Find absolute path from a script

Question

In a script I get in $0 the possible relative path to it. For converting it to absolute I've found this solution which I don't understand:

abspath=$(cd ${0%/*} && echo $PWD/${0##*/})

My problem is the magic inside of ${0%/*} and ${0##*/}. It looks like the former extracts the dirname and the latter extracts the filename, I just don't get how.

Answer 1

Shawn had the simplest solution: readlink -f $0. If you want to be absolutely sure to handle weird file names, you can use this:

absolute_path_x="$(readlink -fn -- "$0"; echo x)"
absolute_path="${absolute_path_x%x}"

Documentation

Answer 2

Definitions:

${string%substring} deletes shortest match of $substring from the end of $string.

${string##substring} deletes longest match of $substring from the start of $string.

Your example:

abspath=$(cd ${0%/*} && echo $PWD/${0##*/})

${0%/*} deletes everything after the last slash, giving you the directory name of the script (which might be a relative path).

${0##*/} deletes everything upto the last slash, giving you just the name of the script.

So, this command changes to the directory of the script and concatenates the current working directory (given by $PWD) and the name of the script giving you the absolute path.

To see what is going on try:

echo ${0%/*}
echo ${0##*/}

Answer 3

Here is a safer and more readable way to do this job:

abspath=$(unset CDPATH && cd "$(dirname "$0")" && echo $PWD/$(basename "$0"))

Notes:

• If $0 is a bare filename with no preceding path, the original script will fail but the one given here will work. (Not a problem with $0 but could be in other applications.)
• Either approach will fail if the path to the file doesn't actually exist. (Not a problem with $0, but could be in other applications.)
• The unset is essential if your user may have CDPATH set.
• Unlike readlink -f or realpath, this will work on non-Linux versions of Unix (e.g., Mac OS X).

Answer 4

If you want to learn Shell Parameter Expansion, you can read it from here, but Expansion isn't always a good choice. In this case, almost every Unix like system has 2 good utils:

basename
dirname

The first will extract the filename, while the second will extract the path, so, if you have $0, say:

dirname $0

And you'ill get the path.

Cheers

Answer 5

Introducing pwd, the bash builtin. Also found in the GNU coreutils package.

$ sh ./cygdrive/c/cygwin/home/../../../../home/jaroslav/tmp/somewhere.sh
$0: ./cygdrive/c/cygwin/home/../../../../home/jaroslav/tmp/somewhere.sh
cheeky binary from /home/jaroslav/tmp (/home/jaroslav/tmp)

$ cat /home/jaroslav/tmp/somewhere.sh

abs=$( cd `dirname "$0"` ; pwd -P)
absBin=$( cd `dirname "$0"` ; /bin/pwd -P)
echo \$0: $0
echo cheeky binary from $abs \($absBin\)