Using the top command with redirection works fine:
top > top.log
Now I want to use grep to filter a certain line:
top | grep "my_program" > top.log
But the log file will remain empty. But grep delivers an output when using
top | grep "my_program"
Where my_program has to be replaced by a running program to see some output.
Why does my approach not work ? And how can I fix it ?
I get the same behavior that you describe. On Ubuntu 11.10
top | grep "my_program" > top.log
does not produce any output.
I believe the reason for this is that grep is buffering its output. To tell GNU grep to spit out output line-by-line, use the --line-buffered option:
top | grep --line-buffered "my_program" > top.log
See also this SO question for other potential solutions.
--line-buffered solves the problem.
-b option is still a good advice from Lev Levitsky, too. That solved some encoding problems with the log file.
you should use:
top -n 1 | grep "blah" > top.log
the "-n 1" runs top for one iteration and then quits instead of continually updating every few seconds
since you're just looking for one line though ps would be a better tool to use.
My workaround for this problem was:
while :;do top -b -n 1 | grep "my_program" >> top.log;done &
By this way I could have a running monitor in the background for my_program and keep all results in the file top.log.
Although both work for me, I think Lev Levitsky's advice is the correct one. Use the -b argument.
There is a good chance that output redirection is the problem and that you're not getting anything through stdout, so try this instead:
top -b 2>&1 | grep "my_program" > top.log
Do note that you might have problems with output buffering aswell. Your shell won't constantly write to the file so it could take a while for top.log to fill.
-boption totopor usingpsinstead.-bdid not solve my problem, but solved some encoding problems. Thank you.