944,141 Members | Top Members by Rank

Software Development > C++ > Making a UNIX Shell, so inexperienced at it
Ad:
  • C++ Discussion Thread
  • Unsolved
  • Views: 7342
  • C++ RSS
Permalink
Jan 31st, 2006
0

Making a UNIX Shell, so inexperienced at it

Expand Post »
So I have to make a UNIX shell that can basically take in any UNIX command and then keep a record of the time each process took, by forking off children processes, recording their time, and recording the parent's time, and then return that time into a linked list or vector so that i can print out all processes and their times when someone wants to see them.

Now the problem I am having is that I have no idea how a regular shell parses out bad input. Say for instance you type "pig" into the command line of your standar tcsh. It will say "pig": Bad command name or something to that effect. I want mine to do that but right now it is just accepting "pig" and calling it another running instance of my current shell. I know i have to do some sort of search for $PATH but no one has ever told me what functions do that kind of search, or even how to search that kind of thing so that I can error check. I pasted my code in here in hopes that perhaps someone can show me where and what the hell to do with making sure bad input stays bad input and doesn't get logged as a process.

C++ Syntax (Toggle Plain Text)
  1. /*
  2.  * Program: This is a shell that reports stats of running times of processes initiated in 
  3.  * the shell as well as the shell itself's process time 
  4.  */
  5.  
  6. #include <sys/types.h>
  7. #include <sys/wait.h>
  8. #include <sys/times.h>
  9. #include <sys/stat.h>
  10. #include <sys/ipc.h>
  11. #include <sys/msg.h>
  12. #include <sys/socket.h>
  13. #include <signal.h>
  14. #include <fcntl.h>
  15. #include <unistd.h>
  16. #include <string.h>
  17. #include <string>
  18. #include <iostream>
  19. #include <iomanip>
  20. #include <cstdlib>
  21.  
  22. using namespace std;
  23.  
  24. /**
  25.  *  method displayID
  26.  *  returns: nothing
  27.  *  parameters: none
  28.  *  Description: This method displays the author information
  29.  */
  30.  
  31. void displayID()
  32. {
  33. }
  34.  
  35. /**
  36.  *  method displayInfo
  37.  *  returns: nothing
  38.  *  parameters: none
  39.  *  Description: This method displays the Game info
  40.  */
  41.  
  42. void displayInfo()
  43. {
  44.         cout << "The function of the shell is to operate child processes" << endl;
  45. 	cout << "And return statistics of process time of each process" << endl;
  46. 	cout << "That has been running in the shell as well as the shell's" << endl;
  47. 	cout << "process time" << endl;
  48. }
  49.  
  50.  
  51. /**
  52.  *  method takeInpt
  53.  *  returns: nothing
  54.  *  parameters: none
  55.  *  Description: This method takes the input for the shell
  56.  */
  57.  
  58. void takeInpt(char* cmnd, char* cmd[], char inpt[])
  59. {
  60. 	//ask for and get the input	
  61.         cout << "statsh$: ";
  62.         cin.getline(inpt,81);
  63. 	cmnd = strtok(inpt, " ");
  64. 	int c = 0;
  65.  
  66. 	while(cmnd != NULL)
  67. 	{
  68. 	    cmd[c] = cmnd;
  69. 	    c++;
  70. 	    cmnd = strtok(NULL, " ");
  71. 	}
  72. }
  73.  
  74. /**
  75.  *  method runProc  
  76.  *  returns: nothing
  77.  *  parameters: none
  78.  *  Description: This method runs exec and forking commands
  79.  */
  80.  
  81. void runProc(char* cmd[])
  82. {
  83.     pid_t pid;
  84.  
  85.     // fork
  86.     / /do i check for bad input here??!! 
  87.     pid = fork();
  88.     if(pid < 0)
  89.     {
  90. 	cout << "Fork Failed" << endl;
  91. 	exit(-1);
  92.     }
  93.     else if( pid == 0)
  94.     {
  95. 	execvp(cmd[0], cmd);
  96.     }
  97.     else 
  98.     {
  99. 	wait(NULL);
  100. 	cout << "Job's Done" << endl;
  101.     }
  102.  
  103. }
  104.  
  105. int main()
  106. {
  107.    char* cmnd;
  108.    char* cmd[40];
  109.    char inpt[81]; 
  110.  
  111.    displayID();
  112.    displayInfo();
  113.  
  114.    while(1)
  115.    {
  116.         //clean out the array each time
  117.         for(int m=0; m < 40; m++)
  118.         {
  119.              cmd[m] = NULL;
  120.         }
  121.  
  122.    	takeInpt(cmnd, cmd, inpt);
  123.  
  124.         if(cmd[0] == NULL)
  125.         {
  126.              cout << "Enter a command please" << endl;
  127.         }
  128.    	else if(strcmp(cmd[0], "exit") == 0 )
  129.    	{
  130.              break;
  131.    	}
  132. 	else
  133. 	{
  134.             runProc(cmd);
  135. 	}
  136. 	/*
  137. 	 * testing print statement to see user input is taken and tokenized   
  138.    	for(int i=0; i < 40; i++)
  139.    	{
  140.        	     if(cmd[i])
  141.        	     {
  142.            	  cout<< cmd[i] << endl;
  143.        	     }
  144.    	}
  145. 	*/
  146.    }
  147.  
  148.    return 1;
  149.  
  150. }

