C语言实现一个迷你Shell

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简介

俄勒冈州立大学（Oregon State University) CS 344作业,使用C语言实现一个Shell.

要求

  • 内置 cd,status和exit 命令
  • 支持输入和输出的重定向
  • 支持后台运行(&)
  • Ctrl-Z 与 Ctrl-C
  • 调用系统其他命令
  • 替换命令中的$$为Shell的PID

参考资料

作业pdf

测试脚本

p3testscriptview raw
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#!/bin/bash

echo "PRE-SCRIPT INFO"
echo "  Grading Script PID: $$"
echo '  Note: your smallsh will report a different PID when evaluating $$'

./smallsh <<'___EOF___'
echo BEGINNING TEST SCRIPT
echo
echo --------------------
echo Using comment (5 points if only next prompt is displayed next)
#THIS COMMENT SHOULD DO NOTHING
echo
echo
echo --------------------
echo ls (10 points for returning dir contents)
ls
echo
echo
echo --------------------
echo ls out junk
ls > junk
echo
echo
echo --------------------
echo cat junk (15 points for correctly returning contents of junk)
cat junk
echo
echo
echo --------------------
echo wc in junk (15 points for returning correct numbers from wc)
wc < junk
echo
echo
echo --------------------
echo wc in junk out junk2; cat junk2 (10 points for returning correct numbers from wc)
wc < junk > junk2
cat junk2
echo
echo
echo --------------------
echo test -f badfile (10 points for returning error value of 1, note extraneous &)
test -f badfile
status &
echo
echo
echo --------------------
echo wc in badfile (10 points for returning text error)
wc < badfile
echo
echo
echo --------------------
echo badfile (10 points for returning text error)
badfile
echo
echo
echo --------------------
echo sleep 100 background (10 points for returning process ID of sleeper)
sleep 100 &
echo
echo
echo --------------------
echo pkill -signal SIGTERM sleep (10 points for pid of killed process, 10 points for signal)
echo (Ignore message about Operation Not Permitted)
pkill sleep
echo
echo
echo --------------------
echo sleep 1 background (10 pts for pid of bg ps when done, 10 for exit value)
sleep 1 &
sleep 1
echo
echo
echo --------------------
echo pwd
pwd
echo
echo
echo --------------------
echo cd
cd
echo
echo
echo --------------------
echo pwd (10 points for being in the HOME dir)
pwd
echo
echo
echo --------------------
echo mkdir testdir$$
mkdir testdir$$
echo
echo
echo --------------------
echo cd testdir$$
cd testdir$$
echo
echo
echo --------------------
echo pwd (5 points for being in the newly created dir)
pwd
echo --------------------
echo Testing foreground-only mode (20 points for entry & exit text AND ~5 seconds between times)
kill -SIGTSTP $$
date
sleep 5 &
date
kill -SIGTSTP $$
exit
___EOF___

源码

smallsh.cview raw
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <signal.h>
#include <unistd.h>

#define MAX_INPUT_LENGTH 2048
#define MAX_ARG_NUMBER 512
#define SPLIT_CHAR " \t\r\n"

int allowBg = 1;

