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 ass

Answer 1

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

Answer 2

grep pattern filename | grep pattern | grep pattern | grep pattern ......

Answer 3

works for me on Mac without --follow=name

bash-3.2$ tail delme.txt | grep po
position.bin
position.lrn
bash-3.2$ tail delme.txt | grep po | grep lr
position.lrn

Answer 4

You do know that tail starts by default with the last ten lines of the file? My guess is everything the cat version found is well into the past. Try tail -n+1 --follow=name file.txt to start from the beginning of the file.

Answer 5

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.