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Full Discussion: Can I pipe stderr to another process
Top Forums Shell Programming and Scripting Can I pipe stderr to another process Post 302414882 by alister on Wednesday 21st of April 2010 07:51:00 AM
Old 04-21-2010
In what folllows, it's important to realize that pipe redirection occurs before any other redirections specified by a command.

You can redirect stderror into a pipe thusly:
Code:
$ command="date; twenty-four=24"
$ eval $command 2>&1 | sed 's/^/stdout and stderr from pipe: /'
stdout and stderr from pipe: Wed Apr 21 07:59:39 EDT 2010
stdout and stderr from pipe: -bash: twenty-four=24: command not found

Note, however, that standard output (from the date command) is still going into that pipe as well. If you don't want stdout to go into the pipe, and instead go to whereever stdout was going before the pipe redirection, before executing the eval pipeline, you can save the original, pre-pipe stdout destination in a file descriptor.

Code:
$ exec 3>&1
$ eval $command 2>&1 1>&3 | sed 's/^/stderr from pipe: /'
Wed Apr 21 08:00:13 EDT 2010
stderr from pipe: -bash: twenty-four=24: command not found

Note that the date command's output did not go through sed, but directly to the terminal, in essence, the usual stdout/stderr behavior of a pipe redirection have been flipped.

Also, it's always a good idea to close file descriptors when they're no longer needed (particularly when using an interactive shell or a long-lived shell-script):
Code:
$ exec 3>&-

Regards,
Alister
Last edited by alister; 04-21-2010 at 09:00 AM..
 

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LEARN ABOUT DEBIAN

rngtest

RNGTEST(1)						      General Commands Manual							RNGTEST(1)

NAME

       rngtest - Check the randomness of data using FIPS 140-2 tests

SYNOPSIS

       rngtest [-c n | --blockcount=n] [-b n | --blockstats=n] [-t n | --timedstats=n] [-p | --pipe] [-?] [--help] [-V] [--version]

DESCRIPTION

       rngtest works on blocks of 20000 bits at a time, using the FIPS 140-2 (errata of 2001-10-10) tests to verify the randomness of the block of
       data.

       It takes input from stdin, and outputs statistics to stderr, optionally echoing blocks that passed the FIPS tests to stdout (when operating
       in pipe mode).  Errors are sent to stderr.

       At  startup,  rngtest will throw away the first 32 bits of data when operating in pipe mode.  It will use the next 32 bits of data to boot-
       strap the FIPS tests (even when not operating in pipe mode).  These bits are not tested for randomness.

       Statistics are dumped to stderr when the program exits.

OPTIONS

       -p, --pipe
	      Enable pipe mode.  All data blocks that pass the FIPS tests are echoed to stdout, and rngtest operates in silent mode.

       -c n, --blockcount=n (default: 0)
	      Exit after processing n input blocks, if n is not zero.

       -b n, --blockstats=n (default: 0)
	      Dump statistics every n blocks, if n is not zero.

       -t n, --timedstats=n (default: 0)
	      Dump statistics every n secods, if n is not zero.

       -?, --help
	      Give a short summary of all program options.

       -V, --version
	      Print program version

STATISTICS

       rngtest will dump statistics to stderr when it exits, and when told to by blockstats or timedstats.

       FIPS 140-2 successes and FIPS 140-2 failures counts the number of 20000-bit blocks either accepted or rejected by  the  FIPS  140-2  tests.
       The  other  statistics  show  a	breakdown of the FIPS 140-2 failures by FIPS 140-2 test.  See the FIPS 140-2 document for more information
       (note that these tests are defined on FIPS 140-1 and FIPS  140-2  errata  of  2001-10-10.  They	were  removed  in  FIPS  140-2	errata	of
       2002-12-03).

       The speed statistics are taken for every 20000-bit block trasferred or processed.

EXIT STATUS

       0 if no errors happen, and no blocks fail the FIPS tests.

       1 if no errors happen, but at least one block fails the FIPS tests.

       10 if there are problems with the parameters.

       11 if an input/output error happens.

       12 if an operating system or resource starvation error happens.

SEE ALSO

       random(4), rngd(8)

       FIPS PUB 140-2 Security Requirements for Cryptographic Modules, NIST,
	      http://csrc.nist.gov/cryptval/140-2.htm

AUTHORS

       Henrique de Moraes Holschuh <hmh@debian.org>

rng-tools 2-unofficial-mt.14					    March 2004								RNGTEST(1)
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