【413】C 语言 Command line

Command-Line Arguments

All the executable programs above have a main(void) program

• more generally, executables take arguments on the command line
• these enter the program via parameters

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  1  int main(int argc, char *argv[])
  

For example:

prompt$ ./a.out -pre 3

means that:

• argc refers to the number of arguments, including the program name

  • argc = 3

• argv[] allows access to arguments represented as strings

  • argv[0] is a pointer to the string "./a.out"

  • argv[1] is a pointer to the string "-pre"

  • argv[2] is a pointer to the string "3"

Command-line argument processing example 1

Write a program that:

• takes an optional numerical command-line argument 1, 2, 3 or 4
• if the argument is not one of these characters (!), a usage message is printed

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   // commarg.c
   #include <stdio.h>
   #include <stdlib.h>
   int main(int argc, char *argv[]) {
       if (argc == 1 ||
          (argc == 2 && atoi(argv[1]) >= 1 && atoi(argv[1]) <= 4)) {
           // we can do something here
       } else {
           printf("Usage: %s [1|2|3|4]\n", argv[0]);
       }
       return EXIT_SUCCESS;
   }
  

• notice that atoi() had to be called to convert the character to a number

• compiling and executing the program:

   prompt$ dcc commarg.c
   prompt$ ./a.out
   prompt$ ./a.out 1
   prompt$ ./a.out 2
   prompt$ ./a.out 3
   prompt$ ./a.out 4
   prompt$ ./a.out 5
   Usage: ./a.out [1|2|3|4]

Command-line argument processing example 2

Write a program that:

• takes an optional command-line switch -reverse

• if the switch is not correct, a usage message is printed

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   // commargrev.c
   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>
   int main(int argc, char *argv[]) {
       if (argc == 1 ||
          (argc == 2 && !strcmp(argv[1], "-reverse"))) {
           // NOTE: strcmp returns 0 if matches.
           // we could do something here
       } else {
           printf("Usage: %s [-reverse]\n", argv[0]);
       }
       return EXIT_SUCCESS;
   }
  

   prompt$ ./a.out
   prompt$ ./a.out -reverse
   prompt$ ./a.out rubbish
   Usage: ./a.out [-reverse]

Makefile