#! /bin/bash
numcheck='^[0-9]\+$'
add_after_line () { { sed "$1q"; echo "$2"; sed "1,$1d"; } < "$3" > /tmp/medtmp; mv /tmp/medtmp "$3"; } #Thanks izabera!
if [ "$1" == "rl" ]; then
        if [[ "$2" =~ $numcheck ]]; then
                echo "error: Not a number" >&2;exit 1
        fi
        NUM="$2"
        cat "$3" | sed "${NUM}q;d" #Read line command
        exit 0
fi
if [ "$1" == "ap" ]; then
        filename="$3"
        echo "$2" >> "$filename" #Append command
        exit 0
fi
if [ "$1" == "hp" ]; then
        echo "View the inside of the file for more info, rl, anyone?"
        exit 0
fi
if [ "$1" == "il" ]; then #insert at
        if [[ "$2" =~ $numcheck ]]; then
                echo "error: Not a number" >&2;exit 1
        fi

        add_after_line "$2" "$3" "$4"
        exit 0
fi