if [ -n "$DRY_RUN" ] ; then alias rm='echo rm' alias cp='echo cp' fi Of course, output redirects will still overwrite the files, since the

if [ -n "$DRY_RUN" ] ; then alias rm='echo rm' alias cp='echo cp' fi Of course, output redirects will still overwrite the files, since the shell does it and IIRC this behaviour can't be changed.