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Re^3: capturing stderr of echo piped to a file

by Crackers2 (Parson)
on Oct 29, 2014 at 19:17 UTC ( #1105535=note: print w/replies, xml ) Need Help??


in reply to Re^2: capturing stderr of echo piped to a file
in thread capturing stderr of echo piped to a file

Perhaps you should just try it?

> { echo out; echo err >&2; } >f1 2>&1 > cat f1 out err > { echo out; echo err >&2; } 2>&1 >f1 err > cat f1 out

Redirection order definitely DOES matter in the shell. (Hmm, unless it's shell-specific? I'm using bash)

And just to do exactly what you said..

> perl -e'print "print\n"; warn "warn"' >f1 2>&1 > cat f1 warn at -e line 1. print > perl -e'print "print\n"; warn "warn"' 2>&1 >f1 warn at -e line 1. > cat f1 print

In the first case both print and warn go to the file, in the second case warn goes to stdout and print to the file.

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Re^4: capturing stderr of echo piped to a file
by boftx (Deacon) on Oct 29, 2014 at 19:26 UTC
    ~/stuff > perl foobar.pl Normal STDOUT Going to STDERR ~/stuff > echo 'Just a starting point > ' >JGBtest.txt ~/stuff > cat JGBtest.txt Just a starting point ~/stuff > perl foobar.pl >>JGBtest.txt 2>&1 ~/stuff > cat JGBtest.txt Just a starting point Going to STDERR Normal STDOUT ~/stuff > cat foobar.pl #!/usr/bin/perl use warnings; use strict; print "Normal STDOUT\n"; warn "Going to STDERR\n"; exit; __END__ ~/stuff >
    You must always remember that the primary goal is to drain the swamp even when you are hip-deep in alligators.

      You didn't run the test with reversed redirection, so the above doesn't actually show anything new. Here's what I get with your code, including also a reversed order test:

      ~> perl foobar.pl Normal STDOUT Going to STDERR ~> echo 'Just a starting point > > ' >JGBtest.txt ~> cat JGBtest.txt Just a starting point > ~> perl foobar.pl >>JGBtest.txt 2>&1 ~> cat JGBtest.txt Just a starting point > Going to STDERR Normal STDOUT ~> perl foobar.pl 2>&1 >>JGBtest.txt Going to STDERR ~> cat foobar.pl #!/usr/bin/perl use warnings; use strict; print "Normal STDOUT\n"; warn "Going to STDERR\n"; exit; __END__ ~>

      Which again shows that the redirection order does matter, and in the 2>&1 >>file case, STDERR goes to the output, not the file.

      Update: And just to illustrate it in one more way...

      ~> perl foobar.pl >>JGBtest.txt 2>&1 & [1] 22571 ~> ll /proc/22571/fd total 0 lrwx------ 1 user group 64 2014-10-29 16:31 0 -> /dev/pts/11 l-wx------ 1 user group 64 2014-10-29 16:31 1 -> /home/user/JGBtest.tx +t l-wx------ 1 user group 64 2014-10-29 16:31 2 -> /home/user/JGBtest.tx +t ~> perl foobar.pl 2>&1 >>JGBtest.txt & [2] 22578 ~> ll /proc/22578/fd total 0 lrwx------ 1 user group 64 2014-10-29 16:32 0 -> /dev/pts/11 l-wx------ 1 user group 64 2014-10-29 16:32 1 -> /home/user/JGBtest.tx +t lrwx------ 1 user group 64 2014-10-29 16:32 2 -> /dev/pts/11 ~> cat foobar.pl #!/usr/bin/perl sleep(100);

        Okay I stand corrected in that order matters. That said, I honestly can't remember ever seeing 2>&1 not coming at the end of the command line, probably because doing so seems to indicate that it won't be captured to STDOUT as one desires.

        ~/stuff > cat JGBtest.txt Just a starting point ~/stuff > perl foobar.pl 2>&1 >>JGBtest.txt Going to STDERR ~/stuff > cat JGBtest.txt Just a starting point Normal STDOUT ~/stuff > perl foobar.pl 2>&1 >>JGBtest.txt | grep STD Going to STDERR ~/stuff > perl foobar.pl 2>&1 | grep STD Going to STDERR Normal STDOUT ~/stuff >

        With all of the foregoing comments by both you and me in mind, I think you're first comment implies that it must come first in order to be captured. But in the interest of being complete even if I'm wrong, I included the final tests that does actually demonstrate what is happening on the command line (though it is not intuitive.) Also, it is possible that setting $| = 1; might alter the output ordering.

        You must always remember that the primary goal is to drain the swamp even when you are hip-deep in alligators.

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