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 `unset' is
not portable (Note:Limitations of 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 $rel && pwd`' break because `abs' receives the path twice.
Setting `CDPATH' to the empty value is not enough for most shells.
A simple path separator is enough except for `zsh', which prefers a
leading dot:
zsh-3.1.6$ 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 `bash' and `zsh' had the same
behavior:
bash-2.02$ mkdir foo && (CDPATH=: cd foo)
bash-2.02$ (CDPATH=:. cd foo)
bash-2.02$ (CDPATH=.: cd foo)
/tmp/foo
Of course, even better style would be to use `PATH_SEPARATOR'
instead of a `:'. Therefore, a portable solution to neutralize
`CDPATH' is
CDPATH=${ZSH_VERSION+.}$PATH_SEPARATOR
Note that since `zsh' supports `unset', you may unset `CDPATH'
using `PATH_SEPARATOR' as a fallback, see Note:Limitations of
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 `"$@"' and some shells then re-interpret (!) the
backslash escapes, so you can end up with backspace and other
strange characters.
`LANG'
`LC_ALL'
`LC_COLLATE'
`LC_CTYPE'
`LC_MESSAGES'
`LC_NUMERIC'
`LC_TIME'
Autoconf-generated scripts normally set all these variables to `C'
because so much configuration code assumes the C locale and POSIX
requires that `LC_ALL' be set to `C' if the C locale is desired.
However, some older, nonstandard systems (notably SCO) break if
`LC_ALL' is set to `C', so when running on these systems
Autoconf-generated scripts first try to unset the variables
instead.
`LANGUAGE'
`LANGUAGE' is not specified by POSIX, but it is a GNU extension
that overrides `LC_ALL' in some cases, so Autoconf-generated
scripts set it too.
`LINENO'
Most modern shells provide the current line number in `LINENO'.
Its value is the line number of the beginning of the current
command. Autoconf attempts to execute `configure' with a modern
shell. If no such shell is available, it attempts to implement
`LINENO' with a Sed prepass that replaces the each instance of the
string `$LINENO' (not followed by an alphanumeric character) with
the line's number.
You should not rely on `LINENO' within `eval', as the behavior
differs in practice. Also, the possibility of the Sed prepass
means that you should not rely on `$LINENO' when quoted, when in
here-documents, or when in long commands that cross line
boundaries. Subshells should be OK, though. In the following
example, lines 1, 6, and 9 are portable, but the other instances of
`LINENO' are not:
$ cat lineno
echo 1. $LINENO
cat <<EOF
3. $LINENO
4. $LINENO
EOF
( echo 6. $LINENO )
eval 'echo 7. $LINENO'
echo 8. '$LINENO'
echo 9. $LINENO '
10.' $LINENO
$ bash-2.05 lineno
1. 1
3. 2
4. 2
6. 6
7. 1
8. $LINENO
9. 9
10. 9
$ zsh-3.0.6 lineno
1. 1
3. 2
4. 2
6. 6
7. 7
8. $LINENO
9. 9
10. 9
$ pdksh-5.2.14 lineno
1. 1
3. 2
4. 2
6. 6
7. 0
8. $LINENO
9. 9
10. 9
$ sed '=' <lineno |
> sed '
> N
> s,$,-,
> : loop
> s,^\([0-9]*\)\(.*\)[$]LINENO\([^a-zA-Z0-9_]\),\1\2\1\3,
> t loop
> s,-$,,
> s,^[0-9]*\n,,
> ' |
> sh
1. 1
3. 3
4. 4
6. 6
7. 7
8. 8
9. 9
10. 10
`NULLCMD'
When executing the command `>foo', `zsh' executes `$NULLCMD >foo'.
The Bourne shell considers `NULLCMD' is `:', while `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'
If it is not set, `configure' will detect the appropriate path
separator for the build system and set the `PATH_SEPARATOR' output
variable accordingly.
On DJGPP systems, the `PATH_SEPARATOR' environment variable can be
set to either `:' or `;' to control the path separator `bash' uses
to set up certain environment variables (such as `PATH'). Since
this only works inside `bash', you want `configure' to detect the
regular DOS path separator (`;'), so it can be safely substituted
in files that may not support `;' as path separator. So it is
recommended to 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 `set'.