Next: , Previous: Slashes, Up: Portable Shell

11.10 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; M4sh takes care of this and provides fallback values, whenever needed, to cater for a very old /bin/sh that does not support unset. (see Portable Shell Programming).

As a general rule, shell variable names containing a lower-case letter are safe; you can define and use these variables without worrying about their effect on the underlying system, and without worrying about whether the shell changes them unexpectedly. (The exception is the shell variable status, as described below.)

Here is a list of names that are known to cause trouble. This list is not exhaustive, but you should be safe if you avoid the name status and names containing only upper-case letters and underscores.

Not all shells correctly reset ‘$?’ after conditionals (see Limitations of Shell Builtins). Not all shells manage ‘$?’ correctly in shell functions (see Shell Functions) or in traps (see Limitations of Shell Builtins). Not all shells reset ‘$?’ to zero after an empty command.
          $ bash -c 'false; $empty; echo $?'
          $ zsh -c 'false; $empty; echo $?'

Many shells reserve ‘$_’ for various purposes, e.g., the name of the last command executed.
In Tru64, if BIN_SH is set to xpg4, subsidiary invocations of the standard shell conform to Posix.
When this variable is set it specifies a list of directories to search when invoking cd with a relative file name that did not start with ‘./’ or ‘../’. Posix 1003.1-2001 says that if a nonempty directory name from CDPATH is used successfully, cd prints the resulting absolute file name. Unfortunately this output can break idioms like ‘abs=`cd src && pwd`’ because abs receives the name 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

You can also avoid output by ensuring that your directory name is absolute or anchored at ‘./’, as in ‘abs=`cd ./src && pwd`’.

Configure scripts use M4sh, which automatically unsets CDPATH if possible, so you need not worry about this problem in those scripts.

When this variable is set, some implementations of tools like ls attempt to add color to their output via terminal escape sequences, even when the output is not directed to a terminal, and can thus cause spurious failures in scripts. Configure scripts use M4sh, which automatically unsets this variable.
In the MKS shell, case statements and file name generation are case-insensitive unless DUALCASE is nonzero. Autoconf-generated scripts export this variable when they start up.
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 Korn shell) gets confused about whether it is interactive, which means that (for example) a PS1 with a side effect can unexpectedly modify ‘$?’. To work around this bug, M4sh scripts (including configure scripts) do something like this:
          (unset ENV) >/dev/null 2>&1 && unset ENV MAIL MAILPATH
          PS1='$ '
          PS2='> '
          PS4='+ '

(actually, there is some complication due to bugs in unset; see see Limitations of Shell Builtins).

The Korn shell uses FPATH to find shell functions, so avoid FPATH in portable scripts. FPATH is consulted after PATH, but you still need to be wary of tests that use PATH to find whether a command exists, since they might report the wrong result if FPATH is also set.
When this variable is set, some implementations of grep honor these options, even if the options include direction to enable colored output via terminal escape sequences, and the result can cause spurious failures when the output is not directed to a terminal. Configure scripts use M4sh, which automatically unsets this variable.
Long ago, shell scripts inherited IFS from the environment, but this caused many problems so modern shells ignore any environment settings for 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 reinterpret (!) 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 ‘$*’; however, note that traditional shells, but also bash-2.04, fail to adhere to this and join with a space anyway.

M4sh guarantees that IFS will have the default value at the beginning of a script, and many macros within autoconf rely on this setting. It is okay to use blocks of shell code that temporarily change the value of IFS in order to split on another character, but remember to restore it before expanding further macros.

Unsetting IFS instead of resetting it to the default sequence is not suggested, since code that tries to save and restore the variable's value will incorrectly reset it to an empty value, thus disabling field splitting:

          unset IFS
          # default separators used for field splitting
          # ...
          # no field splitting performed

You should 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; configure scripts and M4sh do that for you. Export these variables after setting them.
LANGUAGE is not specified by Posix, but it is a GNU extension that overrides LC_ALL in some cases, so you (or M4sh) should set it too.
These locale environment variables are GNU extensions. They are treated like their Posix brethren (LC_COLLATE, etc.) as described above.
Most modern shells provide the current line number in LINENO. Its value is the line number of the beginning of the current command. M4sh, and hence Autoconf, attempts to execute configure with a shell that supports LINENO. 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. In M4sh scripts you should execute AS_LINENO_PREPARE so that these workarounds are included in your script; configure scripts do this automatically in AC_INIT.

