該知道的在實驗指導書都有了，以下是感覺這個實驗重要的地方

• 清楚前臺和後臺的概念，這是shell創造的概念，有外部命令時我們直接去執行的，只不過如果是前臺命令會去等待執行完畢，而後臺不管。所以訊號處理函式都只是對shell而言，按下ctrl+z和ctrl+c都是發給shell的。（這裡有一個程序組的概念，訊號預設是發給前臺程序組的，所以現在發的就是我們自己寫的shell了，為了不讓外部命令建立的子程序接收到這些訊號，要在建立子程序時設定單獨的程序組號）
• 要清楚什麼時候要阻塞什麼訊號，為什麼要阻塞。像建立子程序時阻塞SIGCHLD是因為可能子程序執行完了，父程序才往jobs裡add一個job，為了防止這種情況，要阻塞這個訊號，add完再取消阻塞。但不能阻塞全部訊號，因為可能收到SIGTSTP或SIGINT。
• 而在SIGCHLD的處理函式中，我阻塞了所有訊號，現在看來似乎沒有必要，因為另外兩個訊號處理函式沒有對全域性的jobs進行操作。不過在這個程式裡也沒所謂了。訊號處理函式最好寫得簡單一點，waitpid寫的地方也少一點，不然就會很亂。下面列下書上說的保守的好習慣吧
• 處理程式儘可能簡單。處理程式可能只是簡單地設定全域性標誌並立即返回，所有與接收訊號相關的處理都由主程式執行，主程式週期性檢查並重置這個標誌。
• 在處理程式中只調用非同步訊號安全函式。這裡的非同步訊號安全函式指的是：要麼函式可重入（如至訪問區域性變數），要麼不能被訊號處理程式後中斷。
• 儲存和恢復errno。防止干擾errno
• 阻塞所有訊號。保護對共享全域性資料結構的訪問。像我們這裡只有一個處理函式對jobs進行訪問，其實可以補阻塞所有訊號的。不過這也是一個好習慣吧，畢竟如果哪一天改了，卻忘了這裡有個隱患呢？
• 用volatile宣告全域性變數，volatile宣告的會告訴編譯器補要快取這個變數，總是從內從讀取。
• 使用sig_atomic_t標誌，這可以保證對標量的讀寫是不可中斷的（原子的）。
• 哦，還有就是正確使用waitpid的引數，waitpid是預設阻塞的。攝者WNOHANG可以立即返回，設定WUNTRACED可以檢查已終止和被停止的子程序，這在前臺等待那裡有用。
• 還有就是shell在發訊號到前臺是發到前臺整個程序組的，所以kill的時候記得要負數。
• 訊號處理函式和收到訊號前正在執行的函式是併發的，如果把waitpid這種相對耗時的操作不放在訊號處理函式，會不符合實驗指導書要求。而且似乎把waitpid放在訊號處理函式也更合理。

因為電腦是Windows的，所以敲程式碼在sublime，然後放到CLion上看看有沒有warning，沒有就上傳到雲伺服器的Linux下跑，出錯的話都是先去思考，想不到才去gdb除錯，gdb除錯沒有IDE那麼爽，到斷點不能同時看到幾個變數的值（也許是不會用），只能一個一個去p。感覺按鍵盤也沒有點滑鼠那麼快捷。不過還是要多點熟悉Linux下除錯吧。打算重灌個圖形化虛擬機器，然後用CLion除錯。

把程式碼放到sublime上會發現好看很多哈，感覺比在CLion上還舒服，而且跳轉也方便不少。
在sublime下

在CLion下

/* 
 * tsh - A tiny shell program with job control
 * 
 * <wrc>
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>

/* Misc manifest constants */
#define MAXLINE    1024   /* max line size */
#define MAXARGS     128   /* max args on a command line */
#define MAXJOBS      16   /* max jobs at any point in time */
#define MAXJID    1<<16   /* max job ID */

