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?
>> /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.
Let's break >> /dev/null 2>&1 statement into parts:
Part 1: >> output redirection
This is used to redirect the program output and append the output at the end of the file. More...
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/nullaccepts and discards all input; produces no output (always returns an end-of-file indication on a read). Reference: Wikipedia
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
This is the way to execute a program quietly, and hide all its output.
/dev/null is a special filesystem object that discards everything written into it. Redirecting a stream into it means hiding your program's output.
The 2>&1 part means "redirect the error stream into the output stream", so when you redirect the output stream, error stream gets redirected as well. Even if your program writes to stderr now, that output would be discarded as well.
Let me explain a bit by bit.
0: standard input
1: standard output
2: standard error
>> in command >> /dev/null 2>&1 appends the command output to /dev/null.
command
=> 1 output on the terminal screen
=> 2 output on the terminal screen
command >> /dev/null
=> 1 output to /dev/null
=> 2 output on the terminal screen
/dev/null 2>&1command >> /dev/null 2>&1
=> 1 output to /dev/null
=> 2 output is redirected to 1 which is now to /dev/null
/dev/null is a standard file that discards all you write to it, but reports that the write operation succeeded.
1 is standard output and 2 is standard error.
2>&1 redirects standard error to standard output. &1 indicates file descriptor (standard output), otherwise (if you use just 1) you will redirect standard error to a file named 1. [any command] >>/dev/null 2>&1 redirects all standard error to standard output, and writes all of that to /dev/null.
I use >> /dev/null 2>&1 for a silent cronjob. A cronjob will do the job, but not send a report to my email.
As far as I know, don't remove /dev/null. It's useful, especially when you run cPanel, it can be used for throw-away cronjob reports.
As described by the others, writing to /dev/null eliminates the output of a program. Usually cron sends an email for every output from the process started with a cronjob. So by writing the output to /dev/null you prevent being spammed if you have specified your adress in cron.
instead of using >/dev/null 2>&1 Could you use : wget -O /dev/null -o /dev/null example.com
what i can see on the other forum it says. "Here -O sends the downloaded file to /dev/null and -o logs to /dev/null instead of stderr. That way redirection is not needed at all."
and the other solution is : wget -q --spider mysite.com
https://serverfault.com/questions/619542/piping-wget-output-to-dev-null-in-cron/619546#619546
