oleg@t510 0 ~/work/svn/p4 % ./test.sh
is_number '0' is yes
is_number '1' is yes
is_number '123' is yes
is_number '-1' is yes
is_number '-123' is yes
is_number '-0' is yes
is_number '-123-123' is no
is_number 'bad' is no
is_number '' is no
oleg@t510 0 ~/work/svn/p4 % cat test.sh
#!/bin/bash

function is_number ()
{
    local string=$1
    case $string in
    ''|*[!0-9]*)
        if [ "${string:0:1}" == '-' ]; then
        is_number "${string:1:${#string}-1}"
        return $?
        else
        return 1
        fi
        ;;
    *)
        return 0
        ;;
    esac
}

for value in 0 1 123 -1 -123 -0 -123-123 bad ''; do
    is_number $value
    echo "is_number '$value' is $(is_number ${value} && echo 'yes' || echo 'no')"
done
oleg@t510 0 ~/work/svn/p4 %