/* Job states */
#define UNDEF 0 /* undefined */
#define FG 1    /* running in foreground */
#define BG 2    /* running in background */
#define ST 3    /* stopped */

/* 
 * Jobs states: FG (foreground), BG (background), ST (stopped)
 * Job state transitions and enabling actions:
 *     FG -> ST  : ctrl-z
 *     ST -> FG  : fg command
 *     ST -> BG  : bg command
 *     BG -> FG  : fg command
 * At most 1 job can be in the FG state.
 */

/* Global variables */
extern char **environ;      /* defined in libc */
char prompt[] = "tsh> ";    /* command line prompt (DO NOT CHANGE) */
int verbose = 0;            /* if true, print additional output */
int nextjid = 1;            /* next job ID to allocate */
char sbuf[MAXLINE];         /* for composing sprintf messages */

//是否正在等待的前臺完成
volatile sig_atomic_t waiting =0;
volatile sig_atomic_t waitingPid =-1;//正在等待的前臺的pid，僅當waiting為1時有效

struct job_t {              /* The job struct */
    pid_t pid;              /* job PID */
    int jid;                /* job ID [1, 2, ...] */
    int state;              /* UNDEF, BG, FG, or ST */
    char cmdline[MAXLINE];  /* command line */
};
struct job_t jobs[MAXJOBS]; /* The job list */
/* End global variables */


/* Function prototypes */

/* Here are the functions that you will implement */
void eval(char *cmdline);

int builtin_cmd(char **argv);

void do_bgfg(char **argv);

void waitfg(pid_t pid, sigset_t *set);

void sigchld_handler(int sig);

void sigtstp_handler(int sig);

void sigint_handler(int sig);

//包裝函式
void Sigfillset(sigset_t *);

void Sigemptyset(sigset_t *);

void Sigaddset(sigset_t *, int);

void Sigprocmask(int how, const sigset_t *set, sigset_t *oldsest);


/* Here are helper routines that we've provided for you */
int parseline(const char *cmdline, char **argv);

void sigquit_handler(int sig);

void clearjob(struct job_t *job);

void initjobs(struct job_t *jobs);

int maxjid(struct job_t *jobs);

int addjob(struct job_t *jobs, pid_t pid, int state, char *cmdline);

int deletejob(struct job_t *jobs, pid_t pid);

pid_t fgpid(struct job_t *jobs);

struct job_t *getjobpid(struct job_t *jobs, pid_t pid);

struct job_t *getjobjid(struct job_t *jobs, int jid);

int pid2jid(pid_t pid);

void listjobs(struct job_t *jobs);

void usage(void);

void unix_error(char *msg);

void app_error(char *msg);

typedef void handler_t(int);

handler_t *Signal(int signum, handler_t *handler);

/*
 * main - The shell's main routine
 */
int main(int argc, char **argv) {
    char c;
    char cmdline[MAXLINE];
    int emit_prompt = 1; /* emit prompt (default) */

    /* Redirect stderr to stdout (so that driver will get all output
     * on the pipe connected to stdout) */
    dup2(1, 2);

    /* Parse the command line */
    while ((c = getopt(argc, argv, "hvp")) != EOF) {
        switch (c) {
            case 'h':             /* print help message */
                usage();
                break;
            case 'v':             /* emit additional diagnostic info */
                verbose = 1;
                break;
            case 'p':             /* don't print a prompt */
                emit_prompt = 0;  /* handy for automatic testing */
                break;
            default:
                usage();
        }
    }

    /* Install the signal handlers */

    /* These are the ones you will need to implement */
    Signal(SIGINT, sigint_handler);   /* ctrl-c */
    Signal(SIGTSTP, sigtstp_handler);  /* ctrl-z */
    Signal(SIGCHLD, sigchld_handler);  /* Terminated or stopped child */

    /* This one provides a clean way to kill the shell */
    Signal(SIGQUIT, sigquit_handler);

    /* Initialize the job list */
    initjobs(jobs);

