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shellLab

Shell Lab How to

Read the writeup

General Overview of Unix Shells

  • The child processes created as a result of interpreting a single command line are known collectively as a job. In general, a job can consist of multiple child processes connected by Unix pipes.
  • Typing ctrl-c causes aSIGINT signal to be delivered to each process in the foreground job.
  • typing ctrl-z causes a SIGTSTP signal to be delivered to each process in the foreground job.

The tsh Specification

  • tsh need not support pipes (|) or I/O redirection (< and >).
  • Programs such as more, less, vi, and emacs do strange things with the terminal settings. Don’t run these programs from your shell. Stick with simple text-based programs such as /bin/ls, /bin/ps, and /bin/echo.
  • tsh will be running in a real-world linux shell as foreground job, so if a SIGINT is sent to the shell, all children processes created by tsh will receive the signal. So in order to avoid this situation, you should use setpgid(0,0) after fork but before execve to set the child process’s group id to its process id, then if tsh receive a SIGINT it should forward the signal to its foreground job(processes).

Evaluation

Your score will be computed out of a maximum of 90 points based on the following distribution:

  1. 80 Correctness: 16 trace files at 5 points each.
  2. 10 Style points. We expect you to have good comments (5 pts) and to check the return value of EVERY system call (5 pts).
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$ ./sdriver

Tips

  • Use the codes given by the professors

  • Read the man pages

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    # download man pages
    $ sudo apt-get install manpages-posix manpages-posix-dev

  • Don’t use sleep to solve asynchronous issue, use sigsuspend as alternative

  • The reference shell tshref is very useful

  • Each trace file test a feature, exam the trace files and implement them one by one.

My solution :

I know its a very very bad idea to use printf in signal hander, but it is really handy, I may fix these when I am available

Do not use printf

Trace01

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$ cat trace01.txt
#
# trace01.txt - Properly terminate on EOF.
#
CLOSE
WAIT
$ man feof
The function feof() tests the end-of-file indicator for  the  stream  pointed  to  by `stream`,  returning  nonzero  if  it is set.  The end-of-file indicator can be cleared only by the function clearerr().

Implementation

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if (feof(stdin)) { 
    /* End of file (ctrl-d) */                  
	fflush(stdout);                                            
	exit(0);                                                   
}

Trace02

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$ cat trace02.txt
#
# trace02.txt - Process builtin quit command.
#
quit
WAIT

Implementation

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void eval(char *cmdline) 
{
    char * argvs [MAXARGS];
    int isBG = parseline(cmdline, argvs);
    
    builtin_cmd(argvs);
    return;
}
int builtin_cmd(char **argv) 
{
    char * cmd = argv[0];
    //if tsh> quit, exit the shell
    if(!strcmp(cmd, "quit")){
        exit(0);
    }
    return 0;     /* not a builtin command */
}

Trace 03

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$ cat trace03.txt
#
# trace03.txt - Run a foreground job.
#
/bin/echo tsh> quit
quit

Implementation

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void sigchld_handler(int sig) 
{
    int olderron = errno ;
    wait(NULL); //reap all children

    //debug output
    char * text = "Children reaped\n";
    write(1, text, 17);

    errno = olderron ;
    return;
}
void eval(char *cmdline) 
{

    /* signal stuff */
    sigset_t all ; sigfillset(&all);
    sigset_t childsig ; sigemptyset(&childsig);sigaddset(&childsig, SIGCHLD);
    sigset_t old ;
    /* signal stuff */

    pid_t pid ;

    char * argvs [MAXARGS];
    int isBG = parseline(cmdline, argvs);
    builtin_cmd(argvs);

    if(isBG){
        
    }
    else{
        //run a foreground job
        /* We must block SIGCHLD before fork() 
         * since after fork() the parent may not be able to get control after the child terminate
         */
        sigprocmask(SIG_BLOCK, &childsig, &old);

        pid = fork();

