Waiting for network link to be up before continuing in bash

Question

Is there a way to check for successful network interface link for multiple interfaces in a bash script before continuing?

Something like:

eth0 eth1 eth2 eth3 network interfaces are brought up
Wait for link detection on all 4 interfaces
Proceed

Solution

You can check if the network interfaces' names appear after running ifconfig -s.

Something like:

if [ $( ifconfig -s | grep eth0 ) ]; then echo "eth0 is up!"

Check this link for more details.

---

To make this test ongoing, you can do something like what @jm666 said:

while ! ping -c 1 -W 1 1.2.3.4; do
    echo "Waiting for 1.2.3.4 - network interface might be down..."
    sleep 1
done

Comments

[user1527227]

This is great if I just want to test once, but what if I need to actively wait for until it comes online and then proceed?

[user1527227]

Why do you have a -W flag in there? You didn't specify a timeout for a single ping....

[jimm-cl]

-c 1 -W 1 means "try 1 echo, and wait for it for 1 second at most". If it does not receive the echo back, then enters the if-block, shows the message, and wait for one second before trying again.

[user1527227]

If you didn't specify -W 1... how long would it wait for? I dont see a descrp in the man pages

[jimm-cl]

As far as I understand, -c 1 alone will just send one echo and wait for it to return, no matter how long it takes. Adding the -W 1 will prevent this from waiting too long, allowing just 1 second of time to get a response.

[logidelic]

The while loop as presented (while ! ping -c 1 -W 1 1.2.3.4;) doesn't work for me on Ubuntu 18... It will correctly continue if the network is not available but will also continue if I get a valid ping result. I had to change it to while ! (ping -c 1 -W 1 1.2.3.4 | grep -q 'statistics');