Variables of Shell programming

• Shell Basics

  • What is a shell?
    • Shell is a command-line interpreter
    • The shell is also a very powerful programming language that can invoke Linux commands directly in the shell
  • How the script executes
    • Give execute permission to run directly
      • chmod 755 hello.sh
      • ./hello.sh
    • Execute script via bash call
      • Bash hello.sh
  • Output redirection
    • Commands > Documents
    • Output the correct output of the command to the specified file in a covered manner
    • Commands >> Documents
    • Output the correct output of the command to the specified file in an additional way
  • Sequential execution between multiple commands
    • Command 1; Command 2
    • Multiple command order execution, no logical connection between commands
    • Command 1 && Command 2
    • Command 1 executes correctly, then command 2 executes
    • Command 1 does not execute correctly, then command 2 does not execute
    • Command 1 | | Command 2
    • Command 1 does not execute correctly, then command 2 executes
    • Command 1 is executed correctly, then command 2 does not execute
  • Pipe character
    • Command 1 | Command 2

• Variables of Shell programming

  • Variable command rules
    • The variable name must begin with a letter or underscore, and the middle of the name can consist only of letters, numbers, and underscores
    • In bash, the default type of the variable is the string type
  • Classification of variables
    • User-defined variables: variable-Custom
    • Environment variables: Data variables related to the operating environment of the system
    • Positional parameter variable: A parameter used to pass parameters to a script.
    • Predefined variables: Variables that are already defined in bash, variables cannot be customized, and variables are fixed
  • Defining variables
    • Variable name = variable Value
      • X=5
      • Name= "Jeff Wang"
    • The left and right sides of the equals sign are not allowed to have spaces, and the system will think that this is a command
    • If there are spaces in the variable, be sure to enclose the double or single quotation marks
  • Variable invocation
    • echo $ variable Name
      • Echo $x
      • Echo $name
  • Variable Overlay
    • X=123
    • x= "$x" 456
    • x=${x}789
  • Variable view
    • Set-u
    • If this option is set, an error will be made when a variable is called without declaring it (no hint by default)
  • Variable deletion
    • unset variable Name
    • Be aware that the variable name is not $
      • unset x
      • unset name
  • Environment variables
    • The main difference between an environment variable and a user-defined variable is that the environment variable is a global variable, and the user-defined variable is a local variable
    • Variables can be customized, but the environment variable names and variables that are in effect on the system are fixed
  • Setting environment variables
    • Export variable name = variable Value
    • Or
    • Variable name = variable Value
    • Export variable Name
  • View environment variables
    • Set
      • View all variables
    • Env
      • View environment variables
  • Common environment variables
    • Shell: the current shell
    • Term: Terminal environment
    • Histsize: Number of history command bars
    • Ssh_client: The current operating environment is connected by SSH, where the client IP is logged
    • Ssh_tty:ssh the terminal when connected
  • PATH environment variable
    • Path variable: Paths to System lookup commands
    • Echo $PATH
    • Path= "$PATH":/root/sh
      • Increase the value of the PATH variable
  • PS1 Environment variables
    • PS1 variables: Command prompt settings
      • \u: Displays the current user name
      • \w: Displays the full name of the current directory
      • []\u[]@[]\h[]:[]\w[]$
    • Ls
      • Line break, command too long not to lose the situation
  • Positional parameter variables
    • $n
    • $*
    • [Email protected]
    • $#
  • Pre-defined variables
    • $?
    • $$
    • $!
  • You can use man bash to query the role of positional parameter variables and predefined variables
  • Receive keyboard input
    • read [options] [variable name]
• environment variable configuration file
  • Source Configuration file
  • Or
  • . Configuration file
    • Note there is a space between the point and the configuration file, where the dot is equivalent to source
  • After you modify the configuration file, you must log off to log on again to take effect, and use the source command to log back on
  • Introduction to the configuration file
    • /etc/profile
    • /etc/profile.d/*.sh
    • ~/.bashrc
    • /etc/bashrc
    • Different system file names may be different, but this should be the location
    • You should be able to read these shell script roughly.

Variables of Shell programming