Difference between revisions of "/dev/null discards unwanted output"

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{{navigation
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|previous=Empty file creation
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|next=here documents}}
  
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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:
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<pre>command >/dev/null</pre>
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This syntax redirects the command standard output messages to /dev/null where it is ignored by the shell.
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OR
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<pre>command 2>/dev/null</pre>
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This syntax redirects the command error output messages to /dev/null where it is ignored by the shell.
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OR
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<pre>command &>/dev/null</pre>
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This syntax redirects both standard output and error output messages to /dev/null where it is ignored by the shell.
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==Example==
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Try searching user vivek in [[/etc/passwd]] file:
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<syntaxhighlight lang="bash" >grep vivek /etc/passwd && echo "Vivek found" || "Vivek not found"</syntaxhighlight>
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Sample outputs:
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<pre>vivek:x:1000:1000:Vivek Gite,,,,:/home/vivek:/bin/bash
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Vivek found
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</pre>
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To ignore actual output and just display the message, modify your command as follows:
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<syntaxhighlight lang="bash" >grep vivek /etc/passwd >/dev/null && echo "Vivek found" || "Vivek not found"</syntaxhighlight>
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Sample outputs:
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<pre>Vivek found</pre>
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Consider the following example from [[The exit status of a command|the exit status]] chapter. The output of '''grep "^$username" $PASSWD_FILE > /dev/null''' is send to /dev/null where it is ignored by the shell.
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<syntaxhighlight lang="bash" >#!/bin/bash
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# set var
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PASSWD_FILE=/etc/passwd
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# get user name
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read -p "Enter a user name : " username
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# try to locate username in in /etc/passwd
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#
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grep "^$username" $PASSWD_FILE > /dev/null
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# store exit status of grep
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# if found grep will return 0 exit stauts
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# if not found, grep will return a nonzero exit stauts
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status=$?
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if test $status -eq 0
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then
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echo "User '$username' found in $PASSWD_FILE file."
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else
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echo "User '$username' not found in $PASSWD_FILE file."
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fi</syntaxhighlight>
 
[[Category:Redirection]]
 
[[Category:Redirection]]
 +
{{navigation
 +
|previous=Empty file creation
 +
|next=here documents}}

Latest revision as of 22:50, 29 March 2016

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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

This syntax redirects the command standard output messages to /dev/null where it is ignored by the shell. OR

command 2>/dev/null

This syntax redirects the command error output messages to /dev/null where it is ignored by the shell. OR

command &>/dev/null

This syntax redirects both standard output and error output messages 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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