        if (pid == 0){
            sigprocmask(SIG_UNBLOCK, &childsig, NULL); //child do not need to block SIGCHLD
            setpgid(0, 0); //change child process to another process group
            int loadsuccess =  execve(argvs[0], argvs, environ);
            if(loadsuccess == -1){
               unix_error("Command not found\n"); 
            }
        }
        else{
//parent block all signal before addjob in case of be interrupted by things like SIGINT or SIGSTOP which may result in zombie child
        sigprocmask(SIG_BLOCK, &all, NULL); 
        int alive = waitpid(pid, NULL, WNOHANG);
        if (alive == 0){
            //if still alive
            addjob(jobs, pid, FG, argvs[0]);
            //job added, allow SIGCHLD to come in to perform reaping
            sigprocmask(SIG_SETMASK, &old, NULL);
            //wait until SIGCHLD and let handler to do the reap
            sigsuspend(&old);
        }
        else if (alive == -1){
            unix_error("Error in child\n");
        }
        else{
            //if already exited, it was reaped by waitpid 
            //Just restore the sig mask and process next cmd
            sigprocmask(SIG_SETMASK, &old, NULL);
        }
        }

    }
    
    return;
}
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$ ./tsh
tsh> /bin/echo tsh> quit
tsh> quit
Children reaped
tsh> /bin/ls
....
Children reaped
tsh> /bin/ls -la
total 252
drwxr-xr-x 1 vagrant vagrant  1152 Oct 17  2021 .
drwxr-xr-x 1 vagrant vagrant   224 Oct 16 14:11 ..
......
Children reaped
tsh> /bin/hhhhha
/bin/hhhhha: No such file or directory
Children reaped
tsh> quit
$

Trace 04

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$ cat trace04.txt
#
# trace04.txt - Run a background job.
#
/bin/echo -e tsh> ./myspin 1 \046
./myspin 1 &

Implementation

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void sigchld_handler(int sig) 
{
    int olderron = errno ;
    pid_t diedChild ;
    diedChild = wait(NULL); //reap all children

    deletejob(jobs, diedChild);

    //debug output
    //char * text = "\nChildren reaped\n";
    //write(1, text, 17);

    errno = olderron ;
    return;
}
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void eval(char *cmdline) 
{

    /* signal stuff */
    sigset_t all ; sigfillset(&all);
    sigset_t childsig ; sigemptyset(&childsig);sigaddset(&childsig, SIGCHLD);
    sigset_t old ;
    /* signal stuff */

    pid_t pid ;

    char * argvs [MAXARGS];
    int isBG = parseline(cmdline, argvs);
    builtin_cmd(argvs);

    //do a job
    /* We must block SIGCHLD before fork() 
     * since after fork() the parent may not be able to get control after the child terminate
     */
    sigprocmask(SIG_BLOCK, &childsig, &old);

    pid = fork();

    if (pid == 0){
        sigprocmask(SIG_UNBLOCK, &childsig, NULL); //child do not need to block SIGCHLD
        setpgid(0, 0); //change child process to another process group
        int loadsuccess =  execve(argvs[0], argvs, environ);
        if(loadsuccess == -1){
           unix_error(argvs[0]); 
        }
    }
    else{
//parent block all signal before addjob in case of be interrupted by things like SIGINT or SIGSTOP which may result in zombie child
    sigprocmask(SIG_BLOCK, &all, NULL); 
    int alive = waitpid(pid, NULL, WNOHANG);
    if (alive == 0){
        //if still alive
        int jobstate = isBG ? BG : FG ;
        
        addjob(jobs, pid, jobstate , cmdline );
        //job added, allow SIGCHLD to come in to perform reaping
        sigprocmask(SIG_SETMASK, &old, NULL);

        if (!isBG){
        //wait until SIGCHLD and let handler to do the reap (for foreground job)
        sigsuspend(&old);
        }
        else{
            // print job info and continue evaluation (for background job)
            struct job_t * temp = getjobpid(jobs, pid);
	    printf("[%d] (%d) %s", temp->jid, temp->pid , temp->cmdline );
        }
    }
    else if (alive == -1){
        unix_error("Error in child\n");
    }
    else{
        //if already exited, it was reaped by waitpid 
        //Just restore the sig mask and process next cmd
        sigprocmask(SIG_SETMASK, &old, NULL);
    }
    }
    
