用c语言编写一个简单的shell，其中main包含内部指令cd，clr，dir，environ，help，pause，quit。

好的，以下是一个简单的C语言shell程序，其中包含了内部指令cd，clr，dir，environ，help，pause和quit：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/wait.h>

#define MAX_COMMAND_LENGTH 100
#define MAX_NUM_ARGUMENTS 10

int main()
{
    char *command;
    char *arguments[MAX_NUM_ARGUMENTS];
    char input[MAX_COMMAND_LENGTH];
    int status;

    while (1)
    {
        printf("shell> ");
        fgets(input, MAX_COMMAND_LENGTH, stdin);

        // Remove newline character from input
        input[strlen(input) - 1] = '\0';

        // Parse command and arguments
        command = strtok(input, " ");
        int i = 0;
        while (command != NULL && i < MAX_NUM_ARGUMENTS - 1)
        {
            arguments[i] = command;
            command = strtok(NULL, " ");
            i++;
        }
        arguments[i] = NULL;

        // Handle internal commands
        if (strcmp(arguments[0], "cd") == 0)
        {
            if (arguments[1] == NULL)
            {
                printf("cd: missing argument\n");
            }
            else if (chdir(arguments[1]) != 0)
            {
                printf("cd: %s: No such file or directory\n", arguments[1]);
            }
        }
        else if (strcmp(arguments[0], "clr") == 0)
        {
            system("clear");
        }
        else if (strcmp(arguments[0], "dir") == 0)
        {
            pid_t pid = fork();
            if (pid == 0)
            {
                execlp("ls", "ls", "-l", NULL);
                exit(0);
            }
            else
            {
                waitpid(pid, &status, 0);
            }
        }
        else if (strcmp(arguments[0], "environ") == 0)
        {
            char **env = environ;
            while (*env != NULL)
            {
                printf("%s\n", *env);
                env++;
            }
        }
        else if (strcmp(arguments[0], "help") == 0)
        {
            printf("cd <directory>  Change the current default directory to <directory>\n");
            printf("clr             Clear the screen\n");
            printf("dir             List the contents of the current directory in long format\n");
            printf("environ         List all the environment strings\n");
            printf("help            Display this help message\n");
            printf("pause           Pause operation of the shell until 'Enter' is pressed\n");
            printf("quit            Quit the shell\n");
        }
        else if (strcmp(arguments[0], "pause") == 0)
        {
            printf("Press Enter to continue...");
            getchar();
        }
        else if (strcmp(arguments[0], "quit") == 0)
        {
            exit(0);
        }
        else
        {
            // Handle external commands
            pid_t pid = fork();
            if (pid == 0)
            {
                execvp(arguments[0], arguments);
                printf("%s: command not found\n", arguments[0]);
                exit(0);
            }
            else
            {
                waitpid(pid, &status, 0);
            }
        }
    }

    return 0;
}

希望这个程序能够满足您的需求！