How to read one line from `tail -f` through a pipeline, and then terminate?

Question

I would like to follow changes to a file via tail -f and then print out the first line that matches a grep command before terminating. What's the simplest way to do this? So far I've been experimenting with things like:

tail -f foo.log | grep 'bar' | head -1

However, the pipeline just hangs, presumably due to buffering. I also tried

tail -f foo.log | grep --line-buffered 'bar' | head -1

This prints out the line, but the process does not terminate unless I hit ^C, presumably because a second line of input is needed to terminate head -1. What's the best way to solve this problem?

Answer 1

tail -f foo.log | grep -m 1 bar

if the file foo.log is writen rarily to, you can do:

grep -m 1 bar <( tail -f foo.log )

It should be noted that the tail -f will stay in background until it will get another line to output. If this takes long time it might be problematic. solution in such case is:

grep -m 1 bar <( exec tail -f foo.log ); kill $! 2> /dev/null

kill will kill leftover tail -f process, and we hide errors, because it's possible that the tail will be gone by the time kill will be invoked.

Answer 2

It's as easy as: tail -1 foo.log

Answer 3

I think that applying unbuffer like this,

tail -f foo.log | unbuffer -p grep 'bar' | head -1

would work. I'm not at a system where I can test it, though, and I can't justify why it would work while grep --line-buffered does not. However, I did try this alternative which does seem to work:

tail -f foo.log | sed -n '/bar/{p;q}'

When the match to bar is found, p prints it and q immediately quits.