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(autoconf.info)Special Shell Variables

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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 `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'.