* Structures:: Structures or members that might be missing
* Types:: Types that might be missing
* Compilers and Preprocessors:: Checking for compiling programs
-* C Compiler Characteristics::
-* Fortran 77 Compiler Characteristics::
+* C Compiler:: Checking its characteristics
+* Fortran 77 Compiler:: Checking its characteristics
* System Services:: Operating system services
* UNIX Variants:: Special kludges for specific UNIX variants
Portable Shell Programming
* Shellology:: A zoology of shells
-* Shell Substitutions:: Variable expansions...
+* File Descriptors:: FDs and redirections
+* Shell Substitutions:: Variable and command expansions
* Assignments:: Varying side effects of assignments
* Special Shell Variables:: Variables you should not change
* Limitations of Builtins:: Portable use of not so portable /bin/sh
* Limitations of Usual Tools:: Portable use of portable tools
+* Limitations of Make:: Portable Makefiles
Results of Tests
* Structures:: Structures or members that might be missing
* Types:: Types that might be missing
* Compilers and Preprocessors:: Checking for compiling programs
-* C Compiler Characteristics::
-* Fortran 77 Compiler Characteristics::
+* C Compiler:: Checking its characteristics
+* Fortran 77 Compiler:: Checking its characteristics
* System Services:: Operating system services
* UNIX Variants:: Special kludges for specific UNIX variants
@end menu
@code{AC_CHECK_TYPE}, see @ref{Obsolete Macros}.
-@node Compilers and Preprocessors, C Compiler Characteristics, Types, Existing Tests
+@node Compilers and Preprocessors, C Compiler, Types, Existing Tests
@section Compilers and Preprocessors
@ovindex EXEEXT
@end defmac
-@node C Compiler Characteristics, Fortran 77 Compiler Characteristics, Compilers and Preprocessors, Existing Tests
+@node C Compiler, Fortran 77 Compiler, Compilers and Preprocessors, Existing Tests
@section C Compiler Characteristics
The following macros check for C compiler or machine architecture
-@node Fortran 77 Compiler Characteristics, System Services, C Compiler Characteristics, Existing Tests
+@node Fortran 77 Compiler, System Services, C Compiler, Existing Tests
@section Fortran 77 Compiler Characteristics
The following macros check for Fortran 77 compiler characteristics. To
language other than C/C++.
@end defmac
-@node System Services, UNIX Variants, Fortran 77 Compiler Characteristics, Existing Tests
+@node System Services, UNIX Variants, Fortran 77 Compiler, Existing Tests
@section System Services
The following macros check for operating system services or capabilities.
@menu
* Shellology:: A zoology of shells
-* Shell Substitutions:: Variable expansions...
+* File Descriptors:: FDs and redirections
+* Shell Substitutions:: Variable and command expansions
* Assignments:: Varying side effects of assignments
* Special Shell Variables:: Variables you should not change
* Limitations of Builtins:: Portable use of not so portable /bin/sh
* Limitations of Usual Tools:: Portable use of portable tools
+* Limitations of Make:: Portable Makefiles
@end menu
-@node Shellology, Shell Substitutions, Portable Shell, Portable Shell
+@node Shellology, File Descriptors, Portable Shell, Portable Shell
@subsection Shellology
There are several families of shells, most prominently the Bourne
Below we describe some of the members of the Bourne shell family.
@table @asis
-@item @command{ash}
-@cindex @command{ash}
+@item Ash
+@cindex Ash
@command{ash} is often used on @sc{gnu}/Linux and @sc{bsd} systems as a
-light-weight Bourne-compatible shell. @command{ash} version 0.2 has
-some bugs that are fixed in the 0.3.x series, but portable shell scripts
-should workaround them, since version 0.2 is still shipped with many
-@sc{gnu}/Linux distributions.
+light-weight Bourne-compatible shell. Ash 0.2 has some bugs that are
+fixed in the 0.3.x series, but portable shell scripts should workaround
+them, since version 0.2 is still shipped with many @sc{gnu}/Linux
+distributions.
-To be compatible with @command{ash} 0.2
+To be compatible with Ash 0.2:
@itemize @bullet
@item
@end example
@item
-beware that @command{exit} inside command substitution causes the
-current shell to exit as well. Use parentheses to prevent @command{ash}
-from exiting:
-
-@example
-(`exit 1`) || echo "All right"
-`exit 1` || echo "ash won't print it"
-@end example
+beware that single builtin substitutions are not performed by a sub
+shell, hence their effect applies to the current shell! @xref{Shell
+Substitutions}, item ``Command Substitution''.
@end itemize
-@item @command{bash}
-@cindex @command{bash}
+@item Bash
+@cindex Bash
To detect whether you are running @command{bash}, test if
@code{BASH_VERSION} is set. To disable its extensions and require
@sc{posix} compatibility, run @samp{set -o posix}. @xref{Bash POSIX
There is no extra charge for this package, but it is also not part of a
minimal OS install and therefore some folks may not have it.
-@item @command{zsh}
-@cindex @command{zsh}
+@item Zsh
+@cindex Zsh
To detect whether you are running @command{zsh}, test if
@code{ZSH_VERSION} is set. By default @command{zsh} is @emph{not}
compatible with the Bourne shell: you have to run @samp{emulate sh} and
@sc{posix} standard, the challenge is to find it.
