Tail -f + grep?

Question

Tail has the following options:

-f      The -f option causes tail to not stop when end of file is reached, but rather to wait for additional data to be appended to the
             input.  The -f option is ignored if the standard input is a pipe, but not if it is a FIFO.

I'd like to only grep for something in the tail output.

tail -f <FILE> | grep <SOMETHING> 

Issue is it only run grep once and is done. No other output happens. How can I make grep run correctly with the -f?

Solution 1

You will find another SO Question helpful: How to 'grep' a continuous stream?

Turn on grep's line buffering mode.

tail -f file | grep --line-buffered my_pattern

Solution 2

If this is a logfile it may be rotated. It will then stop giving data.
This will not stop if file is rotated.

tail --follow=name /var/log/syslog | grep "some data"

Comments

[Keith Thompson]

I often do exactly what you're describing, and it works. The grep command doesn't terminate until the tail -f process terminates, but of course it doesn't produce any output until something containing the pattern is appended to the file.

[Rob Napier]

I can confirm the same on several platforms. I've never had trouble using tail+grep exactly as described here. What platform are you having trouble on?

[anubhava]

Issue is it only run grep once and is done no that's not right.

[Chris Stratton]

I don't think you'll hit it here, but be careful in longer piped chains with grep (especially used more than once) that it may defult to a block buffering and thus not produce any output for quite some time, unless you explicitly specify --line-buffered

[Sten Kin]

Ah chaining was the issue! That fixed it @ChrisStratton