How to test if link is up in bash script

Question

using this script I am trying to detect if there is a network link. I am failing in putting multiple commands in one line (ethtool...). What to do?

#!/bin/bash

COMMAND="( /sbin/ethtool eth0 ) | ( /bin/grep \"Link detected: yes\" ) | ( wc -l )"
ONLINE=eval $COMMAND 

if $ONLINE; then 
    echo "Online"
else
    echo "Not online"
fi

Answer 1

You simply need

if /sbin/ethtool eth0 | grep -q "Link detected: yes"; then
    echo "Online"
else
    echo "Not online"
fi

Also if you want to encapsulate checking, simply use a function:

function check_eth {
    set -o pipefail # optional.
    /sbin/ethtool "$1" | grep -q "Link detected: yes"
}

if check_eth eth0; then
    echo "Online"
else
    echo "Not online"
fi

How it works: if simply interprets a command in front of it and check if the return value of $? is 0. grep returns 0 when it finds a match on its search. And so because of this you don't need to use wc and compare its output to 1.