#!/bin/sh
set -eu

file=etc/pam.d/system-auth
line='auth      optional    pam_cap.so'

if [ -f "$file" ]; then
    if ! grep -F -x -q "$line" "$file"; then
        printf '%s\n' "$line" >> "$file"
    fi
else
    printf '%s\n' "$line" > "$file"
fi