Shell Basics-Bash basic features

2. Historical command History
   Option:-C Empty History command-W Save History Command now
   The history command entered under Linux will be saved in the root directory: ~/root/.bash_history file
   Save 1000 by default to modify environment variables in the/etc/profile file
   !ser: Executes the last command that begins with Ser
   !! : Executes the previous command again
4. Defining aliases
   # alias VI = "VIM"//reboot system disappears
   # Unalias VI
   Permanent effect
   # VI/ROOT/.BASHRC
6. Output redirection
   Standard output redirection: command > File override Way command >> file append way
   Standard error Output redirect: Error command 2> file Overwrite method error command 2>> file Append method
   A. Save the correct output and error output to the same file in a covered manner
   Format: command > File 2>&1
   Commands &> Documents
   B. Append the correct output and error output to the same file in Append mode
   Format: Command >> file 2>&1
   Commands &>> Documents
   C, append the correct output to the file 1, error output appended to the file 2
   Format: Commands >> files 1 2>> file 2
8. Script Execution Connector
   ; Perform the order of eg: command 1; Command 2
   && the previous command executed successfully and then executes the next eg: Command 1&& command 2
   || Previous command execution failed to execute eg: command 1| | Command 2
   | Correct output of command 1 as a command two-action object eg: command 1| Command 2
10. grep command
    # grep [Options] "Search content" file
    Options:
    -I: Ignore case
    -N: Output line number
    -V: Reverse Lookup
    –color=auto search keywords are displayed in color
12. Custom variables
    Variables can consist of letters, numbers, underscores, but cannot start with a number
    The type of the variable defaults to the string type, and if a numeric operation is required, the variable type must be modified to be numeric
    The variable is connected by an equal sign, the equals 2 edges cannot have spaces, and if the variable value has a space, a single quotation mark is used to include
    If you need to increase the value of the variable, the variable needs to be enclosed in double quotation marks: "$ variable name" or with ${variable name} containing
    If you assign the result of the command as a variable value, you need to use the inverse quotation mark or the $ () containing command
    Variable settings: a=123
    Variable invocation: Echo $name
    Variable view: Set
    Variable deletion: unset name

13. Pre-defined variables
    
    

    Pre-defined variables	Role
    $?	The return status of the last command executed. If the value of this variable is 0, it proves that the previous command was executed correctly, and if the value of the variable is not 0, the previous command was executed incorrectly. (Echo $?)
    $$	Process number (PID) of the current process
    $!	Process number (PID) of the last process running in the background

    
    

    Receive keyboard input
    # read [options] [variable name]
    Options:
    -P "prompt message" when waiting for the read input, the output prompt message
    -T number of seconds the read command waits for user input, specifying the wait time
    -N Characters The read command will only accept the specified number of characters, and it will execute
    -s hides input data for password input
    code example:

    #! /bin/Bash Read-TTen-P"Please input your name:"name# Prompt Please enter name, wait 10 seconds, save user input to variable name nameEcho "Name is $name"Read-s-tTen-P"Please enter your age:"age# age is privacy, with"- S"option to hide inputEcho-E"\ n"Echo "Age is $age"Read-N1-TTen-P"Please select gender[m/f]:"gender# Use"- N 1"option to receive only one input character will be executed (no carriage return)Echo-E"\ n"Echo "Sex is $gender"

    Code

15. Declare declaring variable types

    # DECLARE [+/-] [option] Variable name

    Options:

    -: Set type attribute to variable

    +: Cancels the Type property of a variable

    -I: Declaring a variable as an integer

    -x: Declaring a variable as an environment variable

    -P: Displays the declared type of the specified variable

    Other methods: Numeric operations using: CC = $ (($aa + $bb) or CC = $[$aa + $BB]

Shell Basics-Bash basic features