How to use linux tail command

In Linux, the tail command is used to view the data at the end of the file. It can display the last part of one or more files or pipe data (default is 10 lines), the syntax is "tail [option] file name"; Common options "-n" are used to specify the number of lines to display, "-c" is used to specify a specific number of bytes to be displayed, and "-f" is used to monitor files for changes. One of the most common uses of the tail command is to monitor and analyze logs and other files that change over time, often in conjunction with other tools such as grep.

#The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

Linux tail command

The tail command displays the last part of one or more files or pipe data (default is 10 lines); can be used to monitor file changes in real time. One of the most common uses of the tail command is to monitor and analyze logs and other files that change over time, often in conjunction with other tools such as grep.

Basic syntax:

tail [OPTION] FILE

• OPTION: tail option, let’s take a look at some of the most common options.

  • -n (--lines) option: Specify the number of lines to display

  • -c (--bytes) option: Specify the number of lines to display Specific number of bytes to display

  • -f (--follow) option: Watch the file for changes

• FILE : Zero or more input file names. If FILE is not specified, or FILE is -, tail reads standard input.

Description:

K here refers to the number of lines. This option means to output the last K lines. On this basis, if -n K is used, it means to output the Kth line from the file. Start output. The K here refers to the number of bytes. This option indicates the content of the last K bytes of the output file. On this basis, using -c K means starting output from the Kth byte of the file. -fOutput the newly added data after the file changes.

Option	Meaning
##-n K
-c K

How to use the tail command?

When used in its simplest form without any options, the tail command will display the last 10 lines.

tail filename.txt

How to display a specific number of rows?

You can use the -n (--lines) option to specify the number of lines to display:

tail -n <NUMBER> filename.txt

You can also omit the letter n and just use the hyphen (-) and numbers (without spaces between them).

To display the last 50 lines of a file named filename.txt, you need to use:

tail -n 50 filename.txt

The following example will display the same results as the above command:

tail -50 filename.txt

How to display a specific number of bytes?

You can use the -c (--bytes) option to display a specific number of bytes:

tail -c <NUMBER> filename.txt

For example, to display the last 500 words in the file filename.txt section data, we can use:

tail -c 500 filename.txt

You can also use a multiplier suffix after the number to specify the number of bytes to be displayed. b is multiplied by 512, kB is multiplied by 1000, K is multiplied by 1024, MB is multiplied by 1000000, M is multiplied by 1048576, and so on.

The following command will display the last two kilobytes (2048) of the file filename.txt:

tail -c 2k filename.txt

How do I view a file for changes?

To monitor a file for changes, you need to use the -f (--follow) option:

tail -f filename.txt

The -f option is especially useful for monitoring log files. For example, to display the last 10 lines of the /var/log/nginx/error.log file and monitor the file for updates you want to use:

tail -f /var/log/nginx/error.log

To interrupt the tail command while monitoring the file, press ctrl c.

To continue monitoring the file as it is recreated, you need to use the -f option:

tail -F filename.txt

The -f option is useful when the tail command follows a rotating log file. When used with the -f option, the tail command will reopen the file when it becomes available again.

How to display multiple files

If multiple files are provided as input to the tail command, it will display the last ten of each file OK.

tail filename1.txt filename2.txt

Users have the same options as when displaying a single file.

For example: display the last 20 lines of files filename1.txt and filename2.txt

tail -c 20 filename1.txt filename2.txt

How to use it with other commands?

The tail command can be used in conjunction with other commands. For example, to monitor the Apache access log file and only display the lines containing the IP address 192.168.42.12, you can use:

tail -f /var/log/apache2/access.log | grep 192.168.42.12

Or display the top ten running processes sorted by CPU usage:

ps aux | sort -nk +3 | tail -5

Recommended related video tutorials: "

Linux Tutorial"

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