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10.9 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.
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.
_ - Many shells reserve ‘$_’ for various purposes, e.g., the name of
the last command executed.
BIN_SH - In Tru64, if BIN_SH is set to
xpg4 , subsidiary invocations of
the standard shell conform to Posix.
Autoconf-generated scripts export this variable when they start up.
CDPATH - When this variable is set it specifies a list of directories to search
when invoking
cd with a relative file name that did not start
with ‘./’ or ‘../’. Posix
1003.1-2001 says that if a nonempty directory name from CDPATH
is used successfully, cd prints the resulting absolute
file name. Unfortunately this output can break idioms like
‘abs=`cd src && pwd`’ because abs receives the name twice.
Also, many shells do not conform to this part of Posix; for
example, zsh prints the result only if a directory name
other than . was chosen from CDPATH.
In practice the shells that have this problem also support
unset, so you can work around the problem as follows:
(unset CDPATH) >/dev/null 2>&1 && unset CDPATH
You can also avoid output by ensuring that your directory name is
absolute or anchored at ‘./’, as in ‘abs=`cd ./src && pwd`’.
Autoconf-generated scripts automatically unset CDPATH if
possible, so you need not worry about this problem in those scripts.
DUALCASE - In the MKS shell, case statements and file name generation are
case-insensitive unless DUALCASE is nonzero.
Autoconf-generated scripts export this variable when they start up.
ENV MAIL MAILPATH PS1 PS2 PS4 - These variables should not matter for shell scripts, since they are
supposed to affect only interactive shells. However, at least one
shell (the pre-3.0 uwin Korn shell) gets confused about
whether it is interactive, which means that (for example) a PS1
with a side effect can unexpectedly modify ‘$?’. To work around
this bug, Autoconf-generated scripts do something like this:
(unset ENV) >/dev/null 2>&1 && unset ENV MAIL MAILPATH
PS1='$ '
PS2='> '
PS4='+ '
IFS - 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.
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 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.
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,$,-,
> t loop
> :loop
> s,^\([0-9]*\)\(.*\)[$]LINENO\([^a-zA-Z0-9_]\),\1\2\1\3,
> t loop
> s,-$,,
> s,^[0-9]*\n,,
> ' |
> sh
1. 1
3. 3
4. 4
6. 6
7. 7
8. 8
9. 9
10. 10
NULLCMD - When executing the command ‘>foo’, zsh executes
‘$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.
PATH_SEPARATOR - 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 .
PWD - 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’.
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. It is common
practice to use $RANDOM as part of a file name, but code
shouldn't rely on $RANDOM expanding to a nonempty string.
status - This variable is an alias to ‘$?’ for
zsh (at least 3.1.6),
hence read-only. Do not use it.
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