SH conditional redirection

Question

I want to be able to redirect some output of my script to /dev/null based on a command line switch. I don't know how should I do it.

In a silly way, it would be something like this (in a too simplified way):

#!/bin/sh

REDIRECT=

if [ $# -ge 1 -a "$1" = "--verbose" ]; then
    echo    "Verbose mode."
    REDIRECT='1>&2 > /dev/null'
fi

echo "Things I want to see regardless of my verbose switch."

#... Other things...

# This command and others along the script should only be seen if I am in verbose mode.
ls -l $REDIRECT

Any clues, please?

Thanks people.

Looks like http://stackoverflow.com/questions/8756535/conditional-redirection-in-bash has the answer you want, more-or-less — DerfK, Aug 06 '12 at 20:46
You are right. This link gives me an answer. Sorry for not being able to find it before. Really thanks. — j4x, Aug 07 '12 at 11:34

score 7 · Accepted Answer · edited Mar 17 '16 at 12:48

Tie STDOUT to another handle if you are in verbose mode, otherwise link those handles to /dev/null. Then write your script so the optional stuff points to the extra handles.

#!/bin/sh

exec 6>/dev/null

if [ $# -ge 1 -a "$1" = "--verbose" ]; then
echo    "Verbose mode."
exec 6>&1
fi

echo "Things I want to see regardless of my verbose switch."

#... Other things...

# This command and others along the script should only be seen if I am in verbose mode.
ls -l >&6 2>&1

That should get you started. I'm not sure if that is BASH specific or not. It was just a memory of long ago. ;-)

DerfK · Answer 2 · 2012-08-06T20:29:08.623

4

I don't know about sh but in bash (not the same!) you'll need to use eval:

$ x='> foo'
$ echo Hi $x
Hi > foo
$ eval echo Hi $x
$ cat foo
Hi

edited Aug 06 '12 at 20:29

answered Aug 06 '12 at 19:21

DerfK

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I don't need it to go backward. I just want to have the evaluation going asortedly on across my script without need to `ifing` or `evaling` everywhere. This script is meant to run on a constrainded embedded device where scripts go really slow. I feel it must be a trick for this. Thanks! – j4x Aug 06 '12 at 19:33

score 0 · Answer 3 · answered Aug 06 '12 at 20:32

0

I believe your test is backwards. You want to redirect to /dev/null when not in verbose mode:

if [ $# -ge 1 -a "$1" = "--verbose" ]; then
    echo    "Verbose mode."
else
    REDIRECT='2>&1 >/dev/null'
fi

answered Aug 06 '12 at 20:32

Michael Hampton

237,123
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477
940

SH conditional redirection

3 Answers3