“watch” the output of a command until a particular string is observed and then exit

Question

I'm looking for a way to programatically watch the output of a command until a particular string is observed and then exit. This is very similar to this question, but instead of tailing a file, I want to 'tail' a command.

Something like:

watch -n1 my_cmd | grep -m 1 "String Im Looking For"

(But this doesn't work for me.)

UPDATE: I need to clarify that 'my_cmd' does not continuously output text but needs to be repeatedly called until the string is found (which is why I thought of the 'watch' command). In this respect, 'my_cmd' is like many other unix commands such as: ps, ls, lsof, last, etc.

Answer 1

Use a loop:

until my_cmd | grep -m 1 "String Im Looking For"; do : ; done

Instead of :, you can use sleep 1 (or 0.2) to ease the CPU.

Answer 2

my_cmd | tail +1f | sed '/String Im Looking For/q'

If tail doesn't support the +1f syntax, try tail -f -n +1. (The -n +1 tells it to start at the beginning; tail -f by default starts with the last 10 lines of output.)

Answer 3

Append the result of your program calls to a file. Then tail -f that file. That way it should work... I hope.

When you restart calling that program you'll have to erase the file or append some gibberish to it just so it doesn't match again right away what you were looking for.

Answer 4

watch -e "! my_cmd | grep -m 1 \"String Im Looking For\""

• ! negates the exit code of the command pipeline
• grep -m 1 exits when string is found
• watch -e returns if any error has occured

But this can be improved to actually display that matched line, which is thrown away so far.