docwhat's avatardocwhat's blog

bash: Checking a port number

Nucella lapillus

Ever needed to check if a given port number is valid in bash? It’s harder than it looks because if you accept any input from the user, you can’t use declare -i since it spews errors and aborts functions with malformed input.

An example:

declare -i foo='10#88f'
bash: declare: 10#88f: value too great for base (error token is "10#88f")

It’s annoying (you can’t use || to get around it) and it’s also wrong. It isn’t a matter of base but an invalid value.


function to_int {
    local -i num="10#${1}"
    echo "${num}"

function port_is_ok {
    local port="$1"
    local -i port_num=$(to_int "${port}" 2>/dev/null)

    if (( $port_num < 1 || $port_num > 65535 )) ; then
        echo "*** ${port} is not a valid port" 1>&2

    echo 'ok'

port_is_ok 1          # => ok
port_is_ok 20         # => ok
port_is_ok 70000      # => *** 70000 is not a valid port
port_is_ok tnenth2    # => *** tnenth2 is not a valid port
port_is_ok thethe     # => *** thethe is not a valid port
port_is_ok 888f88     # => *** 888f88 is not a valid port
port_is_ok 88888f     # => *** 88888f is not a valid port


Without the to_int() function, then you’d get errors from port_is_ok and the function would abort before it gets to the check.

You have to redirect stderr to hide this bash error.

This was tested with bash 4.1.5(1)-release


Edit on GitHub