char *readLine()
{
    size_t maxLength = MAX_INPUT_LENGTH;
    char *line = (char *)malloc(sizeof(char) * maxLength);
    fgets(line, maxLength, stdin);
    int i = 0;
    for (i = 0; i < maxLength; i++)
    {
        if (line[i] == '\n')
        {
            line[i] = '\0';
        }
    }
    return line;
}
char **parse(char *line, int *num, int *back)
{
    size_t maxNumber = MAX_ARG_NUMBER;
    int i = 0;
    char **arr = (char **)malloc(sizeof(char *) * maxNumber);
    char *token;
    token = strtok(line, SPLIT_CHAR);
    while (token != NULL)
    {
        arr[i] = token;
        // replace $$
        int j = 0;
        for (j = 0; j < strlen(arr[i])-1; j++)
        {
            if (arr[i][j] == '$' && arr[i][j + 1] == '$')
            {
                arr[i][j] = '\0';
                arr[i][j+1] = '\0';
                char t[10];
                char newArg[50];
                memset(t, '\0', 10);
                memset(newArg, '\0', 50);
                strncpy(newArg, arr[i], j);
                sprintf(t, "%d", getpid());
                strcat(newArg, t);
                // sprintf(arr[i], newArg);
                arr[i] = newArg;
            }
        }
        i++;
        token = strtok(NULL, SPLIT_CHAR);
    }
    if (i >= 2 && strcmp(arr[i - 1], "&") == 0)
    {
        arr[i - 1] = NULL;
            i--;
        if(allowBg == 1){
            *back = 1;
        }
    }
    *num = i;
    arr[i] = NULL;
    return arr;
}
void printStatus(int status)
{
    if (status == 0 || status == 1)
        printf("exit value %d\n", status);
    else
        printf("terminated by signal %d\n", status);
}
void catchSIGTSTP(int sig)
{
    if (allowBg == 1)
    {
        char *message = "Entering foreground-only mode(& is now ignored)\n";
        write(STDOUT_FILENO, message, 49);
        fflush(stdout);
        allowBg = 0;
    }
    else
    {
        char *message = "Exiting foreground-only mode\n";
        write(STDOUT_FILENO, message, 30);
        fflush(stdout);
        allowBg = 1;
    }
}
void catchSIGINT(int signo)
{
    char message[] = "terminated by signal ";
    char t[10];
    memset(t, '\0', 10);
    sprintf(t, "%d", signo);
    strcat(message, t);
    strcat(message, "\n");
    printf("%s", message);
    fflush(stdout);
    fflush(stdin);
}

int newProcess(char **arr, int num, int back)
{
    pid_t pid = fork();
    int status, fd;
    int input_flag = -1;
    int output_flag = -1;
    int i = 0;
    for (i = 0; i < num; i++)
    {
        if (strcmp(arr[i], "<") == 0)
            input_flag = i;
        else if (strcmp(arr[i], ">") == 0)
            output_flag = i;
    }
    if (pid == 0)
    {
        struct sigaction sigint = {0};
        sigint.sa_handler = catchSIGINT;
        sigemptyset(&sigint.sa_mask);
        sigint.sa_flags = 0;
        sigaction(SIGINT, &sigint, NULL);

