How to asynchronously launch external program from cli and discard its output?

Question

I'm writing a simple desktop initiation script which waits for disk idle, and then launches next external program (like Firefox, Skype or conky) using &, like:

ps cax | grep conky > /dev/null
if [ $? -eq 0 ]; then
  echo "Conky is already running."
else
  wait-for-disk-idle sda
  conky &
fi

That's easy.

The problem is that some programs spew a lot of debug output to the terminal, which gets mixed with the messages produced by my initialization script.

The question: Is there any way to asynchronously launch an external program so that its standard output is discarded?

What I already tried:

• conky & >/dev/null 2>/dev/null
• bash -c conky &

---

The correct answer:

bash -c "conky >/dev/null 2>/dev/null &"

Answer 1

You probably want to discard any STDERR output as well. You can do both like so:

conky > /dev/null 2>&1 &

This statement essentially tells the shell to do the following:

1. conky > /dev/null - redirect all standard output to /dev/null
2. 2>&1 - Redirect standard error to where standard output is currently pointing. Because of the previous redirect, you standard output is pointing to /dev/null, so standard error will follow.
3. & - Run this in a sub-shell (background).
4. Thanks to @alexis for pointing out that my description for & wasn't quite precise:

   (a) Every external command must be run after a (v)fork, not just background processes. (b) Backgrounded processes are not run in a sub-shell, but executed immediately after the fork. The real difference when a process is backgrounded is that the invoking shell doesn't immediately wait(2) for it (but prints a prompt and awaits user input).

When redirecting output, Bash reads the redirects in order, from left to right. Bash also treats the '&' as a command separator, which can be used anywhere ';' would normally be used. What you were doing was telling bash

1. Run conky in the background. More specifically, fork the process and run it in a sub-shell, asynchronously, and return control of the terminal to the user.
2. Bash considers this a new command - this is the same as running >/dev/null on the prompt with nothing preceding the redirect. Nothing happens.
3. Redirect the standard error to /dev/null from the nonexistent command.

Answer 2

Instead of

conky & >/dev/null 2>/dev/null

Use:

conky >/dev/null 2>/dev/null &

Explanation: Because '&' serves as a statement separator, the former is really two commands. It works like this:

conky &
>/dev/null 2>/dev/null

The first command runs conky in the background but does not redirect its output. The second redirects the output on a nonexistent command. To redirect conky's output, you need to run:

conky >/dev/null 2>/dev/null

To redirect conky's output and also run conky in the background, use:

conky >/dev/null 2>/dev/null &