Diff for "BashFAQ/028"

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 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 (BASH syntax):

  [[ $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"

In Bash, version 4.1.7(1)-release, on Linux, it seems bash always opens the script with fd 255 so you can just do:

  HOME="$(dirname "$(readlink /proc/$$/fd/255)")"

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 try to search for the script in all the directories of $PATH.

The following script shows how this could be done:

   1 #!/bin/bash
   2 
   3 myname=$0
   4 if [[ -s "$myname" ]] && [[ -x "$myname" ]]; then
   5     # $myname is already a valid file name
   6 
   7     mypath=$myname
   8 else
   9     case "$myname" in
  10     /*) exit 1;;             # absolute path - do not search PATH
  11     *)
  12         # Search all directories from the PATH variable. Take
  13         # care to interpret leading and trailing ":" as meaning
  14         # the current directory; the same is true for "::" within
  15         # the PATH.
  16 
  17         # Replace leading : with . in PATH, store in p
  18         p=${PATH/#:/.:}
  19         # Replace trailing : with .
  20         p=${p//%:/:.}
  21         # Replace :: with :.:
  22         p=${p//::/:.:}
  23         # Temporary input field separator, see FAQ #1
  24         OFS=$IFS IFS=:
  25         # Split the path on colons and loop through each of them
  26         for dir in $p; do
  27                 [[ -f "$dir/$myname" ]] || continue # no file
  28                 [[ -x "$dir/$myname" ]] || continue # not executable
  29                 mypath=$dir/$myname
  30                 break           # only return first matching file
  31         done
  32         # Restore old input field separator
  33         IFS=$OFS
  34         ;;
  35     esac
  36 fi
  37 
  38 if [[ ! -f "$mypath" ]]; then
  39     echo >&2 "cannot find full path name: $myname"
  40     exit 1
  41 fi
  42 
  43 echo >&2 "path of this script: $mypath"

Note that $mypath is not necessarily an absolute path name. It still can contain relative paths like ../bin/myscript, because $PATH could contain those. If you want to get the directory only from that string, check FAQ 73.

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.

  • BASH_SOURCE is probably a much better idea than $0, gives better results and is better defined. This article should probably be rewritten with BASH_SOURCE in mind. --Lhunath

    • BASH_SOURCE is still unreliable, just as $0 is. If the real script and its accessories are in /opt/foobar but you're running /usr/local/bin/foobar which is a symlink to /opt/foobar/bin/foobar then BASH_SOURCE still shows you /usr/local/bin/foobar, not /opt/foobar/bin/foobar. --GreyCat

CategoryShell