What's '2>&1' and '/dev/null'

Question

I have been browsing through scripts and see 2>&1 & /dev/null everywhere. What are they? After googling, is it different for OS X than for Linux?

Answer 1

There are three 'standard file streams' that Unix processes may make use of - stdin, stdout, and stderr - stdin usually reads input from the keyboard, and stdout and stderr usually go to your terminal window. stderr gets file descriptor number 2, and stdout gets file descriptor number 1. 2>&1 means "redirect all output to stderr to the same place that stdout is going". /dev/null is a character device - it simply discards anything written to it.

If you don't want to store the output of a script anywhere, and you don't want to see it on your terminal, you may redirect stderr to the same place as stdout with 2>&1, and then redirect stdout to /dev/null with > /dev/null

If you only were to redirect stdout with > /dev/null, you would still see errors on your terminal.

Answer 2

2>&1 is the information for the shell executing the script to redirect all the output including errors to specified destination like file or /dev/null

The /dev location in Linux systems provides the system with access to devices. One of those devices is a pseudo-device called null. /dev/null is an empty device - a vacuum if you will. It does nothing besides being a hole where you put something and it disapears. If you copy /dev/null to some file like:

cp /dev/null /tmp/foo

the specified file will be empty.

Detailed information about redirecting output you can be found here.

And here is a full definition of /dev/null:

On UNIX, this is a virtual-file that can be written to. Data written to this file gets discarded. It is similar to the file call NUL on Windows machines. Key point: When rooting a machine, intruders will often redirect logging to /dev/null For example, the command ln -s /dev/null .bash_history will cause the system to stop logging bash commands. Culture: In the vernacular, means much the same thing as black hole. Typical usage: if you don't like what I have to say, please direct your comments to /dev/null.