Linux--Bash and Shell explained

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

1. View the commands executed during the previous login: vim ~/.bash_history

2. Command and file completion function (Tab key)

tab is followed by the first word of a string of instructions, then it is command completion

tab is followed by the second word of a string of instructions After the word, it is file completion

3. Command alias setting function: alias

alias lm='ls -al'

4. Programmed script (shell scripts)

5. Wildcard

6. Newline input \ +[enter]

7. Move forward or backward from the cursor Delete string [crtl]+u / [crtl] +k

8. Move the cursor to the front/end of the entire command string [crtl] +a / [crtl]+e

9. Use env and export to observe environment variables; set can observe all variables in the current bash environment

10. In continuous commands; two commands can be executed continuously:

• && 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

11. Encoding problem: LANG or LC_ALL setting; locale lists the current language.

12. View the original code of the alias: type -a alias command

13. List all keys and key contents: stty -a

14. Set key functions :stty erase ^h : Set crtl + h to delete characters backward

15. Get the core version: echo $(uname -r)

Three , Shell Script

        Concept:Shell script is a program written using the function of the shell. This program uses plain text files. Write some shell syntax and instructions in it, and use functions such as regular expressions, pipeline commands, and data flow redirection to achieve our processing commands.

Personal understanding: In fact, Shell script is a batch command in window. It is a program written by shell instructions and a little script syntax. Shell script is a good tool for system management, but it is not good enough for processing a large number of numerical operations, because shell scripts are slower and use more CPU resources, resulting in the allocation of host resources. bad.

Variable:

Variable type: The bash variable type defaults to string, which can be an integer or array type. There is no floating point type. So there is no decimal, and the result of 1/3 is 0.

Variable declaration: The default is a string, which can be directly declared as variable name = variable content (x=123 )In this way, if you want to declare it as an array or integer type:

• 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

Variable interaction: read -p "Please enter your name:" -t 30 name (-p means Prompt the world, -t means 30 seconds of input time)

Expansion of variable content: PATH="$PATH":/home/bin

Deletion of variable content: echo ${path#/*:} This sentence means to delete the shortest characters from / to:

Replacement of variable content:

Execution: 1. Direct absolute path and relative path execution, the executor must have script execution permission.

2. Global variables can be executed through bash shell.sh or sh shell.sh

Differences in the execution methods of source and sh script ?

sh script or bash script will use a new bash environment to execute the commands in the script. When the subprogram is completed, each variable or action in the subprogram will be End without being transferred back to the parent program

The source script will be executed in the parent program, so all actions will take effect in the original bash

Process control (sequence): Shell scripts are executed in order from top to bottom and from left to right.

Process control (condition):

• 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

  

  

  ## 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

             

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