Tail multiple files while piping one of the files through grep

loggingtail

I want to use tail follow two log files, but one of the log files has too much data, so I want it filtered with grep.

tail -f file1 file2|grep mySearch

The issue with this is that both files are run through the grep or rather the output of tail is run through grep. Only file2 should be filtered with the grep of mySearch. Any ideas?

I have tried named pipes, process substitutions, and compound commands.

Best Answer

can you run the two tails in the background, piping the filtered and unfiltered output to a temp file.

Then use that temp file as the source for whatever you need to dos -

tail -f file1 >> temp &
tail -f file2 | grep mySearch >> temp &


do something with temp now

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.

Macos – Equivalent of ‘clear’ command when using tail -f

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Best Answer

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Monitoring a file until a string is found

A simple POSIX one-liner

Macos – Equivalent of ‘clear’ command when using tail -f

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