This is my (current) favorite bash one-liner. I find myself using it more and more in scripts:

SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )

This sets SCRIPT_DIR as the directory the script is located. This is different from this:

SCRIPT_DIR=$(pwd)

Which sets SCRIPT_DIR to where the script is executing.

For instance if we have a bash script called print_current_directory.sh in ~/files/scripts:

#!/bin/bash
echo $(pwd)
echo $( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )

If we call the script from a ~/files directory we will get:

$ ./scripts/print_current_directory.sh
/home/files
/home/files/scripts

This can be very handy in scripts run in automated processes.