How to exit bash script after while loop gets true?

Question

i'm new at bash scripting and i am trying to write a script to check if an ethernet device is up and if so exit the script. That doesn't work as intended, maybe someone can give me a hint.

I start the script, then plug in my device and the script just seems to hang up in the terminal. It is not getting back to the command line.

When the device is plugged in already and the ethernet dev is up the script just runs perfectly. It then echoes 'Connected' and throws me back to command line.

#! /bin/sh
t1=$(ifconfig | grep -o enxca1aea4347b1)
t2='enxca1aea4347b1'
while [ "$t1" != "$t2" ];
do
    sleep 1;
done
echo "Connected"
exit 0

you can use the exit value of grep to decide if the loop should exit — Diego Torres Milano
– Diego Torres Milano, Commented Mar 7, 2016 at 21:51
thank you!!! now that you mention it i realise my error :D case closed — dynamight
– dynamight, Commented Mar 7, 2016 at 21:52

chepner · Accepted Answer · 2016-03-07 23:25:59Z

3

You don't even need to make the comparison; just check the exit status of grep.

t2='enxca1aea4347b1'
until ifconfig | grep -q "$t2"; do
    sleep 1;
done
echo "Connected"
exit 0

In fact, you don't even need grep:

until [[ "$(ifconfig)" =~ $t2 ]]; do
    sleep 1
done

edited Mar 7, 2016 at 23:25

answered Mar 7, 2016 at 22:12

chepner

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1 Comment

Gordon Davisson Over a year ago

Note that [[ ]] is a bash extension, and not supported by all other shells. If you want to use this, change the shebang to #!/bin/bash or it may (or may not) fail.

user812786 · Accepted Answer · 2016-03-07 22:08:27Z

2

You've made an infinite loop, since you're not updating the value of $t1 inside the while statement.

Instead, try:

#! /bin/sh
t1=$(ifconfig | grep -o enxca1aea4347b1)
t2='enxca1aea4347b1'
while [ "$t1" != "$t2" ];
do
    sleep 1;
    t1=$(ifconfig | grep -o enxca1aea4347b1)
done
echo "Connected"
exit 0

edited Mar 7, 2016 at 22:08

answered Mar 7, 2016 at 21:51

user812786

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