* 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
* 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
@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
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
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 $file_name in
+(*.c) echo "C source code";;
+esac
+@end example
+
+@noindent
+but the @code{(} in this example is not portable to many older Bourne
+shell implementations. It can be omitted safely.
+
Because of a bug in its @code{fnmatch}, Bash fails to properly
handle backslashes in character classes:
that name in Autoconf code is asking for trouble since it is an M4 quote
character.
-If you need to make multiple checks using @code{test}, combine them with
-the shell operators @samp{&&} and @samp{||} instead of using the
-@code{test} operators @option{-a} and @option{-o}. On System V, the
-precedence of @option{-a} and @option{-o} is wrong relative to the unary
-operators; consequently, Posix does not specify them, so using them
-is nonportable. If you combine @samp{&&} and @samp{||} in the same
-statement, keep in mind that they have equal precedence.
+The @option{-a}, @option{-o}, @samp{(}, and @samp{)} operands are not
+portable and should be avoided. Thus, portable uses of @command{test}
+should never have more than four arguments, and scripts should use shell
+constructs like @samp{&&} and @samp{||} instead. If you combine
+@samp{&&} and @samp{||} in the same statement, keep in mind that they
+have equal precedence, so it is often better to parenthesize even when
+this is redundant. For example:
+
+@smallexample
+# Not portable:
+test "X$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
@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:
@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 $ac_feature in
esac
@end example
-Alas, negated character classes are probably not portable, although no
-shell is known to not support the Posix syntax @samp{[!@dots{}]}
-(when in interactive mode, @command{zsh} is confused by the
-@samp{[!@dots{}]} syntax and looks for an event in its history because of
-@samp{!}). Many shells do not support the alternative syntax
-@samp{[^@dots{}]} (Solaris, Digital Unix, etc.).
-
-One solution can be:
-
-@example
-expr "$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&
- @var{action}
-@end example
-
-@noindent
-or better yet
+If the pattern is a complicated regular expression that cannot be
+expressed as a shell pattern, use something like this instead:
@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
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