如何在用 C 编写的 shell 中从命令行执行程序

Question

Im creating a small shell in C programming and if the command is start, it does the following start program [extra parameters] “program” is the program to execute. the argument starts with a “/” (such as /usr/bin/xterm, it should interpret it as a full path. Otherwise, its path starts from the current directory.

it can have optional “parameters”. It uses fork() + exec() to start the program with the corresponding parameters, and waits until the program terminates (use the waitpid() call). For example

 **start /usr/bin/xterm –bg green**

would bring up a terminal with a green background. The prompt would not return until the terminal is closed.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <dirent.h>
#include <errno.h>
#include <signal.h>
#include <sys/wait.h>

// max # args
#define MAX_ARGS 64
// token sparators
#define SEPARATORS " \t\n"

int main (int argc, char ** argv)
{
    char buf[1024];
    char * args[MAX_ARGS];
    char ** arg;
    char *** arguments = malloc(3 * sizeof(char**));
    __pid_t pid;
    int status;
    
    while (1)
    {
        printf("#");
        if (fgets (buf, 1024, stdin ))
        {
            
            arg = args;
            *arg++ = strtok(buf, SEPARATORS);  // tokenize input
            while ((*arg++ = strtok(NULL, SEPARATORS)));
            if (args[0])
            {
                //#byebye command, exist the while loop.
                if (!strcmp(args[0], "byebye")) {
                    break;
                } 
                
                if (!strcmp(args[0], "whereami")) {
                          // arg string input from user matches whereami
                          char cwd[1024];
                          chdir("/path/to/change/directory/to");
                           getcwd(cwd, sizeof(cwd));
                            printf("%s\n", cwd);   
                             continue;
                  }

                   if (!strcmp(args[0], "start")) {
                       if (args[1]) 
                        {
                            arguments[0] = malloc(16 * sizeof(char*));
                             int argument_num = 2;
                              while (args[argument_num]) {
                                          // grab all the parameters
                                          arguments[0][argument_num - 2] = malloc(strlen(args[argument_num]) * sizeof(char));
                                       strcpy(arguments[0][argument_num - 2], args[argument_num]);
                                    argument_num++;
                               }

                           arguments[0][argument_num] = NULL;

                             if (0 == (pid = fork())) {
                                  // child process
                                 if (-1 == execve(args[1], (char **)arguments[0], NULL)) {
                                        fprintf(stdout, "child process execve failed [%m]\n");
                                        break;
                                     }
                               }
                             // wait for child
                             while (0 == waitpid(pid, &status, WNOHANG)) {}
                                                                                  
                              free(arguments[0]);
                          }

                                 continue;
                       }

     
  
                arg = args;
                while (*arg) fprintf(stdout, "%s ", *arg++);
                fputs ("\n", stdout);
            }
        }
    }
    return 0;
} 

For some reason when I test the code it says file or directory not found

Answer 1

The code works fine. If you get directory not found for start /usr/bin/xterm –bg green then it's possibly because /usr/bin/xterm does not exist. You're on a Mac?