    /* Execute the shell's read/eval loop */
    while (1) {

        /* Read command line */
        if (emit_prompt) {
            printf("%s", prompt);
            fflush(stdout);
        }
        if ((fgets(cmdline, MAXLINE, stdin) == NULL) && ferror(stdin))
            app_error("fgets error");
        if (feof(stdin)) { /* End of file (ctrl-d) */
            fflush(stdout);
            exit(0);
        }

        /* Evaluate the command line */
        eval(cmdline);
        fflush(stdout);
        fflush(stdout);
    }

    exit(0); /* control never reaches here */
}

/*
 * eval - Evaluate the command line that the user has just typed in
 *
 * If the user has requested a built-in command (quit, jobs, bg or fg)
 * then execute it immediately. Otherwise, fork a child process and
 * run the job in the context of the child. If the job is running in
 * the foreground, wait for it to terminate and then return.  Note:
 * each child process must have a unique process group ID so that our
 * background children don't receive SIGINT (SIGTSTP) from the kernel
 * when we type ctrl-c (ctrl-z) at the keyboard.
*/
void eval(char *cmdline) {
    char *argv[MAXARGS + 1];
    int bg = parseline(cmdline, (char **) &argv);
    if (!builtin_cmd((char **) &argv)) {
        pid_t pid;
        sigset_t mask_one, prev_one;
        //其實好像不需要阻塞全部訊號
        //Sigfillset(&mask_all);
        Sigemptyset(&mask_one);
        Sigaddset(&mask_one, SIGCHLD);
        //這裡要顯示阻塞CHILD訊號,防止子程序提早執行完但父程序還未addjob
        Sigprocmask(SIG_BLOCK, &mask_one, &prev_one);
        if ((pid = fork()) < 0) {
            unix_error("Fork error");
        }
        if (pid == 0) {//Child runs here
            Sigprocmask(SIG_SETMASK, &prev_one, NULL);//unblock 子程序的SIGCHLD
            setpgid(pid, 0);
            if (execve(argv[0], argv, environ) < 0) {
                printf("%s: Command not found\n", argv[0]);
                exit(1);
            }
        }
        //Sigprocmask(SIG_BLOCK, &mask_all, NULL);
        addjob(jobs, pid, ((bg == 1) ? BG : FG), cmdline);
        //Sigprocmask(SIG_SETMASK, &mask_one, NULL);
        //這裡是不是有點多此一舉？應該不是，畢竟ctrl+z和ctrl+c的訊號不應該在這裡阻塞的，不然就死鎖了
        if (!bg) {
            waiting=1;
            waitingPid=pid;
            waitfg(pid, &prev_one);

        } else {
            printf("[%d] (%d) %s", pid2jid(pid), pid, cmdline);
        }
        Sigprocmask(SIG_SETMASK, &prev_one, NULL);
    }


    return;
}

/*
 * parseline - Parse the command line and build the argv array.
 *
 * Characters enclosed in single quotes are treated as a single
 * argument.  Return true if the user has requested a BG job, false if
 * the user has requested a FG job.
 */
int parseline(const char *cmdline, char **argv) {
    static char array[MAXLINE]; /* holds local copy of command line */
    char *buf = array;          /* ptr that traverses command line */
    char *delim;                /* points to first space delimiter */
    int argc;                   /* number of args */
    int bg;                     /* background job? */

    strcpy(buf, cmdline);
    buf[strlen(buf) - 1] = ' ';  /* replace trailing '\n' with space */
    while (*buf && (*buf == ' ')) /* ignore leading spaces */
        buf++;

    /* Build the argv list */
    argc = 0;
    if (*buf == '\'') {
        buf++;
        delim = strchr(buf, '\'');
    } else {
        delim = strchr(buf, ' ');
    }

    while (delim) {
        argv[argc++] = buf;
        *delim = '\0';
        buf = delim + 1;
        while (*buf && (*buf == ' ')) /* ignore spaces */
            buf++;

        if (*buf == '\'') {
            buf++;
            delim = strchr(buf, '\'');
        } else {
            delim = strchr(buf, ' ');
        }
    }
    argv[argc] = NULL;

    if (argc == 0)  /* ignore blank line */
        return 1;

