The function of login is to log in to the system, and the usage permissions are for all users.

The function of the last command is to display the recent login status of users or terminals, and the usage permissions are for all users. By viewing the log of the program through the last command, the administrator can learn who has or attempted to connect to the system.

last -1 displays the remote address

The exit command is used to exit the system, and its usage permissions are for all users.

The function of the shutdown command is to shut down the computer. The usage permission is super user.

The shutdown command can safely shut down the system. It is very dangerous to shut down the Linux system by directly cutting off the power supply.

Linux is different from Windows. There are many processes running in the background, so forced shutdown may cause data loss of the process, put the system in an unstable state, and even damage hardware devices in some systems.

If you use the shutdown command before shutting down the system, the system administrator will notify all logged-in users that the system will be shut down, and the login command will be frozen, that is, new users will no longer be able to log in.

The function of the halt command is to shut down the system, and the usage permission is super user.

When halt is executed, the application process is killed and the sync (forcibly writing the data stored in the buffer to the hard disk) system call is executed. After the file system write operation is completed, the kernel will be stopped. If the system's run level is 0 or 6, shut down the system: otherwise, use the shutdown command (plus an h parameter) instead.

The sync command can be used to force the data in the memory buffer to be written to disk immediately

The fsck command is used to check and attempt to repair errors in the file system

The super block is located at the front of the block group and describes the data structure of the overall information of the file system. It mainly describes the self-recording of the file system and the static distribution of files, as well as describes the size and quantity of various structures of the file system.

var/log/wtmp is a binary file that records information such as the number and duration of logins for each user

The reboot command is used to restart the computer, and the user rights are system administrators.

The pwd command is used to print out the current working directory

The cd command is used to change the current working directory

Used to list the contents of a directory or file information. The output of this command is sorted by file name by default. If no self-label is specified, the currently self-recorded content is listed.

The mkdir command is used to create self-recording (folder)

mkdir is make directory, which can create multiple directories at once. If the directory already exists, an error will be reported by default. The -p option can make the mkdir command not report an error in this case. The -p option can also be used to automatically create a non-existent parent. Table of contents.

The touch command can be used to create empty files

Can also be used to modify the timestamp of a file

The cp command is used to copy files or directories. It can copy a single file at a time or multiple files at a time (the cp command is a high-risk command, and there is a risk of data loss if used carelessly)

find -name "*.html" -exec cp t /data/html/

find -name "*.html" I xargs cp -t /data/html

The mv command is used to move files or directories (a high-risk command, if used carelessly, there is a risk of data loss)

If the original file and the target file are in the same recording, the effect of the mv command is equivalent to renaming the file.

The rm command is used to delete files or directories

The rm command is a high-risk command. No tool can 100% restore the files deleted by the rm command. When the rm command deletes files, it does not put the files in the "recycle bin" similar to the graphical interface, so there is no "undeletion" operation available.

The cat command is used to read the entire contents of a file or merge several files into one.

head is used to display the beginning of the file. The default head command prints the first 10 lines of the corresponding file.

The tail command is used to read the tail of a file

more will be displayed page by page to facilitate users to read page by page. The most basic command is to press the space key (space) to display the next page, and press the b key to display the page back (back), and There is also a search string function. The more command reads the file from front to back, so the entire file is loaded on startup

The less command reads content and displays it in split screen. Less is similar to more, but more only supports page turning backwards, less supports page turning forward/backward, and less does not load the entire file before viewing.

The find command is used to find files under the specified self-recording

locate can quickly find whether there is a specified file in the file system

Search principle: First establish a database of a person's file name and path, and then query in that person's database when searching.

The which command searches for executable files in the directory specified by PATH

Using the which command, you can see whether someone's system command exists and which person's command is being executed.

The which command is used to quickly determine the absolute path of an external command

gzip is a command commonly used in Linux systems to compress and decompress files.

gzip can be used not only to compress large, rarely used files to save disk space

The tar command can be used to package files and put multiple components into one package to facilitate data movement.

The ln command is used to create linked files

The ln command creates a hard link by default without parameters.

The man command is used to view the documentation manual (manual)

man sleep

-d displays a short topic description of the command

-s displays a short syntax description of the command

tail is used to display the end of the file to the standard output. The default tail command can display the last 10 lines of the file.

Document excerpt

cut is used to display specific columns of files or standard input

Extract columns or fields

Awk is a powerful text analysis tool. Simply put, awk reads files or standard input line by line, slices each line with spaces as the default delimiter, and then performs various analysis and processing on the cut parts.

last -n 51awk‘(print $1} #Display the 5 accounts that have recently logged into the system

Awk basic command format: awk action file name, such as: awk ‘print $oy’test.txt　

Extract columns or fields

The grep command is a powerful text search tool that uses regular expressions to search text and prints matching lines. grep searches for a string template in one or more files. If the template contains spaces, it must be quoted, and all strings following the template are treated as file names. The search results are sent to standard output without affecting the original file content.

Extract keywords

The wc command is used to count word count. Using the wc command, we can calculate the number of bytes, words, or columns of the component. If the file name is not specified, or the file name is "_", the wc command will read data from the standard input device.

text statistics

The sort command can sort files and output the sorted results to standard output. The sort command can take input from a specific file or from stdin

Text sorting

diff compares the similarities and differences of text files line by line. If you specify a directory to compare, diff will compare files with the same file name in the directory, but not subdirectories.

text comparison

The tr command reads data from the standard input device, and after string translation, outputs the result to the standard output device. It is often used to convert or delete characters in the file.

cat text.txt | tr a-z A-z #Convert lowercase to uppercase output

Text manipulation tools

Compared with tr, sed can modify strings. sed is an online editor that can edit text from files and standard input. When executed, sed will read a line from the support or standard input, copy it to the buffer, and after completing the text editing, read the next line until all text lines have been edited. Therefore, the sed command will only change a copy of the text in the buffer. If you want to edit the original file directly, you can use the - option or redirect the result to a new file.

Text manipulation tools