update-file-line.sh

 append_line() {
   set -e
 
   local update line file pat lno
   update="$1"
   line="$2"
   file="$3"
   pat="${4:-}"
   lno=""
 
   echo "Update $file:"
   echo "  - $line"
   if [ -f "$file" ]; then
     if [ $# -lt 4 ]; then
       lno=$(\grep -nF "$line" "$file" | sed 's/:.*//' | tr '\n' ' ')
     else
       lno=$(\grep -nF "$pat" "$file" | sed 's/:.*//' | tr '\n' ' ')
     fi
   fi
   if [ -n "$lno" ]; then
     echo "    - Already exists: line #$lno"
   else
     if [ $update -eq 1 ]; then
       [ -f "$file" ] && echo >> "$file"
       echo "$line" >> "$file"
       echo "    + Added"
     else
       echo "    ~ Skipped"
     fi
   fi
   echo
   set +e
 }