Understanding and learning of shell programming Basics Bash_linux Shell

1. Understanding Bash

Through the shell will input the instructions and kernel communication, let kernel control hardware work
View Shell:cat/etc/shell
A preset shell type is recorded in the/etc/passwd after login and is related to the account
Command alias: Alias lm= ' Ls-al ' #注意有无空格
Help: Man bash
Command types: Type

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Type ls # ls is aliased to ' Ls–color=auto '
Type Cat # cat is hashed (/BIN/CAT)
Type CD # CD is a shell builtin

Command execution:

\[enter]: Escapes [Enter], presses and does not execute, but is a different line

2, the shell's variable function

Concept: The same variable, different users can have different values, so you can use variables to represent this value without having to write a specific value to the program, such as $path

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Display variables: Echo $PATH
Set Variable: path=xxx #注意空格有无
Extension variable: path= $PATH: XXX
Cancel variable: unset MAIL
View Environment variables: env
Viewing environment variables and custom variables: set

PS1: Prompt character setting
Custom variable to environment variable: Export
Why to turn to environment variables: Because subroutines inherit the environment variables of the parent program
Language support
View system supported languages: LOCALE-A
To view language-related variables: locale
Keyboard read user input Read:read atest #将用户输入读入atest变量
Declaring variable types declare:declare-i sum=100+50

3. Command alias and History command

Alias

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Alias lm= ' Ls-al | More
Unalias LM

History command

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History

4, Bash Shell's operating environment

Lookup Order when instruction executes
Relative/absolute path-> alias-> bash built-in-> $PATH
Bash configuration file

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Login Shell/non-login Shell
Login Shell: You need to enter your account password when entering the shell
Non-login Shell: Login via graphical interface, then start shell without entering account password

Configuration file

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Login Shell:
/etc/profile: System Overall setting
The following three files are read in order, if present, and no longer read later
~/.bash_profile
~/.bash_login
~/.profile

Wildcard characters

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*: 0 To infinity characters
? : a character
[]: [ABC] # A or B or C
[-]: [0-9] # 0 to 9
[^]: [^ABC] # is not a character of a,b,c

5, Data Flow redirect

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>: Covering
>>: Cumulative
/dev/null: Receive information but not display
Sequential execution: CMD1,CMD2,CMD3
Based on the results of the previous article, determine the next
CMD1 && CMD2
cmd1 | | Cmd2

6, Pipeline command

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ls | Less
Cut: Same Row data split
echo $PATH | Cut-d ': ' F 5 #以 ': ' Split, 5th data
grep: Returns this line if the required information is available on a line
Sort: Sorting
Uniq: Repeating content is displayed only once
WC: Count the number of words
Tee: Bidirectional redirection
TR: deleting or replacing data
Col: converting to plain text
Join: Bring related information from two files together
Paste: Put two lines together and separate them in the middle with [Tab]
Expand: Convert [Tab] to space
Split: Transfer large files to small files