How do I conditionally redirect the output of a command to /dev/null?

I have a script. I would like to give 开发者_JAVA百科this script a quiet mode and a verbose mode.

This is the equivalent of:

if $verbose
then
  redirect="> /dev/null"
fi

echo "Verbose mode enabled" $redirect # This doesn't work because the redirect isn't evaluated.

I'd really like a better way of doing this than writing if-elses for every statement affected.

eval could work, but has obvious side effects on other variables.

You could write a wrapper function:

redirect_cmd() {
    # write your test however you want; this just tests if SILENT is non-empty
    if [ -n "$SILENT" ]; then
        "$@" > /dev/null
    else
        "$@"
    fi
}

You can then use it to run any command with the redirect:

redirect_cmd echo "unsilenced echo"
redirect_cmd ls -d foo*

SILENT=1
redirect_cmd echo "nothing will be printed"
redirect_cmd touch but_the_command_is_still_run

(If all you need to do is echo with this, you can of course make the function simpler, just echoing the first argument instead of running them all as a command)

Got the idea from another question:

#!/bin/sh

if [ $SILENT ]; then
    exec &>/dev/null
fi

echo "Silence here."

Not perfect, but how about setting redirect to either "/dev/null" or "/dev/tty", and then doing

{
   echo "verbose"
    ....
} > $redirect

Consider whether it would be worth setting set -x for detailed logging to stderr in verbose mode. If that's so, then verbose-only output can be achieved with a no-op : like this.

while getopts "v" o
do case "$o" in
   v) set -x;;
   esac
done

echo "This will always be output"           # goes to stdout
: this will only be output in verbose mode  # goes to stderr

: evaluates it's arguments but does nothing with them. set -x will show what was evaluated on stderr as each statement is executed.

It also lets you split verbose and standard logs by stream.

Might not be what you need here, but it can be a handy trick.