hobbit:/tmp/x$ cat prog
#!/bin/bash
i=0
for f; do
    while IFS= read -r line; do
        let i++
        printf '%d %s\n' "$i" "$line"
    done < "$f" > "$f.tmp"
    mv -- "$f.tmp" "$f"
done
hobbit:/tmp/x$ cat foo
foo1
foo2
foo3
hobbit:/tmp/x$ cat bar
bar alpha
bar beta
hobbit:/tmp/x$ ./prog foo bar
hobbit:/tmp/x$ cat foo
1 foo1
2 foo2
3 foo3
hobbit:/tmp/x$ cat bar
4 bar alpha
5 bar beta