tmutil.completion

#!/usr/bin/env bash 

### 補完関数。
_tmutil(){
  list=$( tmutil | /usr/bin/grep -v ^$ | /usr/bin/grep ^Usage | awk '{print $3}' )
  COMPREPLY=( $(compgen -W "$list" ${COMP_WORDS[COMP_CWORD]}  ) ) 
}
complete -F _tmutil tmutil

Slightly modified

_tmutil(){
    list=$( tmutil | /usr/bin/grep -v ^$ | /usr/bin/grep ^Usage | awk '{print $3}' )

    case $COMP_CWORD in
        1)
            COMPREPLY=( $(compgen -W "$list" ${COMP_WORDS[COMP_CWORD]}) )
            ;;
        *)
            COMPREPLY=()
            # http://stackoverflow.com/questions/12933362/getting-compgen-to-include-slashes-on-directories-when-looking-for-files
        ;;
    esac
}
complete -o default -F _tmutil tmutil