Technically /dev/null is defined as a null device. Anything written to it is lost forever. If you read the wiki description of /dev/null, you will get to know lots of jargons/metaphors written/referred by UNIX techies a lot. In this article, we will concentrate on the importance of /dev/null, why it is there, and how to use it.
When things written to it are lost forever, then why do we need it? This is a valid questions, but we need /dev/null/ to lose information. That’s correct, to explain let’s take an example:
Suppose, you want to list all files of directory /var/tmp/ having word “foo” in its content. To achieve the task, we will write something as:
]$ grep -l foo /var/tmp/* /var/tmp/storagedata /var/tmp/storagedata.0 grep: /var/tmp/td.log: Permission denied
So we got two files having word “foo”; but we also an annoying error message which was part of STDERR. We were not interested in any error message, and we wanted to see only those files which I am permitted to read. So, how do I get rid of this error message? Luckily newer version of grep provides a silent option “-s”, using which we can get rid of this message. But what if I am working on a traditional system, having traditional grep? What if the system command is not having the silent option? The solution is given below:
Capturing STDOUT only
- Using newer grep:
]$ grep -ls foo /var/tmp/* /var/tmp/storagedata /var/tmp/storagedata.0
- Using traditional grep:
]$ grep -l foo /var/tmp/* 2>/dev/null /var/tmp/storagedata /var/tmp/storagedata.0
- In general using any system command :
]$ cmd 2>/dev/null
Most of you should have captured the importance of /dev/null by now. In simple words, by using “2>/dev/null”, we are asking the shell to redirect the STDERR to /dev/null. This is very useful when we execute system command through a program and expect to get only the STDOUT.
There may also be a case, when we are only interested to see the error message (STDERR), and are not at all interested in STDOUT. To get STDERR, and to discard the STDOUT we will redirect the STDERR to STDOUT and will redirect the STDOUT to /dev/null; as given below:
Capturing STDERR only
]$ cmd 2>&1 1>/dev/null
You may be thinking, that if I am redirecting STDERR to STDOUT which in-turn is redirected to /dev/null, so there should not be any output. But that is not the case, and you get the error messages.