What is /dev/null 2>&1?

Question

I found this piece of code in /etc/cron.daily/apf

#!/bin/bash   /etc/apf/apf -f >> /dev/null 2>&1   /etc/apf/apf -s >> /dev/null 2>&1   

It's flushing and reloading the firewall.
I don't understand the >> /dev/null 2>&1 part.

What is the purpose of having this in the cron? It's overriding my firewall rules. Can I safely remove this cron job?

Answer 1

>> /dev/null redirects standard output (stdout) to /dev/null, which discards it.

(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.)

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

Answer 2

Let's break >> /dev/null 2>&1 statement into parts:

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Part 1: >> output redirection

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

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Part 2: /dev/null special file

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). Reference: Wikipedia

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Part 3: 2>&1 (Merges output from stream 2 with stream 1)

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