@cindex parentheses, balancing
@cindex unbalanced parentheses, managing
-One of the pitfalls of portable shell programming is that @command{case}
-statements require unbalanced parentheses (@pxref{case, , Limitations of
-Shell Builtins}). With syntax highlighting
+One of the pitfalls of portable shell programming is that
+if you intend your script to run with obsolescent shells,
+@command{case} statements require unbalanced parentheses.
+@xref{case, , Limitations of Shell Builtins}.
+With syntax highlighting
editors, the presence of unbalanced @samp{)} can interfere with editors
that perform syntax highlighting of macro contents based on finding the
matching @samp{(}. Another concern is how much editing must be done
without proper quoting, each with some benefits and some drawbacks.
@itemize @w{}
+@item Use left parenthesis before pattern
+@example
+AC_DEFUN([my_case],
+[case $file_name in
+ (*.c) echo "C source code";;
+esac])
+@end example
+@noindent
+This is simple and provides balanced parentheses. Although this is not
+portable to obsolescent shells (notably Solaris 10 @command{/bin/sh}),
+platforms with these shells invariably have a more-modern shell
+available somewhere so this approach typically suffices nowadays.
+
@item Creative literal shell comment
@example
AC_DEFUN([my_case],
Expand into a shell @samp{case} statement, where @var{word} is matched
against one or more patterns. @var{if-matched} is run if the
corresponding pattern matched @var{word}, else @var{default} is run.
-Avoids several portability issues (@pxref{case, , Limitations of Shell
-Builtins}).
+@xref{Prerequisite Macros} for why
+this macro should be used instead of plain @samp{case} in code
+outside of an @code{AC_DEFUN} macro, when the contents of the
+@samp{case} use @code{AC_REQUIRE} directly or indirectly.
+@xref{case, , Limitations of Shell Builtins},
+for how this macro avoids some portability issues.
+@xref{Balancing Parentheses}
+for how this macro lets you write code with balanced parentheses
+even if your code must run on obsolescent shells.
@end defmac
@c Deprecated, to be replaced by a better API
argument contains the nonblank characters @code{[]} which expand to
nothing. This restriction on @var{run-if-false} also applies to other
macros with ``if-false'' arguments denoting shell commands.
+
+This macro should be used instead of plain @samp{if} in code
+outside of an @code{AC_DEFUN} macro, when the contents of the @samp{if}
+use @code{AC_REQUIRE} directly or indirectly (@pxref{Prerequisite Macros}).
@end defmac
@defmac AS_MKDIR_P (@var{file-name})
@example
@group
AC_DEFUN([TRAVOLTA],
-[test "$body_temperature_in_celsius" -gt "38" &&
+[test "$body_temperature_in_celsius" -gt 38 &&
dance_floor=occupied])
AC_DEFUN([NEWTON_JOHN],
[test "x$hair_style" = xcurly &&
@group
AC_DEFUN([RESERVE_DANCE_FLOOR],
-[if date | grep '^Sat.*pm' >/dev/null 2>&1; then
+[if test "x`date +%A`" = xSaturday; then
AC_REQUIRE([TRAVOLTA])
AC_REQUIRE([NEWTON_JOHN])
fi])
@end example
@noindent
-does not leave you with a better chance to meet a kindred soul at
-other times than Saturday night since it expands into:
+does not leave you with a better chance to meet a kindred soul on
+days other than Saturday, since the call to @code{RESERVE_DANCE_FLOOR}
+expands to:
@example
@group
-test "$body_temperature_in_Celsius" -gt "38" &&
+test "$body_temperature_in_Celsius" -gt 38 &&
dance_floor=occupied
test "x$hair_style" = xcurly &&
dance_floor=occupied
fi
-if date | grep '^Sat.*pm' >/dev/null 2>&1; then
+if test "x`date +%A`" = xSaturday; then
fi
in C
@end example
-The helper macros @code{AS_IF} and @code{AS_CASE} may be used to
-enforce expansion of required macros outside of shell conditional
-constructs. You are furthermore encouraged, although not required, to
+You can use the helper macros @code{AS_IF} and @code{AS_CASE} in
+top-level code to enforce expansion of required macros outside of shell
+conditional constructs; these helpers are not needed in the bodies of
+macros defined by @code{AC_DEFUN}.
+You are furthermore encouraged, although not required, to
put all @code{AC_REQUIRE} calls
at the beginning of a macro. You can use @code{dnl} to avoid the empty
lines they leave.
release. It is usually called @command{ksh}, but is called @command{sh}
on some hosts if you set your path appropriately.
