Just as confusing as it looks, but it makes a good bit of sense once you learn it.
Say you run:
echo 'hello' | grep 'ello'
That will have an exit code of
0 (no failure, ran successfully).
Say you have a file
answer_to_life.txt. We can grep the contents of the file and use the
$? to see if grep succeeded in finding our matching pattern.
So you can do something like:
cat answer_to_life.txt | grep '42' if [ $? -eq '0' ] then echo 'The answer to life is: 42' else echo 'Apparently the answer to life is not 42' fi
We have the grep run, check for the exit code of the latest operation and if it equals
"0", then we print our known fact. Otherwise we print that we guessed wrong.
Just a neat little trick. Should be useful for a lot of things! 🎉