04-24-2008
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
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FIFO(7) Linux Programmer's Manual FIFO(7)
NAME
fifo - first-in first-out special file, named pipe
DESCRIPTION
A FIFO special file (a named pipe) is similar to a pipe, except that it is accessed as part of the file system. It can be opened by multi-
ple processes for reading or writing. When processes are exchanging data via the FIFO, the kernel passes all data internally without writ-
ing it to the file system. Thus, the FIFO special file has no contents on the file system; the file system entry merely serves as a refer-
ence point so that processes can access the pipe using a name in the file system.
The kernel maintains exactly one pipe object for each FIFO special file that is opened by at least one process. The FIFO must be opened on
both ends (reading and writing) before data can be passed. Normally, opening the FIFO blocks until the other end is opened also.
A process can open a FIFO in nonblocking mode. In this case, opening for read only will succeed even if no-one has opened on the write
side yet, opening for write only will fail with ENXIO (no such device or address) unless the other end has already been opened.
Under Linux, opening a FIFO for read and write will succeed both in blocking and nonblocking mode. POSIX leaves this behavior undefined.
This can be used to open a FIFO for writing while there are no readers available. A process that uses both ends of the connection in order
to communicate with itself should be very careful to avoid deadlocks.
NOTES
When a process tries to write to a FIFO that is not opened for read on the other side, the process is sent a SIGPIPE signal.
FIFO special files can be created by mkfifo(3), and are indicated by ls -l with the file type 'p'.
SEE ALSO
mkfifo(1), open(2), pipe(2), sigaction(2), signal(2), socketpair(2), mkfifo(3), pipe(7)
COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-12-03 FIFO(7)