    return;
}
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# source of run.sh
#!/bin/bash

make clean
make

echo -e '\n\n'
echo "tsh output:"
./sdriver.pl -t trace04.txt -s ./tsh 
echo -e '\n\n'

echo "reference output:"
./sdriver.pl -t trace04.txt -s ./tshref
echo -e '\n\n'
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$ ./run.sh
tsh output:
#
# trace04.txt - Run a background job.
#
tsh> tsh> ./myspin 1 &
tsh> [1] (3854) ./myspin 1 &
tsh> 


reference output:
#
# trace04.txt - Run a background job.
#
tsh> tsh> ./myspin 1 &
tsh> [1] (3858) ./myspin 1 &
tsh>

Trace 05

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$ cat trace05.txt
#
# trace05.txt - Process jobs builtin command.
#
/bin/echo -e tsh> ./myspin 2 \046
./myspin 2 &

/bin/echo -e tsh> ./myspin 3 \046
./myspin 3 &

/bin/echo tsh> jobs
jobs

Implementation

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int builtin_cmd(char **argv) 
{
    int isBuiltIn = 0 ;
    char * cmd = argv[0];
    //if tsh> quit, exit the shell
    if(!strcmp(cmd, "quit")){
        exit(0);
    }
    else if (!strcmp(cmd, "jobs")){
        isBuiltIn = 1 ;
        listjobs(jobs);
    }

    return isBuiltIn;     /* not a builtin command */
}
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// in eval()
if (!isbuiltincmd){
    ....
    //load an ELF as job
}
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$ ./tshref
tsh> ./myspin 10 &
[1] (3965) ./myspin 10 &
tsh> ./myspin 10 &
[2] (3966) ./myspin 10 &
tsh> jobs
[1] (3965) Running ./myspin 10 &
[2] (3966) Running ./myspin 10 &
tsh> quit
$ ./tsh
tsh> ./myspin 10 &
[1] (3968) ./myspin 10 &
tsh> ./myspin 10 &
[2] (3969) ./myspin 10 &
tsh> jobs
[1] (3968) Running ./myspin 10 &
[2] (3969) Running ./myspin 10 &
tsh> quit

Trace 06

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$ cat trace06.txt
#
# trace06.txt - Forward SIGINT to foreground job.
#
/bin/echo -e tsh> ./myspin 4
./myspin 4 

SLEEP 2
INT

Implementation

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void sigint_handler(int sig) 
{
    pid_t current_fjob = fgpid(jobs);
    int jobid = pid2jid(current_fjob);

    int res = kill(current_fjob , sig);
    if(res == 0){
      printf("Job [%d] (%d) terminated by signal %d\n" , jobid , current_fjob , sig)  ;
    }
    else{
        unix_error("Error when sending SIGINT to child\n");
    }
    return;
}
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$ ./tsh
tsh> ./myspin 100
^CJob [1] (4529) terminated by signal 2
tsh> /bin/ps
  PID TTY          TIME CMD
 1907 pts/0    00:00:00 bash
 4528 pts/0    00:00:00 tsh
 4530 pts/0    00:00:00 ps
tsh> quit
$ ./tshref
tsh> ./myspin 100
^CJob [1] (4532) terminated by signal 2
tsh> /bin/ps
  PID TTY          TIME CMD
 1907 pts/0    00:00:00 bash
 4531 pts/0    00:00:00 tshref
 4533 pts/0    00:00:00 ps
tsh> quit

Trace 07

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$ cat trace07.txt
#
# trace07.txt - Forward SIGINT only to foreground job.
#
/bin/echo -e tsh> ./myspin 4 \046
./myspin 4 &

/bin/echo -e tsh> ./myspin 5
./myspin 5 

SLEEP 2
INT

/bin/echo tsh> jobs
jobs

Implementation

Same with trace 06

Trace 08

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$ cat trace08.txt
#
# trace08.txt - Forward SIGTSTP only to foreground job.
#
/bin/echo -e tsh> ./myspin 4 \046
./myspin 4 &