If anyone could help me I would greatly appreciate it. In the meantime I'll keep hittin away at it and try to get it right.
Similar Threads
Reputation Points: 13
Solved Threads: 0
Junior Poster in Training
kharri5 is offline Offline
56 posts
since Jan 2005
Permalink
Jan 31st, 2006
2

Re: Making a UNIX Shell, so inexperienced at it

You can use getenv to get the contents of a certain environment value (such as "PATH"). Use man getenv for details.

The reason it seems like it's running another instance of your current shell is because you've forked your shell into a new process. So the program's doing exactly what you've asked it to do. If execvp fails, it simply returns -1. (If it succeeds, it never returns!) Then your child process keeps running until it finds a reason to exit, and then the parent's wait( ) call comes through, and the parent continues.

If you want to handle this, simply include something to handle it after your execvp call. If the call fails, you can write, "Bad command or filename\n", or whatever you want, and then exit() that process. Then the parent's wait() returns, and the parent continues running.
Team Colleague
Reputation Points: 1135
Solved Threads: 173
Super Senior Demiposter
Rashakil Fol is offline Offline
2,479 posts
since Jun 2005
Permalink
Jan 31st, 2006
0

Re: Making a UNIX Shell, so inexperienced at it

Quote originally posted by Rashakil Fol ...
You can use getenv to get the contents of a certain environment value (such as "PATH"). Use man getenv for details.

The reason it seems like it's running another instance of your current shell is because you've forked your shell into a new process. So the program's doing exactly what you've asked it to do. If execvp fails, it simply returns -1. (If it succeeds, it never returns!) Then your child process keeps running until it finds a reason to exit, and then the parent's wait( ) call comes through, and the parent continues.

If you want to handle this, simply include something to handle it after your execvp call. If the call fails, you can write, "Bad command or filename\n", or whatever you want, and then exit() that process. Then the parent's wait() returns, and the parent continues running.

Thank you first of all for replying to my concern, but I'm afraid I still am not quite getting it. I did this to my procedure. Now it is saying that all of my input is bad input. Am I using getenv wrong? It should search the environment for that variable named there in and return NULL if it doesn't find it right? So says the man pages. So I'm trying to do taht, but it isn't doing it.

