Why no output is shown when using grep twice?

Question

Basically I'm wondering why this doesn't output anything:

tail --follow=name file.txt | grep something | grep something_else 

You can assume that it should produce output I have run another line to confirm

cat file.txt | grep something | grep something_else

It seems like you can't pipe the output of tail more than once!? Anyone know what the deal is and is there a solution?

EDIT: To answer the questions so far, the file definitely has contents that should be displayed by the grep. As evidence if the grep is done like so:

tail --follow=name file.txt | grep something

Output shows up correctly, but if this is used instead:

tail --follow=name file.txt | grep something | grep something

No output is shown.

If at all helpful I am running ubuntu 10.04

Solution 1

You might also run into a problem with grep buffering when inside a pipe. ie, you don't see the output from

   tail --follow=name file.txt | grep something > output.txt

since grep will buffer its own output.

Use the --line-buffered switch for grep to work around this:

tail --follow=name file.txt | grep --line-buffered something > output.txt

This is useful if you want to get the results of the follow into the output.txt file as rapidly as possible.

Solution 2

Figured out what was going on here. It turns out that the command is working it's just that the output takes a long time to reach the console (approx 120 seconds in my case). This is because the buffer on the standard out is not written each line but rather each block. So instead of getting every line from the file as it was being written I would get a giant block every 2 minutes or so.

It should be noted that this works correctly:

tail file.txt | grep something | grep something

It is the following of the file with --follow=name that is problematic.

For my purposes I found a way around it, what I was intending to do was capture the output of the first grep to a file, so the command would be:

tail --follow=name file.txt | grep something > output.txt

A way around this is to use the script command like so:

script -c 'tail --follow=name file.txt | grep something' output.txt

Script captures the output of the command and writes it to file, thus avoiding the second pipe.

This has effectively worked around the issue for me, and I have explained why the command wasn't working as I expected, problem solved.

FYI, These other stackoverflow questions are related:
Trick an application into thinking its stdin is interactive, not a pipe
Force another program's standard output to be unbuffered using Python

Comments

[phuclv]

How to 'grep' a continuous stream?

[radman]

Tried this and it actually works for me too, however the --follow=name part is vital to what I want to achieve

[Martin C.]

Of course it works without the follow part, because this is caused by the buffering, which is flushed once the grep (without follow) exits.

[radman]

Just checked back on this and checked out your solution. Much cleaner and precise than my workaround, marked as correct solution.

[radman]

of course, but the combination with tail is the important part of this problem...