conditional redirection in bash - bash

Question

I have a bash script that I want to be quiet when run without attached tty (like from cron).
I now was looking for a way to conditionally redirect output to /dev/null in a single line.
This is an example of what I had in mind, but I will have many more commands that do output in the script
#!/bin/bash
# conditional-redirect.sh
if tty -s; then
REDIRECT=
else
REDIRECT=">& /dev/null"
fi
echo "is this visible?" $REDIRECT
Unfortunately, this does not work:
$ ./conditional-redirect.sh
is this visible?
$ echo "" | ./conditional-redirect.sh
is this visible? >& /dev/null
what I don't want to do is duplicate all commands in a with-redirection or with-no-redirection variant:
if tty -s; then
echo "is this visible?"
else
echo "is this visible?" >& /dev/null
fi
EDIT:
It would be great if the solution would provide me a way to output something in "quiet" mode, e.g. when something is really wrong, I might want to get a notice from cron.

Answer 1

For bash, you can use the line:
exec &>/dev/null
This will direct all stdout and stderr to /dev/null from that point on. It uses the non-argument version of exec.
Normally, something like exec xyzzy would replace the program in the current process with a new program but you can use this non-argument version to simply modify redirections while keeping the current program.
So, in your specific case, you could use something like:
tty -s
if [[ $? -eq 1 ]] ; then
exec &>/dev/null
fi
If you want the majority of output to be discarded but still want to output some stuff, you can create a new file handle to do that. Something like:
tty -s
if [[ $? -eq 1 ]] ; then
exec 3>&1 &>/dev/null
else
exec 3>&1
fi
echo Normal # won't see this.
echo Failure >&3 # will see this.

Answer 2

I found another solution, but I feel it is clumsy, compared to paxdiablo's answer:
if tty -s; then
REDIRECT=/dev/tty
else
REDIRECT=/dev/null
fi
echo "Normal output" &> $REDIRECT

Answer 3

You can use a function:
function the_code {
echo "is this visible?"
# as many code lines as you want
}
if tty -s; then # or other condition
the_code
else
the_code >& /dev/null
fi

Answer 4

This works well for me. If DUMP_FILE is empty things go to stdout otherwise to the file. It does the job without using explicit redirection, but just uses pipes and existing applications.
function stdout_or_file
{
local DUMP_FILE=${1:-}
if [ -z "${DUMP_FILE}" ]; then
cat
else
sed -n "w ${DUMP_FILE}"
fi
}
function foo()
{
local MSG=$1
echo "info: ${MSG}"
}
foo "bar" | stdout_or_file ${DUMP_FILE}
Of course, you can squeeze this also in one line
foo "bar" | if [ -z "${DUMP_FILE}" ]; then cat; else sed -n "w ${DUMP_FILE}"; fi
Besides sed -n "w ${DUMP_FILE}" another command that does the same is dd status=none of=${DUMP_FILE}

Answer 5

The simplest solution is to use eval (a shell builtin), as it will act on the redirection in the expanded variable... and also act on anything else in the command line, so add extra quoting as required (note the extra single quotes added around the echo string below due to the '?' which would otherwise cause shell filename expansion to be attempted).
#!/bin/bash
# conditional-redirect.sh
if tty -s; then
REDIRECT=
else
REDIRECT=">& /dev/null"
fi
eval echo '"is this visible?"' $REDIRECT