Piping tail output though grep twice

bash shell unix grep tail
Craig Francis picture Craig Francis · Dec 13, 2012 · Viewed 8.4k times · Source

Going with a typical Apache access log, you can run:

tail -f access_log | grep "127.0.0.1"

Which will only show you the logs (as they are created) for the specified IP address.

But why does this fail when you pipe it though grep a second time, to further limit the results?

For example, a simple exclude for ".css":

tail -f access_log | grep "127.0.0.1" | grep -v ".css"

won't show any output.

Answer

Shawn Chin picture Shawn Chin · Dec 13, 2012

I believe the problem here is that the first grep is buffering the output which means the second grep won't see it until the buffer is flushed.

Try adding the --line-buffered option on your first grep:

tail -f access_log | grep --line-buffered "127.0.0.1" | grep -v ".css"

For more info, see "BashFAQ/009 -- What is buffering? Or, why does my command line produce no output: tail -f logfile | grep 'foo bar' | awk ..."

Related questions

How to store command results in a variable in on shellscripting?

I want to find out number of directories and files in home directory and want to store count some variable in a shell script. I am using following set of commands. command="ls -l | grep -c \"rahul.*patle\"" eval $command …

Read More
Waiting for background processes to finish before exiting script

How do I make sure that all my background processes have finished execution before I exit my script (TCL/Bash). I was thinking of writing all my background process pids to a pidfile. And then at the end pgrep the …

Read More
How can I check if a directory exists in a Bash shell script?

What command can be used to check if a directory exists or not, within a Bash shell script?

Read More