There are, by default, three "standard" files open when you run a program, standard input (
stdin), standard output (
stdout), and standard error (
stderr). In Unix, those are associated with "file descriptors" (
stdin = 0,
stdout = 1,
stderr = 2). By default, all three are associated with the device that controls your terminal (the interface through which you see content on screen and type input into the program).
The shell gives you the ability to "redirect" file descriptors. The
> operator redirects output; the
< redirects input.
program_name > /dev/null
Which is equivalent to:
program_name 1> /dev/null
Redirects the output to file descriptor 1 ('stdout') to `/dev/null'
program_name 2> /dev/null
Redirects the output to file descriptor 2 ('stderr') to '/dev/null'
You might want to redirect both
stderr to a single file, so you might think you'd do:
program_name > /dev/null 2> /dev/null
But that doesn't handle interleaving writes to the file descriptors (the details behind this are a different question). To address this, you can do:
program_name > /dev/null 2>&1
Which says "redirect writes to file descriptor 1 to
/dev/null and redirect writes to file descriptor 2 to the same place as the writes to file descriptor 1 are going". This handles interleaving writes to the file descriptors.
That option is so common that some shells include a short-hand that is shorter and functionally equivalent:
program_name &> /dev/null
Finally, you can redirect input in the same way that you redirect output:
program_name < /dev/null
Will redirect file descriptor 0 (
/dev/null, so if the program tries to read input, it'll get EOF.
Putting that all together:
program_name </dev/null &>/dev/null &
Say (1) run
program_name, (2) redirect standard input from
</dev/null), (3) redirect both file descriptors 1 and 2 (
&>/dev/null), and (4) run the program in the background (