在本教程中，我们将学习在C编程语言中使用命令行参数。

什么是命令行参数？

命令行参数是调用它们时提供给程序的参数。

例如，假设我们有一个程序" copyfile"，它带有两个参数，即文件名，其中，第一个文件名是源文件，第二个文件名是目标文件。
然后该程序将第一个文件的内容复制到第二个文件中。

视窗：

E:\TC\BIN>copyfile source_filename destination_filename

Mac/Linux：

$./copyfile source_filename destination_filename

main函数

每个C程序都具有main()函数，该函数标记代码执行的起点。

因此，到目前为止，我们尚未将任何参数传递给main()函数，因此我们设置了void。

int main(void) {
  //some code goes here...

  return 0;
}

argc和argv参数

main()函数可以接受两个参数，分别是argc和argv。

int main(int argc, char *argv[]) {
  //some code goes here...

  return 0;
}

argc是参数计数器，用于对在命令行上传递的参数数量进行计数。

argv是参数向量，表示指向命令行参数的字符指针数组。
并且该指针数组的大小等于" argc"的值。

在以下示例中，我们将执行copyfile程序并传递两个文件名(源和目标)。

Windows：

E:\TC\BIN>copyfile original.txt duplicate.txt

Mac/Linux：

$./copyfile original.txt duplicate.txt

因此，以上示例的" argc"为3。
指针" argv"的数组如下。

注意！命令行参数的第一个参数始终代表程序名称。

用C编写一个程序，将内容从一个文件复制到下一个文件

/**
 * file: copyfile.c
 * date: 2011-04-20
 * description:
 * COMMAND LINE ARGUMENT
 * user defined command: copyfile
 * This command allows the user to copy the content of file1 to file2.
 * if file2 already exists then overwrite only after taking user permission.
 * 
 * syntax:
 * $./copyfile <file1.extension_name> <file2.extension>
 * 
 * where,
 * file1 is the source file
 * file2 is the target file where content of file1 is copied.
 * 
 * example:
 * $./copyfile original.txt duplicate.txt
 */

#include <stdio.h>

int main(int argc, char *argv[]) 
{
  //file pointers
  FILE
    *fptr1,
    *fptr2;

  //variables
  int i;
  char ch;

  printf("Total number of argument passed: %d\n", argc);

  //open source file in read mode
  //if error
  //then, exit
  if( (fptr1 = fopen(argv[1], "r") ) == NULL ) {

    printf("Error...\nCannot open file: %s\n", argv[1]);
    printf("Either the filename is incorrect or it does not exists.\n");

    return -1;

  }

  //check if the destination file exists
  if( (fptr2 = fopen(argv[2], "r") ) != NULL) {

    printf("Warning: File %s already exists.\n", argv[2]);
    printf("Press Y to overwrite. Or any other key to exit: ");
    
    //take user input
    ch = getchar();

    //if user don't want to overwrite
    //then, exit
    if(ch != 'Y' && ch != 'y') {
        
      printf("Terminating the copy process.\n");

      //close connection
      fclose(fptr1);
      fclose(fptr2);
      
      return -1;

    }
    else {
      //close read mode connection
      fclose(fptr2);
        
      //now open destination file in write mode
      fptr2 = fopen(argv[2], "w");
    }

  }
  //if destination file doesn't exists
  //then, open destination file in write mode
  else {
    fptr2 = fopen(argv[2], "w");
  }

  //copy the content of source to destination
  while( !feof(fptr1) ) {
    ch = getc(fptr1);
    if(ch != EOF) {
      putc(ch, fptr2);
    }
  }

  //close connection
  fclose(fptr1);
  fclose(fptr2);

  printf("Content of %s copied to %s\n", argv[1], argv[2]);

  return 0;
}

第一次运行。

-MBP:c-project $gcc copyfile.c -o copyfile
-MBP:c-project $./copyfile original.txt duplicate.txt
Total number of argument passed: 3
Content of original.txt copied to duplicate.txt

第二次运行。

-MBP:c-project $./copyfile original.txt duplicate.txt
Total number of argument passed: 3
Warning: File duplicate.txt already exists.
Press Y to overwrite. Or any other key to exit: y
Content of original.txt copied to duplicate.txt

再次运行，但是这次终止了进程。

-MBP:c-project $./copyfile original.txt duplicate.txt
Total number of argument passed: 3
Warning: File duplicate.txt already exists.
Press Y to overwrite. Or any other key to exit: n
Terminating the copy process.

文件内容。