You should not rely on LINENO within eval or shell functions, as the behavior differs in practice. The presence of a quoted newline within simple commands can alter which line number is used as the starting point for $LINENO substitutions within that command. Also, the possibility of the Sed prepass means that you should not rely on $LINENO when quoted, when in here-documents, or when line continuations are used. Subshells should be OK, though. In the following example, lines 1, 9, and 14 are portable, but the other instances of $LINENO do not have deterministic values:

          $ cat lineno
          echo 1. $LINENO
          echo "2. $LINENO
          3. $LINENO"
          cat <<EOF
          5. $LINENO
          6. $LINENO
          7. \$LINENO
          ( echo 9. $LINENO )
          eval 'echo 10. $LINENO'
          eval 'echo 11. $LINENO
          echo 12. $LINENO'
          echo 13. '$LINENO'
          echo 14. $LINENO '
          15.' $LINENO
          f () { echo $1 $LINENO;
          echo $1 $LINENO }
          f 18.
          echo 19. \
          $ bash-3.2 ./lineno
          1. 1
          2. 3
          3. 3
          5. 4
          6. 4
          7. $LINENO
          9. 9
          10. 10
          11. 12
          12. 13
          13. $LINENO
          14. 14
          15. 14
          18. 16
          18. 17
          19. 19
          $ zsh-4.3.4 ./lineno
          1. 1
          2. 2
          3. 2
          5. 4
          6. 4
          7. $LINENO
          9. 9
          10. 1
          11. 1
          12. 2
          13. $LINENO
          14. 14
          15. 14
          18. 0
          18. 1
          19. 19
          $ pdksh-5.2.14 ./lineno
          1. 1
          2. 2
          3. 2
          5. 4
          6. 4
          7. $LINENO
          9. 9
          10. 0
          11. 0
          12. 0
          13. $LINENO
          14. 14
          15. 14
          18. 16
          18. 17
          19. 19
          $ sed '=' <lineno |
          >   sed '
          >     N
          >     s,$,-,
          >     t loop
          >     :loop
          >     s,^\([0-9]*\)\(.*\)[$]LINENO\([^a-zA-Z0-9_]\),\1\2\1\3,
          >     t loop
          >     s,-$,,
          >     s,^[0-9]*\n,,
          >   ' |
          >   sh
          1. 1
          2. 2
          3. 3
          5. 5
          6. 6
          7. \7
          9. 9
          10. 10
          11. 11
          12. 12
          13. 13
          14. 14
          15. 15
          18. 16
          18. 17
          19. 20

In particular, note that config.status (and any other subsidiary script created by AS_INIT_GENERATED) might report line numbers relative to the parent script as a result of the potential Sed pass.

When executing the command ‘>foo’, zsh executes ‘$NULLCMD >foo’ unless it is operating in Bourne shell compatibility mode and the zsh version is newer than 3.1.6-dev-18. If you are using an older zsh and forget to set NULLCMD, your script might be suspended waiting for data on its standard input.
On DJGPP systems, the PATH_SEPARATOR environment variable can be set to either ‘:’ or ‘;’ to control the path separator Bash uses to set up certain environment variables (such as PATH). You can set this variable to ‘;’ if you want configure to use ‘;’ as a separator; this might be useful if you plan to use non-Posix shells to execute files. See File System Conventions, for more information about PATH_SEPARATOR.
Posix 1003.1-2001 requires that cd and pwd must update the PWD environment variable to point to the logical name of the current directory, but traditional shells do not support this. This can cause confusion if one shell instance maintains PWD but a subsidiary and different shell does not know about PWD and executes cd; in this case PWD points to the wrong directory. Use ‘`pwd`’ rather than ‘$PWD’.
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. It is common practice to use $RANDOM as part of a file name, but code shouldn't rely on $RANDOM expanding to a nonempty string.
This variable is an alias to ‘$?’ for zsh (at least 3.1.6), hence read-only. Do not use it.