What is /Dev/Null in Linux?

If you have been learning shell programming, you may already have come across something like /dev/null. In this article, we will understand what it is and how it is used. Let’s start off by understanding what /dev is.

What is /dev?

In the Linux file system, everything is a file or a directory. Even devices are accessed as files. Your hard drive partitions, Pen drive, speakers, for all of these, there exists a file from which these are accessed. Now to understand how devices are accessed as files, think of it in this way: what do we do with a file? We read data from it and write data to it. A device like a speaker can input data to produce sound, a hard disk can be used to read and write data, a printer takes the input to print files, etc. Files and devices are similar in this way

/dev is a directory that stores all the physical and virtual devices of a Linux system. Physical devices are easy to understand, they are tangible devices like pen-drive, speakers, printers, etc. A Linux system also has virtual devices which act as a device but represent no physical device. 

What is /dev/null?

It is a virtual device, which has a special property: Any data written to /dev/null vanishes or disappears. Because of this characteristic, it is also called bitbucket or blackhole. Let us see a demonstration.

Redirect output to /dev/null

In this demo, you can see that we added some data, but that data was not stored. This particular characteristic of /dev/null has some use cases. Let’s see them.

Usage of /dev/null

Since it discards anything written to it, you can move files to /dev/null to delete them. But this is not a widely used use case. It is mainly used to discard standard output and standard error from an output.

What are standard output and standard error?

The output of a Linux command contains two streams of data: standard output (stdout) and standard error (stderr0), while the standard output is the normal output if the command has no errors, the standard error is the error generated by the command. Output may contain both standard output and standard input. Example of stdout and stderr:

Let’s run the following shell script

# /bin/sh
ls
apt update

The first command ran successfully, but the second one generated error. Since stdout and stderr are two separate data streams, they can be handled separately.

How to access stderr and stdout?

Stdout and stderr are streams of data, both of which are treated as a file in Linux. If you wish to perform any action on these, you use a file descriptor that uniquely identifies the file stream. The file descriptor for stdout is 1 and for stderr is 2.

To access them separately, let’s see the following command which is performed on the same script file which was used in the previous example.

./script.sh 1>stdout.txt

In this, you can see that the output contains standard errors but the standard output was not displayed. This is because the stdout stream was sent to the stdout.txt file, and you can see that the stdout.txt file contains the standard output.

While writing a shell script, we may want to discard the standard error from the output. Whatever stream we may want to suppress from the output, that stream of data can be written into /dev/null. You could write that data into another file, just like the above example, but if you have no use for that data, why would you want to waste memory on it, it’s better to discard it completely. Let’s see the following example

./script.sh 2>/dev/null

And just like that, we have removed the stderr from the output. If you wish to discard the complete output, you can use the following command

command >/dev/null 2>&1

The &> command redirects the output of the file descriptor which is mentioned on the left (in the above command, the output of 2) to the stream of file descriptor mentioned on the right-hand side. So, the output of stderr (2) gets redirected to stdout(1), which in turn gets written in /dev/null and thus gets destroyed.

Secure File Deletion

‘/dev/null’ finds utility in secure file deletion. By redirecting ‘/dev/null’ to a file, you can effectively overwrite its content with null bytes, making it challenging to recover any meaningful data.

For example:

cat /dev/null > sensitive_file

This command replaces the contents of ‘sensitive_file’ with null bytes, ensuring its secure deletion.

Cleaning Log Files

Log files can accumulate substantial data over time. To clear log files without interrupting running services, you can redirect their content to ‘/dev/null.’

For example:

cat /dev/null > /var/log/syslog

This command empties the ‘syslog’ file by redirecting its content to ‘/dev/null,’ effectively cleaning the log without deleting the file itself.

Tips and Tricks:

• Redirecting standard output to ‘/dev/null’: ‘command > /dev/null’
• Redirecting standard error to ‘/dev/null’: ‘command 2> /dev/null’
• Redirecting both output and error to ‘/dev/null’: ‘command >/dev/null 2>&1’

Conclusion

In this article we discussed `/dev/null’ in Linux is a virtual black hole where data disappears when written to it. It is commonly used to discard output and errors, making command execution cleaner. It can also be employed for secure file deletion and log file cleaning. Understanding its functionality allows for efficient command-line operations in Linux.