-Solaris systems have three variants:
+On Solaris 11, @command{/bin/sh} and @command{/usr/bin/ksh} are both
+@samp{ksh93}. On Solaris 10 and earlier, @command{/bin/sh} is a
+pre-Posix Bourne shell and the Korn shell is found elsewhere:
@prindex @command{/usr/bin/ksh} on Solaris
-@command{/usr/bin/ksh} is @samp{ksh88}; it is
-standard on Solaris 2.0 and later.
+@command{/usr/bin/ksh} is @samp{ksh88} on Solaris 2.0 through 10,
@prindex @command{/usr/xpg4/bin/sh} on Solaris
@command{/usr/xpg4/bin/sh} is a Posix-compliant variant of
-@samp{ksh88}; it is standard on Solaris 9 and later.
+@samp{ksh88} on Solaris 9 and later,
@prindex @command{/usr/dt/bin/dtksh} on Solaris
-@command{/usr/dt/bin/dtksh} is @samp{ksh93}.
+and @command{/usr/dt/bin/dtksh} is @samp{ksh93}.
Variants that are not standard may be parts of optional
packages. There is no extra charge for these packages, but they are
not part of a minimal OS install and therefore some installations may
@end example
Don't rely on duplicating a closed file descriptor to cause an
-error. With Solaris @command{/bin/sh}, failed duplication is silently
+error. With Solaris 10 @command{/bin/sh}, failed duplication is silently
ignored, which can cause unintended leaks to the original file
descriptor. In this example, observe the leak to standard output:
@noindent
Make sure you quote the brackets if appropriate and keep the backslash as
-first character (@pxref{case, , Limitations of Shell Builtins}).
+first character. @xref{case, , Limitations of Shell Builtins}.
Also, because the colon is used as part of a drivespec, these systems don't
use it as path separator. When creating or accessing paths, you can use the
and in fact it is even @emph{more} portable: in the first case of the
first attempt, the computation of @code{top_srcdir} is not portable,
since not all shells properly understand @code{"`@dots{}"@dots{}"@dots{}`"},
-for example Solaris 10 ksh:
+for example Solaris 10 @command{ksh}:
@example
$ @kbd{foo="`echo " bar" | sed 's, ,,'`"}
@end example
As with @samp{+} and @samp{-}, @var{value} must be a single shell word,
-otherwise some shells, such as Solaris @command{/bin/sh} or on Digital
+otherwise some shells, such as Solaris 10 @command{/bin/sh} or on Digital
Unix V 5.0, die because of a ``bad substitution''. Meanwhile, Posix
requires that with @samp{=}, quote removal happens prior to the
assignment, and the expansion be the final contents of @var{var} without
@item $@{@var{var}=@var{value}@}
@cindex @code{$@{@var{var}=@var{literal}@}}
-Solaris @command{/bin/sh} has a frightening bug in its handling of
+Solaris 10 @command{/bin/sh} has a frightening bug in its handling of
literal assignments. Imagine you need set a variable to a string containing
-@samp{@}}. This @samp{@}} character confuses Solaris @command{/bin/sh}
+@samp{@}}. This @samp{@}} character confuses Solaris 10 @command{/bin/sh}
when the affected variable was already set. This bug can be exercised
by running:
@end example
@noindent
-@dots{}but beware of the @samp{@}} bug from Solaris (see above). For safety,
-use:
+@dots{}but beware of the @samp{@}} bug from Solaris 10 (see above).
+For safety, use:
@example
test $@{var+y@} || var=@var{@{value@}}
When setting several variables in a row, be aware that the order of the
evaluation is undefined. For instance @samp{foo=1 foo=2; echo $foo}
-gives @samp{1} with Solaris @command{/bin/sh}, but @samp{2} with Bash.
+gives @samp{1} with Solaris 10 @command{/bin/sh}, but @samp{2} with Bash.
You must use
@samp{;} to enforce the order: @samp{foo=1; foo=2; echo $foo}.
IRIX sh sets @samp{$0} to the function name.
It is not portable to pass temporary environment variables to shell
-functions. Solaris @command{/bin/sh} does not see the variable.
+functions. Solaris 10 @command{/bin/sh} does not see the variable.
Meanwhile, not all shells follow the Posix rule that the assignment must
affect the current environment in the same manner as special built-ins.
