Zsh completion basics

File Format

# compdef backup_user                  # <-- program(s) to complete

_arguments \
  '1:firstname:_normal' \              # <-- positional param 1
  '2:file:_normal'                     # <-- positional param 2
  {-h,--help}'[show help]' \           # <-- flag param (no value)
  '-o[output file]::outfile:_files' \  # <-- flag param (with 'file' value)