string2"}} work for any pairs of
strings, but in practice this is not true for troublesome strings that
look like operators or parentheses, or that begin with @samp{-}.
It is best to protect such strings with a leading @samp{X}, e.g.,
@samp{test "X@var{string}" != X} rather than @samp{test -n
"@var{string}"} or @samp{test ! "@var{string}"}.
It is common to find variations of the following idiom:
@ -12864,16 +12901,7 @@ test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" &&
@noindent
to take an action when a token matches a given pattern. Such constructs
should always be avoided by using:
@example
echo "$ac_feature" | grep '[^-a-zA-Z0-9_]' >/dev/null 2>&1 &&
@var{action}
@end example
@noindent
Use @code{case} where possible since it is faster, being a shell builtin:
should be avoided by using:
@example
case $ac_feature in
@ -12881,25 +12909,11 @@ case $ac_feature in
esac
@end example
Alas, negated character classes are probably not portable, although no
shell is known to not support the Posix syntax @samp{[!@dots{}]}
(when in interactive mode, @command{zsh} is confused by the
@samp{[!@dots{}]} syntax and looks for an event in its history because of
@samp{!}). Many shells do not support the alternative syntax
@samp{[^@dots{}]} (Solaris, Digital Unix, etc.).
One solution can be:
If the pattern is a complicated regular expression that cannot be
expressed as a shell pattern, use something like this instead:
@example
expr "$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&
@var{action}
@end example
@noindent
or better yet
@example
expr "X$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&
expr "X$ac_feature" : 'X.*[^-a-zA-Z0-9_]' >/dev/null &&
@var{action}
@end example