@end quotation
+@node File Descriptors, Shell Substitutions, Shellology, Portable Shell
+@subsection File Descriptors
+
+Some file descriptors shall not be used, since some systems, admittedly
+arcane, use them for special purpose:
+
+@table @asis
+@item 3
+some systems may open it to @samp{/dev/tty}.
-@node Shell Substitutions, Assignments, Shellology, Portable Shell
+@item 4
+used on the Kubota Titan.
+@end table
+
+Don't redirect several times the same file descriptor, as you are doomed
+to failure under Ultrix.
+
+@example
+ULTRIX V4.4 (Rev. 69) System #31: Thu Aug 10 19:42:23 GMT 1995
+UWS V4.4 (Rev. 11)
+$ eval 'echo matter >fullness' >void
+illegal io
+$ eval '(echo matter >fullness)' >void
+illegal io
+$ (eval '(echo matter >fullness)') >void
+Ambiguous output redirect.
+@end example
+
+@noindent
+In each case the expected result is of course @file{fullness} containing
+@samp{matter} and @file{void} being empty.
+
+@node Shell Substitutions, Assignments, File Descriptors, Portable Shell
@subsection Shell Substitutions
Contrary to a persistent urban legend, the Bourne shell does not
-systematically split variables and backquoted expressions, in
-particular, the following code:
+systematically split variables and backquoted expressions, in particular
+on the right-hand side of assignments, and the argument of @code{case}.
+For instance the following code:
@example
case "$given_srcdir" in
@end example
@noindent
-is more readable with the right-hand side of the assignments, and the
-argument of @code{case} left without quotes:
+is more readable written as:
@example
case $given_srcdir in
@cindex @samp{"$@@"}
One of the most famous shell portability issues is related to
@samp{"$@@"}: when there are no positional argument, it is supposed to
-be equivalent to nothing. But some shell, for instance under Digital
+be equivalent to nothing. But some shells, for instance under Digital
Unix 4.0 and 5.0, will then replace it with an empty argument. To be
portable, use @samp{$@{1+"$@@"@}}.
@end example
+@item `@var{commands}`
+@cindex `@var{commands}`
+@cindex Command Substitution
+While in general it makes no sense, do not substitute a single builtin
+with side effects as Ash 0.2, trying to optimize, does not fork a sub
+shell to perform the command.
+
+For instance if you wanted to check that @command{cd} is silent, do not
+use @samp{test -z "`cd /tmp`"} because...
+
+@example
+$ pwd
+/tmp
+$ test -n "`cd /`" && pwd
+/
+@end example
+
+@noindent
+The result of @samp{foo=`exit 1`} is left as an exercise to the reader.
+
+
@item $(@var{commands})
@cindex $(@var{commands})
This construct is meant to replace @samp{`@var{commands}`}; they can be
@c -----------------
@cindex @command{exit}
The default value of @command{exit} is supposed to be @code{$?},
-unfortunately some shell, such as the @sc{djgpp} port of Bash 2.04, just
+unfortunately some shells, such as the @sc{djgpp} port of Bash 2.04, just
perform @samp{exit 0}.
@example
@end example
@item @command{if}
+@c ---------------
@cindex @command{if}
Using @samp{!} is not portable. Instead of
fi
@end example
+There are shells which do not reset the exit status out of an
+@command{if}:
+
+@example
+$ if (exit 42); then true; fi; echo $?
+42
+@end example
+
+@noindent
+while a proper shell should have printed @samp{0}. This is especially
+bad in Makefiles since it produces false failures. This is why properly
+written Makefiles, such as Automake's, have such hairy constructs:
+
+@example
+if test -f "$file"; then
+ install "$file" "$dest"
+else
+ :
+fi
+@end example
+
+
@item @command{set}
@c ----------------
@cindex @command{set}
@samp{test ! -r foo || exit 1}.
@item @command{test} (files)
+@c -------------------------
To enable @code{configure} scripts to support cross-compilation, they
shouldn't do anything that tests features of the build system instead of
the host system. But occasionally you may find it necessary to check
have it.
@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"}).
the case of environment variables.
@end table
-@node Limitations of Usual Tools, , Limitations of Builtins, Portable Shell
+@node Limitations of Usual Tools, Limitations of Make, Limitations of Builtins, Portable Shell
@subsection Limitations of Usual Tools
The small set of tools you can expect to find on any machine can still
@end example
@end table
+@node Limitations of Make, , Limitations of Usual Tools, Portable Shell
+@subsection Limitations of Make
+
+Make itself suffers a great number of limitations, only a few of which
+being listed here. First of all, remember that since commands are
+executed by the shell, all its weaknesses are inherited...
+
+@table @asis
+@item @code{VPATH}
+@cindex @code{VPATH}
+Don't use it! For instance any assignment to @code{VPATH} causes Sun
+@command{make} to only execute the first set of double-colon rules.
+@end table
+
@node Multiple Cases, Language Choice, Portable Shell, Writing Tests
@section Multiple Cases
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