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.

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

19,826

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

252,907

SH conditional redirection

3 Answers3