script-directory-use-realpath-or-cd

Script directory: use realpath or cd

To get a script's own directory, we recommend two options:

• Option 1: realpath is more capable but not POSIX yet.

• Option 2: cd is POSIX yet less capable.

Option 1: realpath is more capable but not POSIX yet

Code:

command -v realpath >/dev/null 2>&1 || printf realpath\\n 1>&2 && exit 69
dir=$(dirname "$(realpath "$0")")

The realpath technique is a good solution if your system supports it.

• Pros: Works when the script is run directly, or run via symlink, or sourced.

• Cons: Not POSIX yet; it is on track to be added to the next POSIX update.

Details:

• The realpath command is installed on many major operating systems on the current versions. The command is also able to be installed on older operating systems.

• The code exit 69 is the exit number convention for "Unavailable: a service is unavailable. E.g. a support program or file does not exist."

Help

To install realpath on older macOS via brew:

brew install coreutils

Option 2: cd is POSIX but less capable

Code:

dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd -P)

The cd technique is POSIX and returns the directory of a running script.

Pros: POSIX, and works on a very wide range of systems, including much older systems.

Cons: Does not work if the script is invoked as a symlink, or if the script is sourced.

https://stackoverflow.com/questions/29832037/how-to-get-script-directory-in-posix-sh

Explanation

The CDPATH= is set to a null string so as to ensure that `cd`` never echoes anything.

The pwd -P option is used to print the path with all symlinks resolved. This resolves the resulting directory path to its ultimate target in case the directory and/or its components are symlinks,

How to use both realpath and cd

This code uses realpath if it's available, otherwise falls back to use cd:

if command -v realpath >/dev/null 2>&1; then
    dir=$(dirname "$(realpath "$0")")
else
    dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd -P)
fi

Comparisons

We do not want to use readlink -f because it's not available on some operating systems, such as macOS. The option -f is a GNU capability, and not POSIX.