How do I determine the location of my script? I want to read some config files from the same place.
This is a complex question because there's no single right answer to it. Even worse: it's not possible to find the location reliably in 100% of all cases. All ways of finding a script's location depend on the name of the script, as seen in the predefined variable $0. But providing the script name in $0 is only a (very common) convention, not a requirement.
The suspect answer is "in some shells, $0 is always an absolute path, even if you invoke the script using a relative path, or no path at all". That's not the case in ["BASH"]. But this isn't reliable across shells; some of them return the actual command typed in by the user instead of the fully qualified path. In those cases, if all you want is the fully qualified version of $0, you can use something like this (["POSIX"], non-Bourne):
[[ $0 = /* ]] && echo $0 || echo $PWD/$0
Or the BourneShell version:
case $0 in /*) echo $0;; *) echo `pwd`/$0;; esac
Or a shell-independent variant (needs a readlink supporting -f, though)
readlink -f "$0";
However, this approach has some major drawbacks. The most important is, that the script name (as seen in $0) may not be relative to the current working directory, but relative to a directory from the program search path $PATH (this is often seen with KornShell).
Another drawback is that there is really no guarantee that your script is still in the same place it was when it first started executing. Suppose your script is loaded from a temporary file which is then unlinked immediately... your script might not even exist on disk any more! The script could also have been moved to a different location while it was executing. Or (and this is most likely by far...) there might be multiple links to the script from multiple locations, one of them being a simple symlink from a common PATH directory like /usr/local/bin, which is how it's being invoked. Your script might be in /opt/foobar/bin/script but the naive approach of reading $0 won't tell you that.
(For a more general discussion of the Unix file system and how symbolic links affect your ability to know where you are at any given moment, see [http://www.cs.bell-labs.com/sys/doc/lexnames.html this Plan 9 paper].)
So if the name in $0 is a relative one, i.e. does not start with '/', we can still try to search the script like the shell would have done: in all directories from $PATH.
The following script shows how this could be done:
myname=$0
if [ -s "$myname" ] && [ -x "$myname" ]
then # $myname is already a valid file name
mypath=$myname
else
case "$myname" in
/*) exit 1;; # absolute path - do not search PATH
*)
# Search all directories from the PATH variable. Take
# care to interpret leading and trailing ":" as meaning
# the current directory; the same is true for "::" within
# the PATH.
for dir in `echo "$PATH" | sed 's/^:/.:/g;s/::/:.:/g;s/:$/:./;s/:/ /g'`
do
[ -f "$dir/$myname" ] || continue # no file
[ -x "$dir/$myname" ] || continue # not executable
mypath=$dir/$myname
break # only return first matching file
done
;;
esac
fi
if [ -f "$mypath" ]
then
: # echo >&2 "DEBUG: mypath=<$mypath>"
else
echo >&2 "cannot find full path name: $myname"
exit 1
fi
echo >&2 "path of this script: $mypath"Note that $mypath is not necessarily an absolute path name. It still can contain relative parts like ../bin/myscript.
Generally storing data files in the same directory as their scripts is a bad practice. The Unix file system layout assumes that files in one place (e.g. /bin) are executable programs, while files in another place (e.g. /etc) are data files. (Let's ignore legacy Unix systems with programs in /etc for the moment, shall we....)
It really makes the most sense to keep your script's configuration in a single, static location such as $SCRIPTROOT/etc/foobar.conf. If you need to define multiple configuration files, then you can have a directory (say, /var/lib/foobar or /usr/local/lib/foobar), and read that directory's location from a variable in /etc/foobar.conf. If you don't even want that much to be hard-coded, you could pass the location of foobar.conf as a parameter to the script. If you need the script to assume certain default in the absence of /etc/foobar.conf, you can put defaults in the script itself, and/or fall back to something like $HOME/.foobar.conf if /etc/foobar.conf is missing. (This depends on what your script does. In some cases, it may make more sense to abort gracefully.)