linux-silencing-bash-output

When executing a Bash command, a new process is created. Any errors from
this process are written to the error stream, and any other output is
written to the output stream.

Bash also automatically opens multiple files for each process, denoted
by a numeric File Descriptor (FD).

Two of these FDs are Standard output (stdout) and standard error
(stderr). By default, Bash directs the error stream to stderr and the
output stream to stdout. For both stdout and stderr, any characters
written to these FDs are displayed in the console where the command was
executed.

Additionally, Bash assigns the identifier 1 for the stdout FD and 2
for the stderr FD.

We can change the default destination of error and output streams by
redirecting the output of our command to another FD. We redirect output
using the
redirection
operator: >.

For example, we can write Hello world to the file foo.txt using the
echo command:

Additionally, Linux systems have a specific device, /dev/null, that
does nothing when written to. Therefore, output redirected
to /dev/null will not be written anywhere.

To silence non-error output, we redirect stdout to /dev/null:

By default, the redirection operator redirects stdout so we can omit the
1:

To silence error output, we redirect stderr to /dev/null:

To redirect both stdout and stderr, we must redirect stderr to stdout
and then redirect stdout to /dev/null. To redirect stderr to stdout,
we use the following notation:

We combine this with redirecting stdout to /dev/null to silence all
output:

Thus, stdout is redirected to /dev/null and stderr is redirected to
stdout, causing both streams to be written to /dev/null and silencing
all output from our command.

We can shorten this to the following Bash notation:

Note that this shorthand is not portable and is only supported by Bash
4 or higher.

Although not common, we can also separately redirect stdout and stderr
to /dev/null, but we do not suggest this approach unless we are
auto-generating a Bash script or the previous approaches cannot be used: