oleg@t510 1 ~/work/svn/p4 % cat test.sh
#!/bin/bash

# return 0 if argument is positive number
function is_positive_number ()
{
    case $1 in
        ''|*[!0-9]*) return 1 ;;
        *)           return 0 ;;
    esac
}

# return 0 if argument is number and 1 otherwise
function is_number ()
{
    local string=$1
    is_positive_number $string
    if [ $? -eq 0 ]; then
    return 0
    elif [ "${string:0:1}" == '-' ]; then
    is_positive_number "${string:1:${#string}-1}"
    return $?
    else
        return 1
    fi
}

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