    /* should the job run in the background? */
    if ((bg = (*argv[argc - 1] == '&')) != 0) {
        argv[--argc] = NULL;
    }
    return bg;
}

/*
 * builtin_cmd - If the user has typed a built-in command then execute
 *    it immediately.
 */
int builtin_cmd(char **argv) {
    if (!strcmp(argv[0], "quit")) {
        //是不是還需要回收子程序？
        while (waitpid(-1, NULL, 0) > 0);
        exit(0);
    }
    if (!strcmp(argv[0], "jobs")) {
        listjobs(jobs);
        return 1;
    }
    if (!strncmp(argv[0], "bg", 2) || !strncmp(argv[0], "fg", 2)) {
        do_bgfg(argv);
        return 1;
    }
    return 0;     /* not a builtin command */
}

/*
 * do_bgfg - Execute the builtin bg and fg commands
 */
void do_bgfg(char **argv) {


    int bOrf, isJob, id;
    char *temp;
    if (!strncmp(argv[0], "bg", 2)) {
        bOrf = 0;
    } else {
        bOrf = 1;
    }
    if(argv[1]==NULL){
        if(!bOrf){
            printf("bg command requires PID or %%jobid argument\n");
        }else{
            printf("fg command requires PID or %%jobid argument\n");
        }
        return;
    }
    struct job_t *job;
    if (!strncmp(argv[1], "%%", 1)) {
        isJob = 1;
        temp = argv[1]+1;
        id = atoi(temp);
        job = getjobjid(jobs, id);
    } else {
        isJob = 0;
        temp = argv[1];
        id = atoi(temp);
        job = getjobpid(jobs, id);
    }
    if(id==0){
        if(bOrf){
            printf("fg: argument must be a PID or %%jobid\n");
        }else{
            printf("bg: argument must be a PID or %%jobid\n");
        }
        return;
    }
    if (job == NULL) {
        if (!isJob) {
            printf("(%d): No such process\n",id);
        } else {
            printf("%%%d: No such job\n",id);
        }
        return;
    } else {
        if (0 == bOrf) {//bg
            job->state = BG;
            kill(-job->pid, SIGCONT);
            printf("[%d] (%d) %s", job->jid, job->pid, job->cmdline);
        } else {//fg
            //waitfg
            job->state = FG;
            sigset_t mask, prev;
            Sigemptyset(&mask);
            Sigaddset(&mask, SIGCHLD);
            //這裡要顯示阻塞CHILD訊號,防止子程序喚醒，執行完，回收完了，父程序才去判斷，就會一直等
            Sigprocmask(SIG_BLOCK, &mask, &prev);
            waiting=1;
            waitingPid=job->pid;
            kill(-job->pid, SIGCONT);
            waitfg(job->pid, &prev);
            Sigprocmask(SIG_SETMASK, &prev, NULL);
        }
    }
    return;
}

/*
 * waitfg - Block until process pid is no longer the foreground process
 */
void waitfg(pid_t pid, sigset_t *set) {
    while (waiting==1) {
        sigsuspend(set);
    }
    return;
}

