I use these, but only as what are effectively assertions: [[ -f $input_file ]] || { echo "${input_file} does not exist"; exit 1; } So you can read the left-

I use these, but only as what are effectively assertions: [[ -f $input_file ]] || { echo "${input_file} does not exist"; exit 1; } So you can read the left-hand side bit as an assertion that something is true in the following part of the script, and skim over the right-hand side.