用C语言实现简单的shell程序，包括cd，clr，dir，echo，environ，help，pause，quit指令

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

void cd(char **args);
void clr();
void dir();
void echo(char **args);
void environ();
void help();
void pause_();
void quit();

char *builtin_cmds[] = {
    "cd",
    "clr",
    "dir",
    "echo",
    "environ",
    "help",
    "pause",
    "quit"
};

void (*builtin_funcs[]) (char **) = {
    &cd,
    &clr,
    &dir,
    &echo,
    &environ,
    &help,
    &pause_,
    &quit
};

int num_builtin_cmds() {
    return sizeof(builtin_cmds) / sizeof(char *);
}

int main() {
    char *cmd_line, *cmd;
    char **args;
    int exit_code = 0;

    while (1) {
        printf("> ");
        cmd_line = NULL;
        size_t line_len = 0;
        getline(&cmd_line, &line_len, stdin);

        if (strcmp(cmd_line, "\n") == 0) {
            free(cmd_line);
            continue;
        }

        int arg_idx = 0;
        args = malloc(sizeof(char*) * (arg_idx+1));

        cmd = strtok(cmd_line, " \n");
        while (cmd) {
            args[arg_idx++] = cmd;
            args = realloc(args, sizeof(char*) * (arg_idx+1));
            cmd = strtok(NULL, " \n");
        }
        args[arg_idx] = NULL;

        for (int i = 0; i < num_builtin_cmds(); i++) {
            if (strcmp(args[0], builtin_cmds[i]) == 0) {
                builtin_funcs[i](args);
                break;
            }
        }

        free(args);
        free(cmd_line);

        if (exit_code) {
            exit(exit_code);
        }
    }

    return 0;
}

void cd(char **args) {
    if (args[1] == NULL) {
        fprintf(stderr, "cd: expected argument to \"cd\"\n");
    } else {
        if (chdir(args[1]) != 0) {
            perror("cd");
        }
    }
}

void clr() {
    printf("\033[2J\033[1;1H"); // ANSI escape code for clearing screen
}

void dir() {
    system("ls");
}

void echo(char **args) {
    for (int i = 1; args[i]; i++) {
        printf("%s ", args[i]);
    }
    printf("\n"); // end with newline
}

extern char **environ;

void environ() {
    for (int i = 0; environ[i]; i++) {
        printf("%s\n", environ[i]);
    }
}

void help() {
    printf("Type one of the following commands:\n");
    for (int i = 0; i < num_builtin_cmds(); i++) {
        printf("- %s\n", builtin_cmds[i]);
    }
}

void pause_() {
    printf("Press ENTER to continue...\n");
    getchar();
}

void quit() {
    exit(0);
}

注：为了方便，我将 `pause` 改名为 `pause_`，因为 `pause()` 是一个 POSIX 标准函数。