Detailed explanation of various practical uses of virtual device files in Linux system

Hello everyone, I am Liang Xu.

As we all know, in Linux, everything is a file, including device files. In the process of work, we often see /dev/null , so what is it?

Technically speaking, /dev/null is a virtual device file. For the program, these virtual device files are treated as real files. Programs can request data from this data source, and the resulting data will be provided by the operating system. However, this data is not read from the disk, but is generated dynamically by the operating system. A typical example of a virtual device file is /dev/zero .

However, when you want to write data to /dev/null , whatever data you write to /dev/null will eventually be discarded, just like being thrown into a black hole.

In this case, what is the use of /dev/null ? To understand this problem, you must first have a basic understanding of the standard output and standard error output of Linux systems or Unix and Unix-like systems.

Standard output file and standard error output file

A command-line program can generate two types of output: standard output and standard error output. Standard output is recorded to the standard output file stdout, and standard error output is recorded to the standard error output file stderr.

By default, standard output and standard error files are associated with your terminal window (or console). This means that anything sent to standard output or standard error will be displayed on your screen.

However, through redirection in the shell, you can change this behavior. For example, you can redirect standard output to a file. This way, the standard output information will not be displayed on the screen, it will be saved to that file so that you can view it later, or you can redirect the standard output information to another physical device, such as a digital LED or LCD display.

Some common redirection methods are as follows:

• Using 2> you can redirect standard error output. For example: 2>/dev/null or 2>/home/user/error.log .
• Using 1> you can redirect standard output.
• Using &> you can redirect both standard error and standard output.

Use /dev/null to clear unnecessary output

Since there are two types of output: standard output and standard error output, the first use of /dev/null is to filter out one type of output. The above usage can be more easily understood through the following practical example.

If you want to find files related to power settings by looking for files with the string power in the file name under the /sys folder, you can write the command like this:

$ grep -r power /sys/

Since there are many files in the /sys folder that are inaccessible to non-root users, this will cause many Permission denied errors and be output to the screen.

These error messages can clutter the screen and wash out important information. Since Permission denied error messages are part of the standard error output, you can redirect them to /dev/null .

$ grep -r power /sys/ 2>/dev/null

In this way, the displayed information is much cleaner and clearer.

Sometimes, we may not want to see the contents of standard output, but instead want to see the contents of standard error, so we can redirect standard output to /dev/null .

$ ping baidu.com 1>/dev/null

The above screenshot shows that without redirecting the output, the ping command displays its normal output when the packets can reach the destination host. As for the first command, when the network is connected, the screen shows nothing, but once it is disconnected from the target machine, the screen only shows the error message.
Of course, you can also redirect standard output and standard error output to two different files.

$ ping baidu.com 1>/dev/null 2>error.log

In this example, standard output will not be displayed and error messages will be saved to a file called error.log .

Redirect all output to /dev/null

Sometimes, we may not want to see any output, so there are two ways to do this.

$ grep -r power /sys/ >/dev/null 2>&1

The string >/dev/null means將標準輸出重定向到/dev/null 中, and the second part, 2>&1 , means redirecting standard error to the standard output file. Here you have to write the standard output file &1 instead of simply writing 1 . Writing 2>1 will only redirect standard output to a file named 1 .

One important point to note here is that order is also very important. If you swap the redirect parameters to look like this:

$ grep -r power /sys/ 2>&1 >/dev/null

It just doesn't work the way you expected. That's because once 2>&1 is interpreted by the interpreter first, the standard error output will be redirected to the default standard output file stdout and displayed on the screen. Next, when standard output is redirected to /dev/null , the standard output information will be cleared. The end result is that you will see error messages displayed on the screen. If you forget the correct order, there is an easier one:

$ grep -r power /sys/ &>/dev/null

In this example, &>/dev/null is equivalent to重定向標準輸出與標準錯誤輸出到/dev/null .

Other practical examples of redirecting output to /dev/null

If you want to know how fast your disk can read sequential data, you can use the dd command to test it. But the dd command either outputs to the standard output file or specifies output to a file. In order to exclude the influence of written data on the result, we use the parameter of=/dev/null , so that the dd output can be written to this virtual file instead of the real disk. You don't even need to use shell redirection to do this here.

For the following command, the parameter if= specifies the file name for input; the parameter of= specifies the file name for output, that is, specifies where the output is written.

The following test is not very accurate, but it is enough to illustrate the problem.

$ dd if=debian-disk.qcow2 of=/dev/null status=progress bs=1M iflag=direct

The above is a practical scenario. Here is another scenario.

At some point you might want to know how fast you can download something from a particular server. But you don't want to write unnecessary things to the disk, so at this time, you can write the content to /dev/null .

$ wget -O /dev/null http://ftp.halifax.rwth-aachen.de/ubuntu-releases/18.04/ubuntu-18.04.2-desktop-amd64.iso

in conclusion

This article introduces several practical uses of the /dev/null file. I hope it will be helpful to your work. Do you know of any other interesting uses for this special device file? Leave a message to tell me!

This concludes this article about the various practical uses of virtual device files in Linux systems. For more relevant content on the usage of Linux virtual device files, please search 123WORDPRESS.COM's previous articles or continue to browse the following related articles. I hope everyone will support 123WORDPRESS.COM in the future!