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Top Forums Shell Programming and Scripting Conditional output redirection Post 302972933 by rovf on Thursday 12th of May 2016 02:15:42 AM
Old 05-12-2016
You can't have your cake and eat it. The exit code of a program is available when the program is finished running. The output redirection has to be done before the program produces its output.

You could try to write a wrapper around your program, which catches it standard output internally, and after the program is executed, decides what to do with it. Such a wrapper could, for example, be written with `expect`, if the output is large, or you can stuff it into a shell variable. Of course in both cases it means that the interim output is stored somewhere, be it in a variable or in a file.
 

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ANNOTATE-OUTPUT(1)                                            General Commands Manual                                           ANNOTATE-OUTPUT(1)

NAME
annotate-output - annotate program output with time and stream SYNOPSIS
annotate-output [options] program [args ...] DESCRIPTION
annotate-output will execute the specified program, while prepending every line with the current time and O for stdout and E for stderr. OPTIONS
+FORMAT Controls the timestamp format, as per date(1). Defaults to "%H:%M:%S". -h, --help Display a help message and exit successfully. EXAMPLE
$ annotate-output make 21:41:21 I: Started make 21:41:21 O: gcc -Wall program.c 21:43:18 E: program.c: Couldn't compile, and took me ages to find out 21:43:19 E: collect2: ld returned 1 exit status 21:43:19 E: make: *** [all] Error 1 21:43:19 I: Finished with exitcode 2 BUGS
Since stdout and stderr are processed in parallel, it can happen that some lines received on stdout will show up before later-printed stderr lines (and vice-versa). This is unfortunately very hard to fix with the current annotation strategy. A fix would involve switching to PTRACE'ing the process. Giving nice a (much) higher priority over the executed program could however cause this behaviour to show up less frequently. The program does not work as well when the output is not linewise. In particular, when an interactive program asks for input, the question might not be shown until after you have answered it. This will give the impression that the annotated program has hung, while it has not. SEE ALSO
date(1) SUPPORT
This program is community-supported (meaning: you'll need to fix it yourself). Patches are however appreciated, as is any feedback (posi- tive or negative). AUTHOR
annotate-output was written by Jeroen van Wolffelaar <jeroen@wolffelaar.nl> This manpage comes under the same copyright as annotate-output itself, read /usr/bin/annotate-output (or wherever you install it) for the details. DEBIAN Debian Utilities ANNOTATE-OUTPUT(1)
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