45

I'm looking for a way to programmatically 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.

2
  • I would have thought it was possible to tail -f a program output just as well as a file... Am I wrong?
    – Joanis
    Jan 6, 2012 at 0:52
  • @Joanis. You're right, but in my case 'my_cmd' doesn't continuously produce output and must be repeatedly called (much like most commands: ps, ls, lsof, etc)
    – gdw2
    Jan 6, 2012 at 4:01

6 Answers 6

50

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.

The loop runs until grep finds the string in the command's output. -m 1 means "one match is enough", i.e. grep stops searching after it finds the first match.

You can also use grep -q which also quits after finding the first match, but without printing the matching line.

6
  • 1
    someone else mentioned grep -q which is another option. grep quits after finding the string.
    – Sun
    Mar 22, 2019 at 5:01
  • 1
    note that this command will repeatedly run the command in question, which might or might not be desirable.
    – adrien
    Jun 12, 2019 at 10:03
  • 1
    @A__: It's desirable, as stated in the OP under "Update".
    – choroba
    Jun 12, 2019 at 10:52
  • I recommend inserting a sleep so the CPU is not overly taxed. Example: until my_cmd | grep -q foo; do ; sleep 1; done; echo 'Found'. Depending on the task you might want a much longer sleep period. May 4, 2020 at 18:04
  • 1
    @KeithBennett: That's what I suggested on the third line.
    – choroba
    May 4, 2020 at 19:09
14
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.

7
  • Thanks for the detailed explanation, but it doesn't work for me. My watch command (CentOS) doesn't have the -e flag (which shouldn't really matter). More importantly, though, when the string is found, watch continues to run and does not exit. It seems that when grep -m exits, it only exits kills my_cmd, but not watch.
    – gdw2
    Jan 6, 2012 at 14:05
  • Nope, is does matter!, the "-e" flag is ment for leaving watch when the command has an error code different from 0. Since its not present watch is about to continue on your platform. Anyhow, good to know, on my Ubuntu 11.10 installation everything is fine. I have also sometimes troubles with Mac OSX regarding very very outdated commandline tools and I am using mac ports so far to get more current software.
    – math
    Jan 9, 2012 at 10:00
  • This stops if the pattern is found, but it doesn't show any output until that happens
    – Mark
    Jan 3, 2016 at 19:33
  • 1
    You may employ tee for that, but this introduces a misleading newline, I don't know how to circumvent right now: watch -n1 -e "! date | tee /dev/tty | grep --color -m 1 \"17\""
    – math
    Jan 4, 2016 at 10:02
  • Yeah, this didn't work for me. watch dutifully stops watching when the string is found, but it doesn't actually exit until you press a key. So close.
    – mlissner
    May 1, 2018 at 18:19
10

For those who have a program that is continuously writing to stdout, all you need to do is pipe it to grep with the 'single match' option. Once grep finds the matching string, it will exit, which closes stdout on the process that is being piped to grep. This event should naturally cause the program to gracefully exit so long as the process writes again.

What will happen is that the process will receive a SIGPIPE when it tries writing to closed stdout after grep has exited. Here is an example with ping, which would otherwise run indefinitely:

$ ping superuser.com | grep -m 1 "icmp_seq"

This command will match the first successful 'pong', and then exit the next time ping tries writing to stdout.


However,

It's not always guaranteed that the process will write to stdout again and therefore might not cause a SIGPIPE to be raised (e.g., this can happen when tailing a log file). The best solution i've managed to come up with for this scenario involves writing to a file; please comment if you think you can improve:

$ { tail -f log_file & echo $! > pid; } | { grep -m1 "find_me" && kill -9 $(cat pid) && rm pid; }

Breaking this down:

  1. tail -f log_file & echo $! > pid - tails a file, attaches process to background, and saves the PID ($!) to a file. I tried exporting the PID to a variable instead, but it seems there's a race condition between here and when the PID is used again.
  2. { ... ;} - group these commands together so we can pipe the output to grep while keeping the current context (helps when saving and reusing variables, but wasn't able to get that part working)
  3. | - pipe left side's stdout to right side's stdin
  4. grep -m1 "find_me" - find the target string
  5. && kill -9 $(cat pid) - force kill (SIGKILL) the tail process after grep exits once it finds the matching string
  6. && rm pid - remove the file we created
2
  • Thanks! This worked for me: { docker run --tty --volume $(pwd)/test/jekyll-plantuml:/srv/jekyll --env DEBUG=true swedbankpay/jekyll-plantuml:1.3.5 & echo $! > pid; } | { grep -m1 "Server running" && kill -9 $(cat pid) && rm pid; } May 22, 2020 at 23:06
  • Also, if you'd like to read the entire output from the first command and not just what's matched by grep, you can inject the following between } and {: } | tee /dev/tty | {. If tty is not available (on a build server, for instance), replace /dev/tty with /dev/stderr. May 25, 2020 at 8:28
0
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.)

1
  • Please see my update to the question.
    – gdw2
    Jan 6, 2012 at 4:00
0

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.

0

To watch the output without modifying it, and with a timeout there can be used a "waitforoutput" micro Python application.

pip install waitforoutput
waitforoutput 'String Im Looking For' --command 'watch my_cmd' --timeout 10

On the official README there is more detailed explanation on the behavior: https://pypi.org/project/waitforoutput/

1
  • This may solve the problem– but this is no 'micro python' application if it installs 28 other package dependancies. For those who want to undo isntalling this either pip uninstall waitforoutput Pygments SecretStorage attrs bleach cffi colorama cryptography docutils jeepney jsonschema keyring packaging pbr pycparser pyparsing pyrsistent requests rfc3986 rkd rkd tabulate tqdm twine typing waitforoutput webencodings readme-renderer python-dotenv importlib-metadata or install another package pip install pip-autoremove and pip-autoremove waitforoutput to safely remove this. Nov 9, 2021 at 19:36

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