returns 0 if string is a number (bash function)

lock.bash

# returns 0 if string is a number
# @param string input

function isnumber()
{   #look for a number, returns 0 on success, 1 otherwise
    [ -z "$1" ] && return 1
    printf "%s\\n" "$@" | grep -v "[^0-9]" >/dev/null
}