/dev/null discards unwanted output

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All data written on a /dev/null or /dev/zero special file is discarded by the system. Use /dev/null to send any unwanted output from program/command and syntax is:

command >/dev/null

OR

command 2>/dev/null

The last syntax redirects the the command error to /dev/null where it is ignored by the shell.

Example

Try searching user vivek in /etc/passwd file:

grep vivek /etc/passwd && echo "Vivek found" || "Vivek not found"

Sample outputs:

vivek:x:1000:1000:Vivek Gite,,,,:/home/vivek:/bin/bash
Vivek found

To ignore actual output and just display the message, modify your command as follows:

grep vivek /etc/passwd >/dev/null && echo "Vivek found" || "Vivek not found"

Sample outputs:

Vivek found

Consider the following example from the exit status chapter. The output of grep "^$username" $PASSWD_FILE > /dev/null is send to /dev/null where it is ignored by the shell.

#!/bin/bash
# set var 
PASSWD_FILE=/etc/passwd

# get user name
read -p "Enter a user name : " username

# try to locate username in in /etc/passwd 
# 
grep "^$username" $PASSWD_FILE > /dev/null

# store exit status of grep
# if found grep will return 0 exit stauts
# if not found, grep will return a nonzero exit stauts
status=$?

if test $status -eq 0
then
	echo "User '$username' found in $PASSWD_FILE file."
else
	echo "User '$username' not found in $PASSWD_FILE file."
fi

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