/bin/echo -e tsh> ./myspin 5
./myspin 5 

SLEEP 2
TSTP

/bin/echo tsh> jobs
jobs

Implementation:

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void sigtstp_handler(int sig) 
{
    pid_t current_fjob = fgpid(jobs);

    if(current_fjob == 0){
        return; 
    }
    int res = kill(current_fjob , sig);

    if (res == -1){
        unix_error("Error when sending SIGSTP to child\n");
    }
    return;
}
void sigchld_handler(int sig) 
{
    int olderron = errno ;
    pid_t diedChild ;
    int exitstat = 0;
    diedChild = waitpid(-1, &exitstat , WNOHANG | WUNTRACED); //reap all children

    int jobid = pid2jid(diedChild);

    if(WIFSIGNALED(exitstat)){

        int causing_signum = WTERMSIG(exitstat);
        deletejob(jobs, diedChild);
        printf("Job [%d] (%d) terminated by signal %d\n" , jobid , diedChild , causing_signum);
    }
    else if (WIFEXITED(exitstat)){
        deletejob(jobs, diedChild);
    }
    else if (WIFSTOPPED(exitstat)){
        int causing_signum = WTERMSIG(exitstat);
        struct job_t * temp = getjobpid(jobs, diedChild);
        temp->state = ST;
        printf("Job [%d] (%d) stopped by signal %d\n" , jobid , diedChild , causing_signum);
    }

    //debug output
    //char * text = "\nChildren reaped\n";
    //write(1, text, 17);

    errno = olderron ;
    return;
}
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$ ./tsh
tsh> ./myspin 100 &
[1] (5217) ./myspin 100 &
tsh> /bin/echo -e "hwjefwe \x51"
"hwjefwe Q"
tsh> jobs
[1] (5217) Running ./myspin 100 &
tsh> ./myspin 200 &
[2] (5220) ./myspin 200 &
tsh> jobs
[1] (5217) Running ./myspin 100 &
[2] (5220) Running ./myspin 200 &
tsh> ./myspin 30
^Ctsh> Job [3] (5221) terminated by signal 2
jobs
[1] (5217) Running ./myspin 100 &
[2] (5220) Running ./myspin 200 &
tsh> ./myspin 300
^Ztsh> Job [3] (5222) stopped by signal 127
jobs
[1] (5217) Running ./myspin 100 &
[2] (5220) Running ./myspin 200 &
[3] (5222) Stopped ./myspin 300
tsh> /bin/kill -9 5217 5220 5222
Job [2] (5220) terminated by signal 9
tsh> Job [1] (5217) terminated by signal 9
Job [3] (5222) terminated by signal 9
jobs
tsh> quit

Trace 09 & 10

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$ cat trace09.txt trace10.txt
#
# trace09.txt - Process bg builtin command
#
/bin/echo -e tsh> ./myspin 4 \046
./myspin 4 &

/bin/echo -e tsh> ./myspin 5
./myspin 5 

SLEEP 2
TSTP

/bin/echo tsh> jobs
jobs

/bin/echo tsh> bg %2
bg %2

/bin/echo tsh> jobs
jobs

#
# trace10.txt - Process fg builtin command. 
#
/bin/echo -e tsh> ./myspin 4 \046
./myspin 4 &

SLEEP 1
/bin/echo tsh> fg %1
fg %1

SLEEP 1
TSTP

/bin/echo tsh> jobs
jobs

/bin/echo tsh> fg %1
fg %1

/bin/echo tsh> jobs
jobs

Implementation

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void do_bgfg(char **argv) 
{
    struct job_t * temp = NULL ;
    int jobid = 0;
    int currentpid = 0;

    if (*(char*)(argv[1]) == '%'){
        sscanf(argv[1] + 1, "%d" , &jobid);
        temp = getjobjid(jobs, jobid);
    }
    else{
        sscanf(argv[1], "%d" , &currentpid);
        temp = getjobpid(jobs, currentpid);
    }

    if(temp == NULL){ printf("No such job\n");
    }

    if(temp->state == ST){
        kill(temp->pid, SIGCONT);
    }

    if(!strcmp(argv[0], "fg")){
        //cmd fg
        temp->state = FG;
        waitfg(temp->pid);