/*****************
 * Signal handlers
 *****************/

/*
 * sigchld_handler - The kernel sends a SIGCHLD to the shell whenever
 *     a child job terminates (becomes a zombie), or stops because it
 *     received a SIGSTOP or SIGTSTP signal. The handler reaps all
 *     available zombie children, but doesn't wait for any other
 *     currently running children to terminate.
 */
void sigchld_handler(int sig) {
    int temp = errno;
    pid_t cpid;
    int status;
    sigset_t mask_all, prev_one;
    Sigfillset(&mask_all);
    while ((cpid = waitpid(-1, &status, WUNTRACED | WNOHANG)) > 0) {
        //這裡應該判斷是掛起還是終止，根據不同來更新或刪除job
        Sigprocmask(SIG_BLOCK, &mask_all, &prev_one);
        if(cpid==waitingPid)
            waiting=0;
        if (WIFEXITED(status)) {
            deletejob(jobs, cpid);
        } else if (WIFSIGNALED(status)) {
            printf("Job [%d] (%d) terminated by signal %d\n", pid2jid(cpid), cpid, WTERMSIG(status));
            deletejob(jobs, cpid);
        } else if (WIFSTOPPED(status)) {
            struct job_t *fjob = getjobpid(jobs, cpid);
            fjob->state = ST;
            printf("Job [%d] (%d) stopped by signal 20\n", pid2jid(cpid), cpid);
        }
        Sigprocmask(SIG_SETMASK, &prev_one, NULL);
    }
    errno = temp;
    return;
}

/*
 * sigint_handler - The kernel sends a SIGINT to the shell whenver the
 *    user types ctrl-c at the keyboard.  Catch it and send it along
 *    to the foreground job.
 */
void sigint_handler(int sig) {
    int temp = errno;
    pid_t fpid;
    fpid = fgpid(jobs);
    if (fpid > 0) {
        kill(-fpid, SIGINT);//向整個程序組發
    }
    errno = temp;
    return;
}

/*
 * sigtstp_handler - The kernel sends a SIGTSTP to the shell whenever
 *     the user types ctrl-z at the keyboard. Catch it and suspend the
 *     foreground job by sending it a SIGTSTP.
 */
void sigtstp_handler(int sig) {
    pid_t fpid;
    fpid = fgpid(jobs);
    if (fpid > 0) {
        kill(-fpid, SIGTSTP);//向整個程序組發
    }
    return;
}

/*********************
 * End signal handlers
 *********************/

/***********************************************
 * Helper routines that manipulate the job list
 **********************************************/

/* clearjob - Clear the entries in a job struct */
void clearjob(struct job_t *job) {
    job->pid = 0;
    job->jid = 0;
    job->state = UNDEF;
    job->cmdline[0] = '\0';
}

/* initjobs - Initialize the job list */
void initjobs(struct job_t *jobs) {
    int i;

    for (i = 0; i < MAXJOBS; i++)
        clearjob(&jobs[i]);
}

/* maxjid - Returns largest allocated job ID */
int maxjid(struct job_t *jobs) {
    int i, max = 0;

    for (i = 0; i < MAXJOBS; i++)
        if (jobs[i].jid > max)
            max = jobs[i].jid;
    return max;
}

/* addjob - Add a job to the job list */
int addjob(struct job_t *jobs, pid_t pid, int state, char *cmdline) {
    int i;

    if (pid < 1)
        return 0;

    for (i = 0; i < MAXJOBS; i++) {
        if (jobs[i].pid == 0) {
            jobs[i].pid = pid;
            jobs[i].state = state;
            jobs[i].jid = nextjid++;
            if (nextjid > MAXJOBS)
                nextjid = 1;
            strcpy(jobs[i].cmdline, cmdline);
            if (verbose) {
                printf("Added job [%d] %d %s\n", jobs[i].jid, jobs[i].pid, jobs[i].cmdline);
            }
            return 1;
        }
    }
    printf("Tried to create too many jobs\n");
    return 0;
}

/* deletejob - Delete a job whose PID=pid from the job list */
int deletejob(struct job_t *jobs, pid_t pid) {
    int i;

    if (pid < 1)
        return 0;

    for (i = 0; i < MAXJOBS; i++) {
        if (jobs[i].pid == pid) {
            clearjob(&jobs[i]);
            nextjid = maxjid(jobs) + 1;
            return 1;
        }
    }
    return 0;
}

