Bo2SS

[How to implement command options? Think of the input parameters argc, 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 of the main function
    • Parameter optstring: Option string
  • View man manual: man 3 getopt
    • Return value is character [option] or -1
      • When returning, it will update the external variable optind and the static variable nextchar
      • If there are no options left, return -1, optind moves to the position of the first element of argv [not an option]
    • argc, argv
      • Directly passed from the main function parameters
    • optstring
      • Meaning: A string composed of all valid optional characters
      • A single character represents an option
      • If a single character is followed by a colon
        • One colon means that this option must be followed by a parameter value
          • The parameter follows immediately after the option or is separated by a space
          • [Similar to the existence of option parameter in commands]
        • Two colons mean that the parameter after the option is optional
          • If there is a parameter, it must follow the option and cannot be separated by a space
        • The pointer to the parameter is assigned to optarg [For two colons, defaults to 0←null]
      • Example
        • 
        • x, y, z represent 3 options
    • Repeated calls to getopt() can read multiple options, such as -al -h
    • Some global variables
      • 
      • optarg: Can extract option parameters when there is a colon
      • optind
        • Meaning: Index of the next element [character] in the argv array
        • When the caller resets it to 1, it can rescan argv
        • [PS] External variable; like a pointer
      • Can be used directly [no need to declare], as these variables are already defined in the 【included header file】
        • Concept of external variables

Code Demonstration#

getopt#

• 
• Learn to use flag, switch
• 【Meaning of colon】 Required/Optional option parameters
• Pay attention to global variables related to getopt, which can be used directly
• exit() can use non-zero values to indicate abnormal exit
• Program Results
  • 
  • For reference

Additional Knowledge Points#

Points for Consideration#

Tips#

• ⭐Small Assignment: Implement the ls -al command in C language
  • Learn to use flowcharts to clarify ideas, recommended ProcessOn——online flowchart drawing website
    • 
    • Simple flowchart from the dorm ship
• When first learning commands, do not focus too much on variations
• In vim, enter to view the structured view of the source code
• Common ctags operations: ctrl + ] to jump to definition, ctrl + o to return