Built-in variables for common shell scripting

Processing of parameter processing-shell incoming parameters

1. $#The number of arguments passed to the script
2. $*Displays all parameters passed to the script in a single string. Unlike positional variables, this option parameter can be more than 9
3. $$The current process ID number for the script to run
4. $!Process ID number of the last process running in the background
5. [email protected]With the $# same, but quoted when used, and returns each parameter in quotation marks
6. $-Displays the current options used by the shell, same as the SET command function
7. $?Displays the exit status of the last command. 0 means there is no error, and any other value indicates an error.
   Variable meaning
   $0Script Name
   $1Positional parameters #1
   $2- $9 Position parameter #2-#9
   ${10}Positional parameters #10
   $#Number of positional parameters
   "$*"All positional parameters (as a single string) *
   "[email protected]"All positional parameters (each as a separate string)
   ${#*}The number of command-line arguments passed to the script
   ${#@}The number of command-line arguments passed to the script
   $?return value
   $$Process ID (PID) of the script
   $-Flags passed to script (using set)
   $_Last parameter of the previous command
   $!Process ID (PID) of the last job running in the background

Built-in variables for common shell scripting