Bash reading STDOUT stream in real-time

Question

I have searched for this and expected to find hundreds of solutions yet found none !

I would like to read the STDOUT stream and wait for a specific string to appear,

Answer 1

You can pipe into grep to do the matching and have it exit when it encounters a match. That will also exit the program producing the output.

if mycommand | grep something -q; then
    dosomething
fi

The above will quit if anything matches (-q), but not display the result. If you want to see the output, you can quit on the first match (using -m1):

RESP=$(mycommand | grep something -m1)

Read the man-pages for grep for more information.

---

If you don't want to cancel the program producing the output, you could try writing it to file in the background, then tail that file:

mycommand > output &
if tail -f output | grep something -q; then
    dosomething
fi

Answer 2

From this unix.stackexchange answer I got following solution:

cmd_which_streams_new_lines \
| while read -r line; do
  # process your line here
  echo "${line}"

  # when you're done, break the loop (be prepared for messy stderr)
done