Weird tail + grep bug

Aeiri · 07-28-2005, 10:52 PM

I just found a weird bug in the way tail and grep work together. I've tested this on Slackware -current and Fedora Core 2. Run this code in your terminal:

Code:

$ echo bob jones > bob
$ tail -n 1 | grep bob | grep bob
bob jones
$ tail -fn 1 | grep bob
bob jones
<CTRL+C>
$ tail -fn 1 | grep bob | grep bob
<CTRL+C>

The last command outputs NOTHING!. This should output "bob jones" like the others, what is going on?

I just noticed this while tailing my syslog file, watching iptables' output.

I know this used to work, so when did this all of a sudden happen?

This only happens with the "-f" option, and two piped greps to it. (Note: the -n 1 isn't really needed)

rhoekstra · 07-29-2005, 02:09 AM

you forgot the filename 'bob' on the tail command line, but okay..

Further, the tail -f tries to follow on a file and passes information in chunks to the grep, and grep does the same ..

To my knowledge, if you put some more lines in 'bob' you WOULD see something, but perhaps not EVERYTHING. the content is stuck in some buffer held to be processed once more info is getting into the file bob while tail is being watching it.

I am not an expert on the way tail and grep work, but I'd say it isn't a bug, but rather you're experiencing the buffered output each program has..

rebuke · 12-15-2006, 05:30 AM

I hit a similar problem, a mate managed to solve it, you need to add --line-buffered to any grep thats piping to another, so in your example (with the filename added in):

tail -fn 1 bob | grep --line-buffered bob | grep bob

I realise this is an old post, so sorry for resurrecting it, but just in case other people find it like I did with google, thought it might help...