How to use bash return code in conditional?

The return code is available in the special parameter $? after the command exits. Typically, you only need to use it when you want to save its value before running another command:

valid_ip "$IP1"status1=$?valid_ip "$IP2"if [ $status1 -eq 0 ] || [ $? -eq 0 ]; then

or if you need to distinguish between various non-zero statuses:

valid_ip "$IP"case $? in    1) echo valid_IP failed because of foo ;;    2) echo valid_IP failed because of bar ;;    0) echo Success ;;esac

Otherwise, you let the various operators check it implicitly:

if valid_ip "$IP"; then    echo "OK"fivalid_IP "$IP" && echo "OK"

Here is a simple, idiomatic way of writing valid_ip:

valid_ip () {    local ip=$1    [[ $ip =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]] && {        IFS='.' read a b c d <<< "$ip"        (( a < 255 && b < 255 && c < 255 && d << 255 ))    }}

There are two expressions, the [[...]] and the { ... }; the two are joined by &&. If the first fails, then valid_ip fails. If it suceeds, then the second expression (the compound statement) is evaluated. The read splits the string into four variables, and each is tested separately inside the arithmetic expression. If all are true, then the ((...)) succeeds, which means the && list succeeds, which means that valid_ip succeeds. No need to store or return explicit return codes.

No parentheses needed if the exit status is inspected:

if valid_ip $IP ; then    ...

Just call the function in the way you would call any other command.