    }else{
        //cmd bg
        temp->state = BG ;

        printf("[%d] (%d) %s", temp->jid, temp->pid, temp->cmdline);
    }

    return;
}

/* 
 * waitfg - Block until process pid is no longer the foreground process
 */
void waitfg(pid_t pid)
{
    sigset_t emp ;
    sigemptyset(&emp);
    sigsuspend(&emp);
    return;
}
  • trace11.txt - Forward SIGINT to every process in foreground process group
  • trace12.txt - Forward SIGTSTP to every process in foreground process group
  • trace13.txt - Restart every stopped process in process group
  • trace14.txt - Simple error handling
  • trace15.txt - Putting it all together
  • trace16.txt - Tests whether the shell can handle SIGTSTP and SIGINT signals that come from other processes instead of the terminal

這幾個 trace 基本上是用來檢驗前面幾個步驟是否有疏漏，在此不再贅述

Reference

hackmd

github

you can go way more further:

gnu

gnu

Full code(might be buggy since only pass hardcoded test cases)

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/* 
 * tsh - A tiny shell program with job control
 * 
 * <Put your name and login ID here>
 */
#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>



int printterminate = 0;
int printstop = 0;
pid_t diedChild = 0 ;
int jobid = 0;
int causing_signum = 0;


void clear(){
    printterminate = 0;
    printstop = 0;
    diedChild = 0 ;
    jobid = 0;
    causing_signum = 0;
}






/* 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 */

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);

void sigchld_handler(int sig);
void sigtstp_handler(int sig);
void sigint_handler(int sig);

/* 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 */

        //print the "tsh> " prompt
	if (emit_prompt) {
	    printf("%s", prompt);
	    fflush(stdout);
	}

        //if something wrong with `fgets` or `stdin` exit
	if ((fgets(cmdline, MAXLINE, stdin) == NULL) && ferror(stdin))
	    app_error("fgets error");

        
	if (feof(stdin)) { /* End of file (ctrl-d) */
	    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) 
{
    
    if(!strcmp(cmdline, "\n")){
        return;
    }

    /* signal stuff */
    sigset_t all ; sigfillset(&all);
    sigset_t childsig ; sigemptyset(&childsig);sigaddset(&childsig, SIGCHLD);
    sigset_t old ;
    /* signal stuff */

    pid_t pid ;

    char * argvs [MAXARGS];
    int isBG = parseline(cmdline, argvs);
    int isbuiltincmd = builtin_cmd(argvs);

    if (!isbuiltincmd){
        //load a ELF file as job
        /* We must block SIGCHLD before fork() 
         * since after fork() the parent may not be able to get control after the child terminate
         */
        sigprocmask(SIG_BLOCK, &childsig, &old);

        pid = fork();

        if (pid == 0){
            sigprocmask(SIG_UNBLOCK, &childsig, NULL); //child do not need to block SIGCHLD
            setpgid(0, 0); //change child process to another process group
            int loadsuccess =  execve(argvs[0], argvs, environ);
            if(loadsuccess == -1){
               unix_error(argvs[0]); 
            }
        }
        else{
    //parent block all signal before addjob in case of be interrupted by things like SIGINT or SIGSTOP which may result in zombie child
        sigprocmask(SIG_BLOCK, &all, NULL); 
        int alive = waitpid(pid, NULL, WNOHANG);
        if (alive == 0){
            //if still alive
            int jobstate = isBG ? BG : FG ;
            
            addjob(jobs, pid, jobstate , cmdline );
            //job added, allow SIGCHLD to come in to perform reaping
            sigprocmask(SIG_SETMASK, &old, NULL);

