Diff for "BashFAQ/028"

Anchor(faq28)

How do I determine the location of my script? I want to read some config files from the same place.

This topic comes up frequently. This answer covers not only the expression used above ("configuration files"), but also several variant situations. If you've been directed here, please read this entire answer before dismissing it.

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". But this isn't reliable across shells; some of them (including ["BASH"]) return the actual command typed in by the user instead of the fully qualified path. And this is just the tip of the iceberg!

Your script may not actually be on a locally accessible disk at all. Consider this:

  ssh remotehost bash < ./myscript

The shell running on remotehost is getting its commands from a pipe. There's no script anywhere on any disk that bash can see.

Moreover, even if your script is stored on a local disk and executed, it could move. Someone could mv the script to another location in between the time you type the command and the time your script checks $0. Or someone could have unlinked the script during that same time window, so that it doesn't actually have a link within a file system any more.

Even in the cases where the script is in a fixed location on a local disk, the $0 approach still 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). Or (and this is most likely problem 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 -- it may say /usr/local/bin/script instead.

(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].)

Having said all that, if you still want to make a whole slew of naive assumptions, and 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(1) supporting -f, though, so it's OS-dependent):

  readlink -f "$0"

If we want to account for the cases where the script's relative pathname (in $0) may be relative to a $PATH component instead of the current working directory (as mentioned above), 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.

            # Replace leading : with . in PATH, store in p
            p=${PATH/#:/.:}
            # Replace trailing : with .
            p=${p//%:/:.}
            # Replace :: with .
            p=${p//::/:.:}
            # Temporary input field separator
            OFS=$IFS; IFS=$'\n'

            # Splits the path in newlines and loops through each of them
            for dir in ${p//:/$'\n'}; do
                [ -f "$dir/$myname" ] || continue # no file
                [ -x "$dir/$myname" ] || continue # not executable
                mypath=$dir/$myname
                break           # only return first matching file
            done
            IFS=$OFS
            ;;
        esac
        fi

        if [ ! -f "$mypath" ]; then
        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.

Are you starting to see how ridiculously complex this problem is becoming? And this is still just the simplistic case where we've made a lot of assumptions about the script not moving and not being piped in!

Generally, storing data files in the same directory as their programs is a bad practise. 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....)

Here are some common sense alternatives you should consider, instead of attempting to perform the impossible:

• It really makes the most sense to keep your script's configuration in a single, static location such as /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 fixed place such as /etc/foobar.conf.

• If you don't even want that much to be hard-coded, you could pass the location of foobar.conf (or of your configuration directory itself) 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, or fall back to something like $HOME/.foobar.conf if /etc/foobar.conf is missing.

• When you install the script on a target system, you could put the script's location into a variable in the script itself. The information is available at that point, and as long as the script doesn't move, it will always remain correct for each installed system.
• In most cases, it makes more sense to abort gracefully if your configuration data can't be found by obvious means, rather than going through arcane processes and possibly coming up with wrong answers.