Question

I am trying to implement a script that wait for a specific message in a log file. Once the message is logged then I want to continue the script.

Here’s what I am trying out with tail -f and grep -q:

# tail -f logfile | grep -q 'Message to continue'

The grep never quit and so it waits forever even if ‘Message to continue’ is logged in the file.

When I run this without -f it seems to work fine.

Answer 1

tail -f will read a file and display lines later added, it will not terminate (unless a signal like SIGTERM is sent). grep is not the blocking part here, tail -f is. grep will read from the pipe until it is closed, but it never is because tail -f does not quit and keep the pipe open.

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A solution to your problem would probably be (not tested and very likely to perform badly):

tail -f logfile | while read line; do
  echo $line | grep -q 'find me to quit' && break;
done