Ubuntu – tail a log file but show only specific lines

command linegrep

I'm tailing a log file with -f flag. Then I'm piping this to grep, to find only lines that contain "X". That's working perfectly fine.
Now I want to pipe this again into another grep, that will remove all the lines containing "Y". When I add the second pipe, the file stop refreshing and it looks like no data is coming.

This is the command that works: tail -f my_file.log | grep "X"

This is the command that doesn't: tail -f my_file.log | grep "X" | grep -v "Y"

How should I structure this so that the command works?

Best Answer

As the output of grep is buffered, use --line-buffered option of grep to enable line buffering:

tail -f /path/to/log | grep --line-buffered 'X' | grep -v 'Y'

If your grep does not have the option, you can use stdbuf as an alternative:

tail -f /path/to/log | stdbuf -oL grep 'X' | grep -v 'Y'

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