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

bashlinux

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.

Best Answer

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.

Related Solutions

Bash – Using watch with pipes

Surround the command with quotes

watch -n 1 'tail -n 200 log/site_dev.log | fgrep Doctrine'

Linux – Filter tail command through multiple grep commands to separate files

It might be easier if you use more than one line to do this. You could write a bash script:

#!/bin/bash

tail -f test.log | while read line; do
    if echo "$line" | grep -q "Error"; then
        echo "$line" >> error.log
    elif echo "$line" | grep -q "Warning"; then
        echo "$line" >> warning.log
    # The following is in case you want to print out lines that do not match 
    else
        echo "$line"
    fi
done

Best Answer

Related Solutions

Bash – Using watch with pipes

Linux – Filter tail command through multiple grep commands to separate files

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