0 Course Introduction and Command Line Parsing Functions

[How to implement command options? You can think of the input parameters argc and argv in the main function]

Course Content#

getopt Function#

[Command Line Parsing Function]

<unistd.h> → Unix Standard
⭐Function Prototype
- int getopt(int argc, char * const argv[], const char *optstring);
- Guess
  - Return value int
    - ① 0: success; -1: failure
    - ② ≥0: success; <0: failure
  - Parameters argc, argv: similar to the parameters in the main function
  - Parameter optstring: option string
- Check the man page: man 3 getopt
  - The return value is a character [option] or -1
    - The external variable optind and the static variable nextchar will be updated upon return
    - If there are no more options, -1 is returned and optind moves to the first element of argv [not an option]
  - argc, argv
    - Passed directly from the main function arguments
  - optstring
    - Meaning: a string composed of all valid optional characters
    - Single character represents an option
    - If a single character is followed by a colon
      - One colon means that the option must be followed by an argument value
        
        The argument is immediately after the option or separated by a space
        
        [Similar to the existence of option arguments in commands]
      - Two colons mean that the argument after the option is optional
        
        If there is an argument, it must follow the option and cannot be separated by a space
      - The pointer to the argument is assigned to optarg [For two colons, it defaults to 0←null]
    - Example
      - x, y, z represent three options
  - Repeatedly calling getopt() can read multiple options, such as -al -h
  - Some global variables
    - optarg: When there is a colon, the option argument can be extracted
    - optind
      - Meaning: the index of the next element [character] in the argv array
      - When the caller resets it to 1, argv can be rescanned
      - [PS] External variable; like a pointer
    - Can be used directly [no need to declare], because these variables have been defined in the [referenced header file]
      - Concept of external variables

Code Demonstration#

getopt#

Learn to use flags and switches
Meaning of the colon: required/optional option argument
Pay attention to the global variables related to getopt, they can be used directly
exit() can use a non-zero value to indicate abnormal exit
Program result
- For reference

Additional Knowledge#

Points for Consideration#

Tips#

⭐Homework: Implement the ls -al command in C language
- Learn to use flowcharts to clarify your thoughts, recommended ProcessOn - an online flowchart drawing website
  - A simple flowchart from the ship
When learning commands, don't focus too much on variations
In vim, enter to view the structured view of the source code
Common operations of ctags: ctrl + ] to jump to the definition, ctrl + o to return