1. What is Shell?
The shell in the narrow sense refers to command line software, including the basic Linux operating window Bash, etc. The shell in the broad sense includes graphical interface software, because the graphical interface can actually operate various drivers. The program calls the core to do the work.
The legal shells of the system are written in the /etc/shells file.
2. Basic operating skills of Bash Shell

&& The previous instruction must have no error message
|| If correct, the second instruction will not be executed. If the error occurs, execute the second item

Variables and variable contents are connected with an equal sign =, a=123
The two sides of the equal sign cannot be connected directly The space character
consists of English letters and numbers, but the beginning cannot be a number
If there are special characters in the variable content, you can use "" Or '' combine the contents of variables
" "The $ variable can retain the original characteristics
' 'The $ variable can be used as To process ordinary characters
, you can use the escape character \ to treat unknown special characters as ordinary characters
`` or () Meaning: `` or the instructions in () are executed first, as external input information
Change - into + to perform the [cancel] action: declare +x sum :
List the types of variables separately: declare -p sum
- ##Upgrade to environment variables: export PATH
- Unset variable: unset a
- Array definition: var[1]="123" var[2]="456" var[3]=" 789"
- The meaning of `` or (): The instructions in `` or () are executed first as external input information
- Variable $$: The number that appears is your PID number
- Variable?: The error code of the previous error instruction, if there is no error, it will be 0

Replacement of variable content:

- test -e file file name exists
- test -f file Whether the file name exists and is a file
- test -d directory Whether the file name exists and is a directory
- test -r -w -x filename Whether it has read, write, and executable permissions
- test n1 -eq n2 The two values are equal
- test n1 -ne n2 The two values are not equal
-Ge greater than equal to
--Le small than or equal to
- ##
Script variable interaction can also use ${0}, ${1}, ${2}, ${3} To proceed, ${0} represents the executed script name, ${1} represents the first variable entered, ${2} represents the second variable entered, ${3} represents the third variable entered
if conditional judgment
# case conditional judgment##Flow control (loop):
while loop## for loop
Function:Note: Unlike traditional languages, functions must be defined at the front of the script so that the function can be recognized when the program is executed. Function parameters are also passed using ${1}, ${2}, and ${3}. That is to say, ${1}/${2}/${3} within the function are passed in when calling the function. actual parameters. Instead of the parameters passed when executing the script
The above is the detailed content of Linux--Bash and Shell explained. For more information, please follow other related articles on the PHP Chinese website!

Maintenance mode plays a key role in Linux system management, helping to repair, upgrade and configuration changes. 1. Enter maintenance mode. You can select it through the GRUB menu or use the command "sudosystemctlisolaterscue.target". 2. In maintenance mode, you can perform file system repair and system update operations. 3. Advanced usage includes tasks such as resetting the root password. 4. Common errors such as not being able to enter maintenance mode or mount the file system, can be fixed by checking the GRUB configuration and using the fsck command.

The timing and reasons for using Linux maintenance mode: 1) When the system starts up, 2) When performing major system updates or upgrades, 3) When performing file system maintenance. Maintenance mode provides a safe and controlled environment, ensuring operational safety and efficiency, reducing impact on users, and enhancing system security.

Indispensable commands in Linux include: 1.ls: list directory contents; 2.cd: change working directory; 3.mkdir: create a new directory; 4.rm: delete file or directory; 5.cp: copy file or directory; 6.mv: move or rename file or directory. These commands help users manage files and systems efficiently by interacting with the kernel.

In Linux, file and directory management uses ls, cd, mkdir, rm, cp, mv commands, and permission management uses chmod, chown, and chgrp commands. 1. File and directory management commands such as ls-l list detailed information, mkdir-p recursively create directories. 2. Permission management commands such as chmod755file set file permissions, chownuserfile changes file owner, and chgrpgroupfile changes file group. These commands are based on file system structure and user and group systems, and operate and control through system calls and metadata.

MaintenanceModeinLinuxisaspecialbootenvironmentforcriticalsystemmaintenancetasks.Itallowsadministratorstoperformtaskslikeresettingpasswords,repairingfilesystems,andrecoveringfrombootfailuresinaminimalenvironment.ToenterMaintenanceMode,interrupttheboo

The core components of Linux include kernel, file system, shell, user and kernel space, device drivers, and performance optimization and best practices. 1) The kernel is the core of the system, managing hardware, memory and processes. 2) The file system organizes data and supports multiple types such as ext4, Btrfs and XFS. 3) Shell is the command center for users to interact with the system and supports scripting. 4) Separate user space from kernel space to ensure system stability. 5) The device driver connects the hardware to the operating system. 6) Performance optimization includes tuning system configuration and following best practices.

The five basic components of the Linux system are: 1. Kernel, 2. System library, 3. System utilities, 4. Graphical user interface, 5. Applications. The kernel manages hardware resources, the system library provides precompiled functions, system utilities are used for system management, the GUI provides visual interaction, and applications use these components to implement functions.

Linux maintenance mode can be entered through the GRUB menu. The specific steps are: 1) Select the kernel in the GRUB menu and press 'e' to edit, 2) Add 'single' or '1' at the end of the 'linux' line, 3) Press Ctrl X to start. Maintenance mode provides a secure environment for tasks such as system repair, password reset and system upgrade.


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