10.8 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 (see Limitations of Builtins) and a fallback value is needed. We list these values below.

CDPATH

When this variable is set it specifies a list of directories to search when invoking cd with a relative filename. POSIX 1003.1-2001 says that if a nonempty directory name from CDPATH is used successfully, cd prints the resulting absolute filename. Unfortunately this output can break idioms like `abs=`cd src && pwd`' because abs receives the path twice. Also, many shells do not conform to this part of POSIX; for example, zsh prints the result only if a directory name other than . was chosen from CDPATH.

In practice the shells that have this problem also support unset, so you can work around the problem as follows:

          (unset CDPATH) >/dev/null 2>&1 && unset CDPATH
     

Autoconf-generated scripts automatically unset CDPATH if possible, so you need not worry about this problem in those scripts.

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.

The proper value for IFS (in regular code, not when performing splits) is `<SPC><TAB><RET>'. The first character is especially important, as it is used to join the arguments in `@*'.

LANG

LC_ALL

LC_COLLATE

LC_CTYPE

LC_MESSAGES

LC_MONETARY

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 locale environment variables be set to `C' if the C locale is desired. However, some older, nonstandard systems (notably SCO) break if locale environment variables are set to `C', so when running on these systems Autoconf-generated scripts 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.

LC_ADDRESS

LC_IDENTIFICATION

LC_MEASUREMENT

LC_NAME

LC_PAPER

LC_TELEPHONE

These locale environment variables are GNU extensions. They are treated like their POSIX brethren (LC_COLLATE, etc.) as described above.

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 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 to be `:', 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.

ENV

MAIL

MAILPATH

PS1

PS2

PS4

These variables should not matter for shell scripts, since they are supposed to affect only interactive shells. However, at least one shell (the pre-3.0 uwin ksh) gets confused about whether it is interactive, which means that (for example) a PS1 with a side effect can unexpectedly modify `$?'. To work around this bug, Autoconf-generated scripts do something like this:

          (unset ENV) >/dev/null 2>&1 && unset ENV MAIL MAILPATH
          PS1='$ '
          PS2='> '
          PS4='+ '
     

PWD

POSIX 1003.1-2001 requires that cd and pwd must update the PWD environment variable to point to the logical path to the current directory, but traditional shells do not support this. This can cause confusion if one shell instance maintains PWD but a subsidiary and different shell does not know about PWD and executes cd; in this case PWD will point to the wrong directory. Use ``pwd`' rather than `$PWD'.

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 each time it is 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.