#!/bin/sh -e if test $# -eq 0 then echo "usage: $0 shellname [shellname ...]" 1>&2 exit 1 fi file=/etc/shells # I want this to be GUARANTEED to be on the same filesystem as $file tmpfile="${file}.tmp" otmpfile="${file}.tmp2" set -o noclobber cleanup() { rm -f "$tmpfile" "$otmpfile" } trap cleanup EXIT if ! cat "$file" > "$tmpfile" then cat 1>&2 <<EOF Either another instance of $0 is running, or it was previously interrupted. Please examine ${tmpfile} to see if it should be moved onto ${file}. EOF exit 1 fi # this is supposed to be reliable, not pretty for i do REALDIR="$(dirname "$(realpath -m "$i")")/$(basename "$i")" for j in "$i" "$REALDIR" do grep -v "^${j}$" "$tmpfile" > "$otmpfile" || true mv "$otmpfile" "$tmpfile" done done chmod --reference="${file}" "${tmpfile}" || chmod $(stat -c %a "${file}") "${tmpfile}" chown --reference="${file}" "${tmpfile}" || chown $(stat -c %U "${file}") "${tmpfile}" mv -Z "${tmpfile}" "${file}" || mv "${tmpfile}" "${file}" trap "" EXIT exit 0