            if (!isBG){
            //wait until SIGCHLD and let handler to do the reap (for foreground job)
            sigsuspend(&old);
            }
            else{
                // print job info and continue evaluation (for background job)
                struct job_t * temp = getjobpid(jobs, pid);
                printf("[%d] (%d) %s", temp->jid, temp->pid , temp->cmdline );
            }
        }
        else if (alive == -1){
            unix_error("Error in child\n");
        }
        else{
            //if already exited, it was reaped by waitpid 
            //Just restore the sig mask and process next cmd
            sigprocmask(SIG_SETMASK, &old, NULL);
            }
        }
    }
    


    if(printterminate){
    printf("Job [%d] (%d) terminated by signal %d\n" , jobid , diedChild , causing_signum);
    }
    else if (printstop){
    printf("Job [%d] (%d) stopped by signal %d\n" , jobid , diedChild , causing_signum);
    }
    clear();
    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) 
{
    int isBuiltIn = 0 ;
    char * cmd = argv[0];
    //if tsh> quit, exit the shell
    if(!strcmp(cmd, "quit")){
        exit(0);
    }
    else if (!strcmp(cmd, "jobs")){
        isBuiltIn = 1 ;
        listjobs(jobs);
    }
    else if (!strcmp(cmd, "fg") || !strcmp(cmd, "bg")){
        isBuiltIn = 1;
        do_bgfg(argv);
    }

    return isBuiltIn;     /* not a builtin command */
}

/* 
 * do_bgfg - Execute the builtin bg and fg commands
 */
void do_bgfg(char **argv) 
{
    struct job_t * temp = NULL ;
    int jobid = 0;
    int currentpid = 0;

    if (*(char*)(argv[1]) == '%'){
        sscanf(argv[1] + 1, "%d" , &jobid);
        temp = getjobjid(jobs, jobid);
    }
    else{
        sscanf(argv[1], "%d" , &currentpid);
        temp = getjobpid(jobs, currentpid);
    }

    if(temp == NULL){ printf("%s :No such job\n" , argv[1]);
                    return;
    }


    if(temp->state == ST){
        kill(-(temp->pid), SIGCONT);
    }

    if(!strcmp(argv[0], "fg")){
        //cmd fg
        temp->state = FG;
        waitfg(temp->pid);

    }else{
        //cmd bg
        temp->state = BG ;

        printf("[%d] (%d) %s", temp->jid, temp->pid, temp->cmdline);
    }

    return;
}

/* 
 * waitfg - Block until process pid is no longer the foreground process
 */
void waitfg(pid_t pid)
{
    sigset_t emp ;
    sigemptyset(&emp);
    sigsuspend(&emp);
    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 olderron = errno ;
    int exitstat = 0;
    diedChild = waitpid(-1, &exitstat , WNOHANG | WUNTRACED); //reap any terminated children

    jobid = pid2jid(diedChild);

    if(WIFSIGNALED(exitstat)){

        causing_signum = WTERMSIG(exitstat);
        deletejob(jobs, diedChild);
        printterminate = 1;
    }
    else if (WIFEXITED(exitstat)){
        deletejob(jobs, diedChild);
    }
    else if (WIFSTOPPED(exitstat)){
        causing_signum = WTERMSIG(exitstat);
        struct job_t * temp = getjobpid(jobs, diedChild);
        temp->state = ST;
        printstop = 1;
    }

    //debug output
    //char * text = "\nChildren reaped\n";
    //write(1, text, 17);

    errno = olderron ;
    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) 
{
    pid_t current_fjob = fgpid(jobs);

    if(current_fjob == 0){
        return; 
    }
    int res = kill(-current_fjob , sig);
    sigset_t emp ; sigemptyset(&emp); sigsuspend(&emp);
    if(res == -1){
        unix_error("Error when sending SIGINT to child\n");
    }
    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 current_fjob = fgpid(jobs);

    if(current_fjob == 0){
        return; 
    }
    int res = kill(-current_fjob , sig);
    sigset_t emp ; sigemptyset(&emp); sigsuspend(&emp);

    if (res == -1){
        unix_error("Error when sending SIGSTP to child\n");
    }
    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);
}