@prindex @command{!}
The Unix version 7 shell did not support
negating the exit status of commands with @command{!}, and this feature
-is still absent from some shells (e.g., Solaris @command{/bin/sh}).
+is still absent from some shells (e.g., Solaris 10 @command{/bin/sh}).
Other shells, such as FreeBSD @command{/bin/sh} or @command{ash}, have
bugs when using @command{!}:
@end example
@noindent
-but the @code{(} in this example is not portable to many Bourne
+but the @code{(} in this example is not portable to a few obsolescent Bourne
shell implementations, which is a pity for those of us using tools that
-rely on balanced parentheses. For instance, with Solaris
+rely on balanced parentheses. For instance, with Solaris 10
@command{/bin/sh}:
@example
Do not use backslashes in the arguments, as there is no consensus on
their handling. For @samp{echo '\n' | wc -l}, the @command{sh} of
-Solaris outputs 2, but Bash and Zsh (in @command{sh} emulation mode) output 1.
+Solaris 10 outputs 2,
+but Bash and Zsh (in @command{sh} emulation mode) output 1.
The problem is truly @command{echo}: all the shells
understand @samp{'\n'} as the string composed of a backslash and an
@samp{n}. Within a command substitution, @samp{echo 'string\c'} will
nonzero status, the trap also exits with nonzero status so that the
invoker can tell that an error occurred.
-Unfortunately, in some shells, such as Solaris @command{/bin/sh}, an exit
+Unfortunately, in some shells, such as Solaris 10 @command{/bin/sh}, an exit
trap ignores the @code{exit} command's argument. In these shells, a trap
cannot determine whether it was invoked by plain @code{exit} or by
@code{exit 1}. Instead of calling @code{exit} directly, use the
received by the shell when it is launched should be imported as a shell
variable marked as exported.
-Alas, many shells, such as Solaris @command{/bin/sh},
+Alas, many shells, such as Solaris 10 @command{/bin/sh},
IRIX 6.3, IRIX 5.2,
AIX 4.1.5, and Digital Unix 4.0, forget to
@command{export} the environment variables they receive. As a result,
item @samp{$@@}, for more.
Posix requires support for a @command{for} loop with no list after
-@code{in}. However, Solaris @command{/bin/sh} treats that as a syntax
+@code{in}. However, Solaris 10 @command{/bin/sh} treats that as a syntax
error. It is possible to work around this by providing any shell word
that expands to nothing, or by ignoring an obvious sentinel.
b
@end example
-In Solaris @command{/bin/sh}, when the list of arguments of a
+In Solaris 10 @command{/bin/sh}, when the list of arguments of a
@command{for} loop starts with @emph{unquoted} tokens looking like
variable assignments, the loop is not executed on those tokens:
@item @command{read}
@c -----------------
@prindex @command{read}
-No options are portable, not even support @option{-r} (Solaris
+No options are portable, not even support @option{-r} (Solaris 10
@command{/bin/sh} for example). Tru64/OSF 5.1 @command{sh} treats
@command{read} as a special built-in, so it may exit if input is
redirected from a non-existent or unreadable file.
users not to use @samp{sh -e}.
When @samp{set -e} is in effect, a failed command substitution in
-Solaris @command{/bin/sh} cannot be ignored, even with @samp{||}.
+Solaris 10 @command{/bin/sh} cannot be ignored, even with @samp{||}.
@example
$ @kbd{/bin/sh -c 'set -e; foo=`false` || echo foo; echo bar'}
Posix also 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, AIX 3.2, UNICOS 10.0.0.6,
+shells (such as Solaris 10, AIX 3.2, UNICOS 10.0.0.6,
Digital Unix 4, etc.)@: get confused if
@var{string} looks like an operator:
Posix says that @samp{trap - 1 2 13 15} resets the traps for the
specified signals to their default values, but many common shells (e.g.,
-Solaris @command{/bin/sh}) misinterpret this and attempt to execute a
+Solaris 10 @command{/bin/sh}) misinterpret this and attempt to execute a
``command'' named @command{-} when the specified conditions arise.
Posix 2008 also added a requirement to support @samp{trap 1 2 13 15} to
reset traps, as this is supported by a larger set of shells, but there
@command{exit} itself?''
Bash considers @command{exit} to be the last command, while Zsh and
-Solaris @command{/bin/sh} consider that when the trap is run it is
+Solaris 10 @command{/bin/sh} consider that when the trap is run it is
@emph{still} in the @command{exit}, hence it is the previous exit status
that the trap receives:
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