What does '>/dev/null 2>&1' mean in this article of crontab basics?

> is for redirect

/dev/null is a black hole where any data sent, will be discarded

2 is the file descriptor for Standard Error

> is for redirect

& is the symbol for file descriptor (without it, the following 1 would be considered a filename)

1 is the file descriptor for Standard Out

Therefore >/dev/null 2>&1 redirects the output of your program to /dev/null. Include both the Standard Error and Standard Out.

Much more information is available at The Linux Documentation Project's I/O Redirection page.

cron will only email you if there is some output from you job. With everything redirected to null, there is no output and hence cron will not email you.

/dev/null is a device file that acts like a blackhole. Whatever that is written to it, get discarded or disappears. When you run a script that gives you an output and if we add a > /dev/null 2>&1 at the end of the script, we are asking the script to write whatever that is generated from the script (both the output and error messages) to /dev/null.

To break it up:

• 2 is the handle for standard error or STDERR
• 1 is the handle for standard output or STDOUT

2>&1 is asking to direct all the STDERR as STDOUT, (ie. to treat all the error messages generated from the script as its standard output). Now we already have > /dev/null at the end of the script which means all the standard output (STDOUT) will be written to /dev/null. Since STDERR is now going to STDOUT (because of 2>&1) both STDERR and STDOUT ends up in the blackhole /dev/null. In other words, the script is silenced.

By the way, you need to have a > in front of /dev/null 2>&1. It should be:

x * * * * /path/to/my/script > /dev/null 2>&1

This is standard I/O redirection.

There are always three default files open.

• stdin (0)
• stdout (1)
• stderr (2)

So in this example, the stdout (1) is being redirected to /dev/null.

The null device is a device file that discards all data written to it.

Then stderr is then being redirected into stdout (2>&1), therefore, both stdout and stderr will go to /dev/null

So placing this at the end of a crontab job will suppress all output and errors from the command.

Reference

I/O Redirection