I would want the bash scripting to run the following command
iptables -t nat -A PREROUTING -p tcp --destination-port 80 -j REDIRECT --to-port 10000
if there is no output of it found using
iptables -t nat --list
How can I use the If-Else to look for the output. Can i use 'cat' ?
Use $() to capture the output of a command and -z to determine if it is empty:
output=$(iptables -t nat --list)
if [ -z $output ] # returns true if the length of $output is 0
then
output=$(iptables -t nat -A PREROUTING -p tcp --destination-port 80 -j REDIRECT --to-port 10000)
fi
iptables -t nat --list to be empty, it pretty much never will be since it prints headers if nothing else. I think they want to check if that routing rule is already there, which would be the "of it" in their expression--though of course I can't be sure that's what they mean
You could use grep with the iptables list, depending on how you're trying to match it.
if iptables -t nat --list PREROUTING | grep -- '--destintation-port 80' | grep -q -- '--to-port 10000'
iptables -t nat -A PREROUTING -p tcp --destination-port 80 -j REDIRECT --to-port 10000
fi
This will look if there is a PREROUTING entry that concerns both --destination-port 80 and --to-port 10000. If the output string is more predictable you could use a single grep for it, but I don't know iptables well enough to offer that as part of the solution
