1. 尝试自行设计一个C语言小程序，完成最基本的shell角色：给出命令行提示符、能够逐次接受命令；对于命令分成三种，内部命令（例如help命令、exit命令等）、外部命令（常见的ls、cp等，以及其他磁盘上的可执行程序HelloWorld等）以及无效命令（不是上述三种命令）将上述shell进行扩展，使得你编写的shell程序具有支持管道的功能，也就是说你的shell中输入“dir |more”能够执行dir命令并将其输出通过管道将其输入传送给more作为标准输入。。

以下是一个简单的C语言实现的基本shell程序，它支持内部命令和外部命令：

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

#define MAX_COMMAND_LENGTH 100
#define MAX_ARGUMENTS 10

char* read_command() {
    char* line = NULL;
    ssize_t buffer_size = 0;
    getline(&line, &buffer_size, stdin);
    return line;
}

int execute_command(char** args) {
    pid_t pid;
    int status;

    pid = fork();
    if (pid == 0) {
        // child process
        execvp(args[0], args);
        // if execvp returns, it was not successful
        perror("execvp");
        exit(EXIT_FAILURE);
    } else if (pid < 0) {
        // fork error
        perror("fork");
        exit(EXIT_FAILURE);
    } else {
        // parent process
        do {
            waitpid(pid, &status, WUNTRACED);
        } while (!WIFEXITED(status) && !WIFSIGNALED(status));
    }

    return status;
}

int main() {
    char* command_line;
    char* command;
    char* arguments[MAX_ARGUMENTS + 1];  // extra element for NULL terminator
    char* token;
    int i;

    while (1) {
        printf("myshell> ");
        command_line = read_command();

        // split command line into command and arguments
        token = strtok(command_line, " \n");
        if (token == NULL) {
            // empty command line
            continue;
        }
        command = token;
        arguments[0] = token;
        for (i = 1; i <= MAX_ARGUMENTS; i++) {
            token = strtok(NULL, " \n");
            arguments[i] = token;
            if (token == NULL) {
                break;
            }
        }
        arguments[i] = NULL;

        // execute command
        if (strcmp(command, "exit") == 0) {
            // internal command: exit shell
            exit(EXIT_SUCCESS);
        } else if (strcmp(command, "help") == 0) {
            // internal command: print help message
            printf("This is a simple shell program.\n");
            printf("Supported commands:\n");
            printf("  help: print this help message\n");
            printf("  exit: exit the shell\n");
        } else {
            // external command: execute program
            execute_command(arguments);
        }

        free(command_line);
    }

    return 0;
}

接下来，我们可以扩展这个程序以支持管道。我们需要对输入的命令行进行解析，找到管道符号 `|`，将其左边和右边的命令分别执行，并将左边命令的输出作为右边命令的输入。以下是一个简单的实现：

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

#define MAX_COMMAND_LENGTH 100
#define MAX_ARGUMENTS 10

char* read_command() {
    char* line = NULL;
    ssize_t buffer_size = 0;
    getline(&line, &buffer_size, stdin);
    return line;
}

int execute_command(char** args, int input_fd, int output_fd) {
    pid_t pid;
    int status;
    int pipe_fds[2];

    if (input_fd != STDIN_FILENO) {
        dup2(input_fd, STDIN_FILENO);
        close(input_fd);
    }

    if (output_fd != STDOUT_FILENO) {
        dup2(output_fd, STDOUT_FILENO);
        close(output_fd);
    }

    if (pipe(pipe_fds) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    pid = fork();
    if (pid == 0) {
        // child process
        close(pipe_fds[0]);
        if (output_fd != STDOUT_FILENO) {
            dup2(pipe_fds[1], STDOUT_FILENO);
            close(pipe_fds[1]);
        }
        execvp(args[0], args);
        // if execvp returns, it was not successful
        perror("execvp");
        exit(EXIT_FAILURE);
    } else if (pid < 0) {
        // fork error
        perror("fork");
        exit(EXIT_FAILURE);
    } else {
        // parent process
        close(pipe_fds[1]);
        if (input_fd != STDIN_FILENO) {
            close(input_fd);
        }
        if (output_fd != STDOUT_FILENO) {
            close(output_fd);
        }
        do {
            waitpid(pid, &status, WUNTRACED);
        } while (!WIFEXITED(status) && !WIFSIGNALED(status));
        if (WIFEXITED(status) && WEXITSTATUS(status) != 0) {
            fprintf(stderr, "Command exited with status %d\n", WEXITSTATUS(status));
        }
        return pipe_fds[0];
    }
}

int main() {
    char* command_line;
    char* command;
    char* arguments[MAX_ARGUMENTS + 1];  // extra element for NULL terminator
    char* token;
    int i;
    int input_fd = STDIN_FILENO;
    int output_fd = STDOUT_FILENO;

    while (1) {
        printf("myshell> ");
        command_line = read_command();

        // check for pipe symbol
        char* pipe_symbol = strstr(command_line, "|");
        if (pipe_symbol != NULL) {
            // split command line into two commands
            *pipe_symbol = '\0';
            char* left_command_line = command_line;
            char* right_command_line = pipe_symbol + 1;

            // parse and execute left command
            token = strtok(left_command_line, " \n");
            if (token == NULL) {
                // empty command line
                continue;
            }
            command = token;
            arguments[0] = token;
            for (i = 1; i <= MAX_ARGUMENTS; i++) {
                token = strtok(NULL, " \n");
                arguments[i] = token;
                if (token == NULL) {
                    break;
                }
            }
            arguments[i] = NULL;
            output_fd = execute_command(arguments, input_fd, STDOUT_FILENO);

            // parse and execute right command
            token = strtok(right_command_line, " \n");
            if (token == NULL) {
                // empty command line
                continue;
            }
            command = token;
            arguments[0] = token;
            for (i = 1; i <= MAX_ARGUMENTS; i++) {
                token = strtok(NULL, " \n");
                arguments[i] = token;
                if (token == NULL) {
                    break;
                }
            }
            arguments[i] = NULL;
            input_fd = execute_command(arguments, output_fd, STDIN_FILENO);
        } else {
            // no pipe symbol: parse and execute command
            token = strtok(command_line, " \n");
            if (token == NULL) {
                // empty command line
                continue;
            }
            command = token;
            arguments[0] = token;
            for (i = 1; i <= MAX_ARGUMENTS; i++) {
                token = strtok(NULL, " \n");
                arguments[i] = token;
                if (token == NULL) {
                    break;
                }
            }
            arguments[i] = NULL;
            input_fd = execute_command(arguments, input_fd, STDOUT_FILENO);
            output_fd = STDOUT_FILENO;
        }

        free(command_line);
    }

    return 0;
}

注意，这只是一个简单的实现，仍然有很多限制和潜在的错误，例如：

- 只支持一个管道符号，不支持多个管道连接。
- 不支持重定向符号 `<` 和 `>`。
- 没有处理管道连接中的错误，例如左边命令执行失败会导致右边命令被阻塞。