What is /dev/null in Linux ?

The /dev/null device is a special file that discards all data written to it but reports that the write operation succeeded. It is typically used for disposing of unwanted output streams of a process. This is usually done by redirection.

/dev/null is creaetd every time on system boot. From the below, we can see that /dev/null is a character device, so it processes data character by character.

$ ls -l /dev/null
crw-rw-rw- 1 root root 1, 3 Feb 18 17:24 /dev/null

Size of /dev/null is 0 as we can from the stat command.

$ stat /dev/null
  File: '/dev/null'
  Size: 0               Blocks: 0          IO Block: 4096   character special file
Device: 6h/6d   Inode: 6           Links: 1     Device type: 1,3
Access: (0666/crw-rw-rw-)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2021-02-18 17:24:07.571207521 +0530
Modify: 2021-02-18 17:24:07.571207521 +0530
Change: 2021-02-18 17:24:07.571207521 +0530
 Birth: -

Usage in Script

/dev/null is commonly used in scripting. You can tell if an operation is successful. For example, in the script excerpt below, we touch a file and then check to see if the file was updated or created by examining the return code. If operation succeeded, the return code will always be 0.

touch $file 2> /dev/null
if [ $? != 0 ]; then
    echo File creation failed
    exit 1
else

In the below example, we extract from a tar file, but hide possible errors from the user. This is the kind of thing that many sysadmins will do in a script to reduce the output that their scripts will generate.

#!/bin/bash

cd /usr/local/apps
tar xf /var/tmp/app.tar 2 >/dev/null
if [ $? != 0 ]; then
    echo extract failed
    exit 1
fi

Redirecting Output

2 means Standard error and 1 means Standard output. The & means file descriptor1. So 2>&1 redirects standard error to whatever standard output currently points at, while 2>1 redirects standard error into a file called 1. >> means append while > means truncate and write. Either appending to or writing to /dev/null has the same net effect.

Following example, redirects standard error to point at what standard output currently points at, then redirects stdout to /dev/null.

2>&1 >/dev/null

General forms for /dev/null use

• Send standard out to /dev/null
  command > /dev/null
• Send standard error to /dev/null
  command 2> /dev/null
• Send both standard out and standard error to /dev/null
  command > /dev/null 2>&1