/* fgpid - Return PID of current foreground job, 0 if no such job */
pid_t fgpid(struct job_t *jobs) {
    int i;

    for (i = 0; i < MAXJOBS; i++)
        if (jobs[i].state == FG)
            return jobs[i].pid;
    return 0;
}

/* getjobpid  - Find a job (by PID) on the job list */
struct job_t *getjobpid(struct job_t *jobs, pid_t pid) {
    int i;

    if (pid < 1)
        return NULL;
    for (i = 0; i < MAXJOBS; i++)
        if (jobs[i].pid == pid)
            return &jobs[i];
    return NULL;
}

/* getjobjid  - Find a job (by JID) on the job list */
struct job_t *getjobjid(struct job_t *jobs, int jid) {
    int i;

    if (jid < 1)
        return NULL;
    for (i = 0; i < MAXJOBS; i++)
        if (jobs[i].jid == jid)
            return &jobs[i];
    return NULL;
}

/* pid2jid - Map process ID to job ID */
int pid2jid(pid_t pid) {
    int i;

    if (pid < 1)
        return 0;
    for (i = 0; i < MAXJOBS; i++)
        if (jobs[i].pid == pid) {
            return jobs[i].jid;
        }
    return 0;
}

/* listjobs - Print the job list */
void listjobs(struct job_t *jobs) {
    int i;

    for (i = 0; i < MAXJOBS; i++) {
        if (jobs[i].pid != 0) {
            printf("[%d] (%d) ", jobs[i].jid, jobs[i].pid);
            switch (jobs[i].state) {
                case BG:
                    printf("Running ");
                    break;
                case FG:
                    printf("Foreground ");
                    break;
                case ST:
                    printf("Stopped ");
                    break;
                default:
                    printf("listjobs: Internal error: job[%d].state=%d ",
                           i, jobs[i].state);
            }
            printf("%s", jobs[i].cmdline);
        }
    }
}
/******************************
 * end job list helper routines
 ******************************/


/***********************
 * Other helper routines
 ***********************/

/*
 * usage - print a help message
 */
void usage(void) {
    printf("Usage: shell [-hvp]\n");
    printf("   -h   print this message\n");
    printf("   -v   print additional diagnostic information\n");
    printf("   -p   do not emit a command prompt\n");
    exit(1);
}

/*
 * unix_error - unix-style error routine
 */
void unix_error(char *msg) {
    fprintf(stdout, "%s: %s\n", msg, strerror(errno));
    exit(1);
}

/*
 * app_error - application-style error routine
 */
void app_error(char *msg) {
    fprintf(stdout, "%s\n", msg);
    exit(1);
}

/*
 * Signal - wrapper for the sigaction function
 */
handler_t *Signal(int signum, handler_t *handler) {
    struct sigaction action, old_action;

    action.sa_handler = handler;
    sigemptyset(&action.sa_mask); /* block sigs of type being handled */
    action.sa_flags = SA_RESTART; /* restart syscalls if possible */

    if (sigaction(signum, &action, &old_action) < 0)
        unix_error("Signal error");
    return (old_action.sa_handler);
}

/*
 * sigquit_handler - The driver program can gracefully terminate the
 *    child shell by sending it a SIGQUIT signal.
 */
void sigquit_handler(int sig) {
    printf("Terminating after receipt of SIGQUIT signal\n");
    exit(1);
}

void Sigfillset(sigset_t *set) {
    if (sigfillset(set) < 0) {
        unix_error("sigfillset error");
    }
}

void Sigemptyset(sigset_t *set) {
    if (sigemptyset(set) < 0) {
        unix_error("sigemptyset error");
    }
}

void Sigaddset(sigset_t *set, int num) {
    if (sigaddset(set, num) < 0) {
        unix_error("sigaddset error");
    }
}

void Sigprocmask(int how, const sigset_t *set, sigset_t *oldsest) {
    if (sigprocmask(how, set, oldsest) < 0) {
        unix_error("sigprocmask error");
    }
}