How do I get the absolute directory of a file in bash?

I have written a bash script that takes an input file as an argument and reads it.
This file contains some paths (relative to its location) to additional files used.

I would like the script to go to the folder containing the input file, to execute further commands.

So, how do I get the folder (and just the folder) from an input file? (In linux.)

To get the full path use:

readlink -f relative/path/to/file

To get the directory of a file:

dirname relative/path/to/file

You can also combine the two:

dirname $(readlink -f relative/path/to/file)

If readlink -f is not available on your system you can use this^*:

function myreadlink() {
  (
  cd "$(dirname $1)"         # or  cd "${1%/*}"
  echo "$PWD/$(basename $1)" # or  echo "$PWD/${1##*/}"
  )
}

Note that if you only need to move to a directory of a file specified as a relative path, you don't need to know the absolute path, a relative path is perfectly legal, so just use:

cd $(dirname relative/path/to/file)

if you wish to go back (while the script is running) to the original path, use pushd instead of cd, and popd when you are done.

* While myreadlink above is good enough in the context of this question, it has some limitation relative to the readlink tool suggested above. For example it doesn't correctly follow a link to a file with different basename.

Take a look at realpath which is available on GNU/Linux, FreeBSD and NetBSD, but not OpenBSD 6.8. I use something like:

CONTAININGDIR=$(realpath ${FILEPATH%/*})

to do what it sounds like you're trying to do.