Discard output in Bash

Output Redirection >|>>

This is used to redirect the program output and append the output at the end of the file.

> redirects output to a file, overwriting the file.

>> redirects output to a file appending the redirected output at the end.

The >> seems sort of superfluous, since >> means append while > means truncate and write, and either appending to or writing to /dev/null has the same net effect. I usually just use > for that reason.

Special File /dev/null

This is a Pseudo-devices special file.

Command ls -l /dev/null will give you details of this file:


crw-rw-rw-. 1 root root 1, 3 Mar 20 18:37 /dev/null

Did you observe crw?

Which means it is a pseudo-device file which is of character-special-file type that provides serial access.

/dev/null accepts and discards all input; produces no output (always returns an end-of-file indication on a read). 

File Descriptor 2>&1

2>&1 redirects standard error (2) to standard output (1), which then discards it as well since standard output has already been redirected.

Whenever you execute a program, the operating system always opens three files, standard input, standard output, and standard error as we know whenever a file is opened, the operating system (from kernel) returns a non-negative integer called a file descriptor. The file descriptor for these files are 0, 1, and 2, respectively.

So 2>&1 simply says redirect standard error to standard output.

& means whatever follows is a file descriptor, not a filename.

In short, by using this command you are telling your program not to shout while executing.

What is the importance of using 2>&1?

If you don’t want to produce any output, even in case of some error produced in the terminal.

To explain more clearly, let’s consider the following example:


$ ls -l > /dev/null

For the above command, no output was printed in the terminal, but what if this command produces an error:


$ ls -l file_doesnot_exists > /dev/null

ls: cannot access file_doesnot_exists: No such file or directory

Despite I’m redirecting output to /dev/null, it is printed in the terminal.

It is because we are not redirecting error output to /dev/null, so in order to redirect error output as well, it is required to add 2>&1:


$ ls -l file_doesnot_exists > /dev/null 2>&1

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