HI all,

I want to capture cpu data in batch mode of "top" command and redirect to a file like this:

top -b > cpu.dat

top -b | grep ^Cpu >cpu.dat

Hard to explain if we do not know input data.
Anyway - 'grep ^Cpu' should work. Empty output file tells only that there are no lines with word "Cpu" on the beggining of a line.

hi,
the direct answer for your 'why' is
->the output is not going to std out it is going to std err


secondly how to accomplish it
->method 1
$top 2>outputOfTop
or
$top >& outputOfTop

then
$grep pattern outputOfTop

->method 2
open two terminal term1 and term2

in term1
$mkfifo pipe
$top 2>pipe

in term2
$grep pattern pipe

make sure the working directory is same in both the terminal
i hope this should work

Best Regards,
Rakesh UV

uvrakesh: if "top -b >cpu.dat" worked then the output was not being written to stderr.

My copy of top (Ubuntu Linux 7.10) writes to stdout. But it keeps on looping even with the -b option, so I don't have exactly the same version.

fongthai: if top -b >cpu.dat works then does grep ^Cpu cpu.dat work? That would be a workaround at least.

Yes, I got them.
I don't want to get data first then using grep, because it make output file become a huge file. I want to run top -b for one or two days, that's the issue.

If you run it without redirection, do you see the results on your screen?

If you terminate it with ctrl-c, the output might still be buffered somewhere (weak hunch, might be wrong) -- if you let it run for long enough, does the output become visible?

Tested here; looks like I'm right. If I hit ctrl-c after five seconds (enough to produce about two matching lines, I guess) the output file is empty. If I run it for a minute or so, I still don't get output in the file. Then all of a sudden, lots of output. It's output buffering at play.

If you have grep --line-buffered, try with that, just to see that it works. If you run it for long enough, you probably want to take it out again, though.