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Special Shell Variables

Some shell variables should not be used, since they can have a deep influence on the behavior of the shell. In order to recover a sane behavior from the shell, some variables should be unset, but @command{unset} is not portable (see section Limitations of Shell Builtins) and a fallback value is needed. We list these values below.

CDPATH

When this variable is set cd is verbose, so idioms such as `abs=`cd $rel && pwd`' break because abs receives the path twice. Setting CDPATH to the empty value is not enough for most shells. A simple colon is enough except for zsh, which prefers a leading dot:

zsh-3.1.6 % mkdir foo && (CDPATH=: cd foo)
/tmp/foo
zsh-3.1.6 % (CDPATH=:. cd foo)
/tmp/foo
zsh-3.1.6 % (CDPATH=.: cd foo)
zsh-3.1.6 %

(of course we could just unset CDPATH, since it also behaves properly if set to the empty string). Life wouldn't be so much fun if @command{bash} and @command{zsh} had the same behavior:

bash-2.02 % (CDPATH=:. cd foo)
bash-2.02 % (CDPATH=.: cd foo)
/tmp/foo

Therefore, a portable solution to neutralize `CDPATH' is

CDPATH=${ZSH_VERSION+.}:

Note that since @command{zsh} supports @command{unset}, you may unset `CDPATH' using `:' as a fallback, see section Limitations of Shell Builtins.

IFS

Don't set the first character of IFS to backslash. Indeed, Bourne shells use the first character (backslash) when joining the components in `"$@"' and some shells then re-interpret (!) the backslash escapes, so you can end up with backspace and other strange characters.

LANG

LC_ALL

LC_TIME

LC_CTYPE

LANGUAGE

LC_COLLATE

LC_NUMERIC

LC_MESSAGES

These must not be set unconditionally because not all systems understand e.g. `LANG=C' (notably SCO). Fixing @env{LC_MESSAGES} prevents Solaris @command{sh} from translating var values in set! Non-C @env{LC_CTYPE} values break the ctype check. Fixing @env{LC_COLLATE} makes scripts more portable in some cases. For example, it causes the regular expression `[a-z]' to match only lower-case letters on ASCII platforms. However, `[a-z]' does not work in general even when @env{LC_COLLATE} is fixed; for example, it does not work for EBCDIC platforms. For maximum portability, you should use regular expressions like `[abcdefghijklmnopqrstuvwxyz]' that list characters explicitly instead of relying on ranges. If one of these variables is set, you should try to unset it, using `C' as a fall back value. see section Limitations of Shell Builtins, builtin @command{unset}, for more details.

NULLCMD

When executing the command `>foo', @command{zsh} executes `$NULLCMD >foo'. The Bourne shell considers NULLCMD is `:', while @command{zsh}, even in Bourne shell compatibility mode, sets NULLCMD to `cat'. If you forgot to set NULLCMD, your script might be suspended waiting for data on its standard input.

status

This variable is an alias to `$?' for zsh (at least 3.1.6), hence read-only. Do not use it.

PATH_SEPARATOR

On DJGPP systems, the PATH_SEPARATOR variable can be set to either `:' or `;' to control the path separator @command{bash} uses to set up certain environment variables (such as PATH). Since this only works inside bash, you want autoconf to detect the regular DOS path separator `;', so it can be safely substituted in files that may not support `;' as path separator. So either unset this variable or set it to `;'.

RANDOM

Many shells provide RANDOM, a variable that returns a different integer when used. Most of the time, its value does not change when it is not used, but on IRIX 6.5 the value changes all the time. This can be observed by using @command{set}.

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