        // input redirect
        if (input_flag > -1)
        {
            fd = open(arr[input_flag + 1], O_RDONLY, 0644);
            if (fd == -1)
            {
                printf("cannot open %s for input\n", arr[input_flag + 1]);
                exit(1);
            }
            else
            {
                if (dup2(fd, 0) == -1)
                {
                    printf(": Could not redirect stdin for input file %s\n", arr[input_flag + 1]);
                    exit(1);
                }
                arr[input_flag] = NULL;
                close(fd);
            }
        }
        //output redirect
        if (output_flag > -1)
        {
            fd = open(arr[output_flag + 1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
            if (fd == -1)
            {
                printf("cannot not open %s for output\n", arr[output_flag + 1]);
                exit(1);
            }
            else
            {
                if (dup2(fd, 1) == -1)
                {
                    printf("Could not redirect stdout for output file %s\n", arr[output_flag + 1]);
                    exit(1);
                }
                arr[output_flag] = NULL;
                close(fd);
            }
        }
        if (back == 1 && input_flag < 0)
        {
            fd = open("/dev/null", O_RDONLY, 0644);
            if (fd == -1)
            {
                printf("Could not open \"/dev/null\"\n");
                exit(1);
            }
            else
            {
                if (dup2(fd, 0) == -1)
                {
                    printf("Could not redirect stdin to \"/dev/null\"\n");
                    exit(1);
                }
                close(fd);
            }
        }
        // printf("%c\n", arr[0][0]);
        if (execvp(arr[0], arr) == -1 && arr[0][0] >= 'a' && arr[0][0] <= 'z')
        {
            printf("%s: no such file or directory\n", arr[0]);
            fflush(stdout);
            exit(1);
        }
    }
    else if (pid < 0)
    {
        printf("Smallsh !\n");
        return 1;
    }
    else
    { // parent
        if (back == 1)
        {
            printf("Background pid is %d.\n", pid);
            fflush(stdout);
        }
        else
        {
            waitpid(pid, &status, WUNTRACED);
            if (WIFEXITED(status))
                return (WEXITSTATUS(status));
            else if (WIFSIGNALED(status))
                return (WTERMSIG(status));
        }
    }
    return status;
}
int executeCommand(char **arr, int status, int num, int back)
{
    if (arr[0] == NULL || strcmp("#", arr[0]) == 0)
        return status;
    else if (strcmp(arr[0], "exit") == 0)
        exit(0);
    else if (strcmp(arr[0], "status") == 0)
        printStatus(status);
    else if (strcmp(arr[0], "cd") == 0)
    {
        char *directory;
        if (arr[1] == NULL)
            directory = getenv("HOME");
        else
            directory = arr[1];
        if (chdir(directory) == -1)
        {
            printf("cd: No such file or directory\n");
            fflush(stdout);
        }
        return status;
    }
    else
    {
        int t = newProcess(arr, num, back);
        // printf("------------%d---\n",t);
        // fflush(stdout);
        return t;
    }
}


int main()
{
    char *input;
    char **arr;
    int status = 0;

    struct sigaction sigint = {0};
    sigint.sa_handler = SIG_IGN;
    sigemptyset(&sigint.sa_mask);
    sigint.sa_flags = 0;
    sigaction(SIGINT, &sigint, NULL);

    struct sigaction sigtstp = {0};
    sigtstp.sa_handler = catchSIGTSTP;
    sigfillset(&sigtstp.sa_mask);
    sigtstp.sa_flags = 0;
    sigaction(SIGTSTP, &sigtstp, NULL);
    while (1)
    {
        int num = 0;
        int back = 0;
        printf(": ");
        fflush(stdout);
        input = readLine();
        arr = parse(input, &num, &back);
        status = executeCommand(arr, status, num, back);

        //background
        pid_t childPid = waitpid(-1, &status, WNOHANG);
        if (childPid > 0)
        {
            printf("background pid %d is done: ", childPid);
            if (WIFEXITED(status))
                printf("exit value %d\n", WEXITSTATUS(status));
            else
                printf("terminated by signal %d\n", status);
            fflush(stdout);
        }
        fflush(stdin);
        free(input);
        free(arr);
    }
    return 0;
}

总结

wait()函数

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pid_t wait(int* status);
pid_t waitpid(pid_t pid, int* status, int options);
  • WNOHANG 如果该子进程没有结束，则以非阻塞方式等待子进程，并且返回0;如果该子进程已经结束，返回其pid.如果没有子进程退出，则立刻返回-1
  • WUNTRACED 若子进程进入暂停状态，则马上返回

WIFEXTED与WEXITSTATUS

  • WIFEXTED(status) 这个宏用来获取是否正常退出，正常退出获得true
  • WEXITSTATUS(status) 只可在WIFEXITED为true时使用，获取正常退出的状态码

WIFSIGNALED与WTERMSIG

  • WIFSIGNALED(status) 这个宏用来获取是否异常退出，异常退出获得true
  • WTERMSIG(status) 只可在WIFSIGNALED为true时使用，获取异常退出的状态码
    smallsh.cview raw
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    waitpid(pid, &status, WUNTRACED);
                if (WIFEXITED(status))
                    return (WEXITSTATUS(status));
                else if (WIFSIGNALED(status))
                    return (WTERMSIG(status));
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