The following shell script is used to check if the input IP address is invalid or not. Rs = 1 while [$rs -gt 0]; do clear echo -n Please input the ip read ip #use expr to check if the input IP address follows the IP address pattern is IP = if; then replace each dot by a space in string ip ip = isvalid = 0; #loop over the words in string ip for n in; do if [-0 $n -1t 0]; then echo "ERROR: The number of the IP should not be greater less than 255 and less than 0" break; done if; then echo'----- This is a valid IP-'. Fi else echo "ERROR: The IP format you in is wrong, the format should be like 192.168.100.1 fi echo echo -n Press 0 to exit and other numbers to continue....." read rs done