autoconf: Portable Shell
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1 11 Portable Shell Programming
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1 When writing your own checks, there are some shell-script programming
1 techniques you should avoid in order to make your code portable. The
1 Bourne shell and upward-compatible shells like the Korn shell and Bash
1 have evolved over the years, and many features added to the original
1 System7 shell are now supported on all interesting porting targets.
1 However, the following discussion between Russ Allbery and Robert Lipe
1 is worth reading:
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1 Russ Allbery:
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1 The GNU assumption that `/bin/sh' is the one and only shell leads
1 to a permanent deadlock. Vendors don't want to break users'
1 existing shell scripts, and there are some corner cases in the
1 Bourne shell that are not completely compatible with a Posix
1 shell. Thus, vendors who have taken this route will _never_
1 (OK..."never say never") replace the Bourne shell (as `/bin/sh')
1 with a Posix shell.
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1 Robert Lipe:
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1 This is exactly the problem. While most (at least most System
1 V's) do have a Bourne shell that accepts shell functions most
1 vendor `/bin/sh' programs are not the Posix shell.
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1 So while most modern systems do have a shell _somewhere_ that
1 meets the Posix standard, the challenge is to find it.
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11 For this reason, part of the job of M4sh (⇒Programming in
M4sh) is to find such a shell. But to prevent trouble, if you're not
1 using M4sh you should not take advantage of features that were added
1 after Unix version 7, circa 1977 (⇒Systemology); you should not
1 use aliases, negated character classes, or even `unset'. `#' comments,
1 while not in Unix version 7, were retrofitted in the original Bourne
1 shell and can be assumed to be part of the least common denominator.
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1 On the other hand, if you're using M4sh you can assume that the shell
1 has the features that were added in SVR2 (circa 1984), including shell
1 functions, `return', `unset', and I/O redirection for builtins. For
1 more information, refer to `http://www.in-ulm.de/~mascheck/bourne/'.
1 However, some pitfalls have to be avoided for portable use of these
1 constructs; these will be documented in the rest of this chapter. See
DONTPRINTYET 1 in particular ⇒Shell Functions and *noteLimitations of Shell
1DONTPRINTYET 1 in particular ⇒Shell Functions and ⇒Limitations of Shell
Builtins Limitations of Builtins.
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1 Some ancient systems have quite small limits on the length of the
1 `#!' line; for instance, 32 bytes (not including the newline) on SunOS
1 4. However, these ancient systems are no longer of practical concern.
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1 The set of external programs you should run in a `configure' script
11 is fairly small. ⇒Utilities in Makefiles (standards)Utilities in
Makefiles, for the list. This restriction allows users to start out
1 with a fairly small set of programs and build the rest, avoiding too
1 many interdependencies between packages.
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1 Some of these external utilities have a portable subset of features;
1 see ⇒Limitations of Usual Tools.
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1 There are other sources of documentation about shells. The
1 specification for the Posix Shell Command Language
1 (http://www.opengroup.org/susv3/utilities/xcu_chap02.html), though more
1 generous than the restrictive shell subset described above, is fairly
1 portable nowadays. Also please see the Shell FAQs
1 (http://www.faqs.org/faqs/unix-faq/shell/).
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