Bak function: swap file <-> file.bak and file.bak <-> file

Function definition:

# swap file <-> file.bak and file.bak <-> file
bak() {
    [[ "$#" -ne 1 ]] && { echo >&2 '1 path must be supplied'; return 1; }
    [[ "$1" = *'.bak' ]] && other="${1%.bak}" || other="$1.bak"
    [[ -e "$1" ]] && [[ -e "$other" ]] && { echo >&2 "Both $1 and $other already exist"; return 1; }
    ! [[ -e "$1" ]] && ! [[ -e "$other" ]] && { echo >&2 "Neither $1 nor $other exist"; return 1; }
    if [[ -e "$other" ]]; then
        mv "$other" "$1"
    else
        mv "$1" "$other"
    fi
}

Example usage:

Use case 1: Swap a regular file to .bak and back 

% ls
custom.conf
% bak custom.conf
% ls
custom.conf.bak
% bak custom.conf
% ls
custom.conf

Use case 2: Swap a .bak file to a regular file and back 

% ls
custom.conf.bak
% bak custom.conf.bak
% ls
custom.conf
% bak custom.conf.bak
% ls
custom.conf.bak

Breakdown

  • [[ "$#" -ne 2 ]] && { echo >&2 '2 paths must be supplied'; return 1; }
    • Ensure there are 2 arguments to the swap function
  • { [[ -e "$1" ]] || [[ -e "$2" ]]; } || { echo >&2 'Neither file exists'; return 1; }
    • Ensure at least 1 argument exists
  • [[ "$1" = "$2" ]] && { echo >&2 "Can't swap file to itself"; return 1; }
    • Ensure both arguments are distinct; swapping a file to itself would be a no-op
  • if [[ -e "$1" ]] && [[ -e "$2" ]]; then
    • The case for swapping 2 currently-existing files. Do both files exist?
    • [[ -e "$1.$$" ]] && { echo >&2 'Swap temp file already exists. Aborting'; return 1; }
      • $$ expands to the pid of the current shell.
      • "$1.$$" is being used as a temporary location when swapping two files that exist.
      • An example of what "$1.$$" might be is "/etc/nginx/conf.d/site.conf.3941"
    • mv "$1" "$1.$$" &&
      • Move A to the temporary location
    • mv "$2" "$1" &&
      • Move B to the previous location of A
    • mv "$1.$$" "$2"
      • Move A from the temporary location to the previous location of B
  • elif [[ -e "$1" ]]; then
    • The case for moving A to B
    • mv "$1" "$2"
  • elif [[ -e "$2" ]]; then
    • The case for moving B to A
    • mv "$2" "$1"