Reliable way for a Bash script to get the full pat

Question

This question already has an answer here:

• Getting the source directory of a Bash script from within 52 answers

I have a Bash script that needs to know its full path. I'm trying to find a broadly-compatible way of doing that without ending up with relative or funky-looking paths. I only need to support Bash, not sh, csh, etc.

What I've found so far:

1. The accepted answer to Getting the source directory of a Bash script from within addresses getting the path of the script via dirname $0, which is fine, but that may return a relative path (like .), which is a problem if you want to change directories in the script and have the path still point to the script's directory. Still, dirname will be part of the puzzle.

2. The accepted answer to Bash script absolute path with OS X (OS X specific, but the answer works regardless) gives a function that will test to see if $0 looks relative and if so will pre-pend $PWD to it. But the result can still have relative bits in it (although overall it's absolute) — for instance, if the script is t in the directory /usr/bin and you're in /usr and you type bin/../bin/t to run it (yes, that's convoluted), you end up with /usr/bin/../bin as the script's directory path. Which works, but...

3. The readlink solution on this page, which looks like this:

   # Absolute path to this script. /home/user/bin/foo.sh
   SCRIPT=$(readlink -f $0)
   # Absolute path this script is in. /home/user/bin
   SCRIPTPATH=`dirname $SCRIPT`
   

   But readlink isn't POSIX and apparently the solution relies on GNU's readlink where BSD's won't work for some reason (I don't have access to a BSD-like system to check).

So, various ways of doing it, but they all have their caveats.

What would be a better way? Where "better" means:

• Gives me the absolute path.
• Takes out funky bits even when invoked in a convoluted way (see comment on #2 above). (E.g., at least moderately canonicalizes the path.)
• Relies only on Bash-isms or things that are almost certain to be on most popular flavors of *nix systems (GNU/Linux, BSD and BSD-like systems like OS X, etc.).
• Avoids calling external programs if possible (e.g., prefers Bash built-ins).
• (Updated, thanks for the heads up, wich) It doesn't have to resolve symlinks (in fact, I'd kind of prefer it left them alone, but that's not a requirement).

Answer 1

Try this:

cd $(dirname $([ -L $0 ] && readlink -f $0 || echo $0))

Answer 2

I have used the following approach successfully for a while (not on OS X though), and it only uses a shell built-in and handles the 'source foobar.sh' case as far as I have seen.

One issue with the (hastily put together) example code below is that the function uses $PWD which may or may not be correct at the time of the function call. So that needs to be handled.

#!/bin/bash

function canonical_path() {
  # Handle relative vs absolute path
  [ ${1:0:1} == '/' ] && x=$1 || x=$PWD/$1
  # Change to dirname of x
  cd ${x%/*}
  # Combine new pwd with basename of x
  echo $(pwd -P)/${x##*/}
  cd $OLDPWD
}

echo $(canonical_path "${BASH_SOURCE[0]}")

type [
type cd
type echo
type pwd

Answer 3

Yet another way to do this:

shopt -s extglob

selfpath=$0
selfdir=${selfpath%%+([!/])}

while [[ -L "$selfpath" ]];do
  selfpath=$(readlink "$selfpath")
  if [[ ! "$selfpath" =~ ^/ ]];then
    selfpath=${selfdir}${selfpath}
  fi
  selfdir=${selfpath%%+([!/])}
done

echo $selfpath $selfdir

Answer 4

Perhaps the accepted answer to the following question may be of help.

How can I get the behavior of GNU's readlink -f on a Mac?

Given that you just want to canonicalize the name you get from concatenating $PWD and $0 (assuming that $0 is not absolute to begin with), just use a series of regex replacements along the line of abs_dir=${abs_dir//\/.\//\/} and such.

Yes, I know it looks horrible, but it'll work and is pure Bash.

Answer 5

More simply, this is what works for me:

MY_DIR=`dirname $0`
source $MY_DIR/_inc_db.sh

Answer 6

I'm surprised that the realpath command hasn't been mentioned here. My understanding is that it is widely portable / ported.

Your initial solution becomes:

SCRIPT=`realpath $0`
SCRIPTPATH=`dirname $SCRIPT`

And to leave symbolic links unresolved per your preference:

SCRIPT=`realpath -s $0`
SCRIPTPATH=`dirname $SCRIPT`