How to write to file using tail -f through a pipe to grep?

[ndedhia1]

Hi -- I'm looking to write to a file after piping output from tail -f through to grep:

#write to a file for all lines with "searchtext" within in error_log:

tail -f /var/error_log | grep searchtext > output.txt
 

The above command fails even though the following command::

tail -f /var/error_log | grep searchtext
 

does produce output, as expected. Nothing is getting through while redirecting to files after a tail -f... The following work:

tail -f /var/error_log > output.txt                //without grep
tail /var/error_log | grep searchtext > output.txt   //without -f
 

So it's just the combination of grep and tail with specifically the -f (following) option that causes the problem. It would be ideal to be able to do this with a single inline command. Any ideas?

thanks for the help!!

[mac11]

It's working, you just don't see anything for a while because of buffering. I think grep's the one doing the buffering, but I'm not sure.

I tested as follows (you need 3 commands going at once):

Producer:

while [ 1 ]; do date >> in.txt; done

Filter:

tail -f in.txt | grep Feb > out.txt

Show the results as they come out:

tail -f out.txt

If you run it like this you will see stuff coming out from the tail on out.txt (like expected), but if you put a sleep into the first command (slow up the producer) it will take quite a while for anything to show up in the tail of out.txt

Edit: I checked (man grep) and it's got the --line-buffered option, which will help you see output quicker