Monitoring a file until a string is found

bashgreplogfilestail

I am using tail -f to monitor a log file that is being actively written to. When a certain string is written to the log file, I want to quit the monitoring, and continue with the rest of my script.

Currently I am using:

tail -f logfile.log | grep -m 1 "Server Started"

When the string is found, grep quits as expected, but I need to find a way to make the tail command quit too so that the script can continue.

Best Answer

A simple POSIX one-liner

Here is a simple one-liner. It doesn't need bash-specific or non-POSIX tricks, or even a named pipe. All you really need is to decouple the termination of tail from grep. That way, once grep ends, the script can continue even if tail hasn't ended yet. So this simple method will get you there:

( tail -f -n0 logfile.log & ) | grep -q "Server Started"

grep will block until it has found the string, whereupon it will exit. By making tail run from it's own sub-shell, we can place it in the background so it runs independently. Meanwhile, the main shell is free to continue execution of the script as soon as grep exits. tail will linger in its sub-shell until the next line has been written to the logfile, and then exit (possibly even after the main script has terminated). The main point is that the pipeline no longer waits for tail to terminate, so the pipeline exits as soon as grep exits.

Some minor tweaks:

The option -n0 to tail makes it start reading from the current last line of logfile, in case the string exists earlier in the logfile.
You might want to give tail -F rather than -f. It is not POSIX, but it allows tail to work even if the log is rotated while waiting.
Option -q rather than -m1 makes grep quit after the first occurrence, but without printing out the trigger line. Also it is POSIX, which -m1 isn't.

Related Solutions

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

Linux – Tail, cat and grep a file at the same time

I don't know if this is what you are looking for but you might try this:

grep "pattern you are looking for" log.txt;tail -n 0 -f log.txt | grep "pattern you are looking for"

it's calling two commands one after another. first you grep the pattern you are interested in from the log file and then you start tail -f at the end of the file redirecting output to grep.

however I suggest you do it using one command only, you can specify the number of lines to be displayed by tail using -n parameter (+1 means from the first line):

tail -n +1 -f log.txt | grep "pattern you are looking for"

Best Answer

A simple POSIX one-liner

Related Solutions

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

Linux – Tail, cat and grep a file at the same time

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