C++ Syntax (Toggle Plain Text)
  1. void runProc(char* cmd[], int argSz) //argSz should be the max amount of args they entered counted in another method
  2. {
  3.     tms strt, end;
  4.     long s,e;
  5.     pid_t pid;
  6.     bool badEx = false;
  7.  
  8.     for(int i=0; i < argSz; i++)   
  9.     {
  10.         //cout << "What's wrong here" << endl;
  11.         if(getenv(cmd[i]) == NULL)
  12.         {
  13.             cout << "The error occured at: " << i << endl;
  14.             badEx = true;  
  15.             break;
  16.         }
  17.     }
  18.     if(badEx == true)
  19.     {
  20.         cerr << "Bad Command or file name" << endl;
  21.     }
  22.     else
  23.     {
  24.     // fork
  25.     times(&strt);   
  26.     pid = fork();   
  27.     switch(pid) 
  28.     {
  29.         case -1:
  30.             cout << "Fork Failed" << endl;
  31.             exit(-1);
  32.         case  0:   
  33.             execvp(cmd[0], cmd);
  34.  
  35.         default: 
  36.  
  37.             wait(NULL);
  38.             times(&end);
  39.             cout << "Process took: " << (end.tms_utime-strt.tms_utime)/100;
  40.             cout << " seconds" << endl;
  41.             cout << "Child has finished" << endl;
  42.     }
  43. } //end else

And lastly, I don't get what you mean by putting a line after the execvp line. What would the line contain? An exit(0) statement? I'm a little confused about that.
Reputation Points: 13
Solved Threads: 0
Junior Poster in Training
kharri5 is offline Offline
56 posts
since Jan 2005
Permalink
Jan 31st, 2006
2

Re: Making a UNIX Shell, so inexperienced at it

You don't actually need getenv. If execvp fails, it returns. That's how you know it fails. But as a side note, you should be using getenv with
C++ Syntax (Toggle Plain Text)
  1. char * pathstr = getenv("PATH");
Then pathstr contains the null memory address, or it contains the memory address of a string such as "/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games". That's the output of this miniprogram:

C++ Syntax (Toggle Plain Text)
  1. #include <stdlib.h>
  2. #include <stdio.h>
  3.  
  4. int main(void) {
  5.  
  6.   char * s = getenv("PATH");
  7.  
  8.   if (s) {
  9.     printf("%s\n", s);
  10.   }
  11.  
  12.   return 0;
  13. }


Quote ...
And lastly, I don't get what you mean by putting a line after the execvp line. What would the line contain? An exit(0) statement? I'm a little confused about that.
If execvp returns, you still have two processes running. The parent is wait()ing for the child process to terminate. What do you want your child process to do?
Team Colleague
Reputation Points: 1135
Solved Threads: 173
Super Senior Demiposter
Rashakil Fol is offline Offline
2,479 posts
since Jun 2005
Permalink
Jan 31st, 2006
0

Re: Making a UNIX Shell, so inexperienced at it

Quote originally posted by Rashakil Fol ...
You don't actually need getenv. If execvp fails, it returns. That's how you know it fails. But as a side note, you should be using getenv with
C++ Syntax (Toggle Plain Text)
  1. char * pathstr = getenv("PATH");
Then pathstr contains the null memory address, or it contains the memory address of a string such as "/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games". That's the output of this miniprogram:

C++ Syntax (Toggle Plain Text)
  1. #include <stdlib.h>
  2. #include <stdio.h>
  3.  
  4. int main(void) {
  5.  
  6.   char * s = getenv("PATH");
  7.  
  8.   if (s) {
  9.     printf("%s\n", s);
  10.   }
  11.  
  12.   return 0;
  13. }


Let me know if I that is wrong. I think it seems to be working right now, but I need to remember that it must work for later commands when I add piping and duping crap...whcih as of yet I don't konw how to do.

Thanks again for the help btw. Good show!



If execvp returns, you still have two processes running. The parent is wait()ing for the child process to terminate. What do you want your child process to do?

Hmm, did you mean something like the following. Since execvp returns -1 when it fails, then this should work for exiting the process that isn't a real process right?

C++ Syntax (Toggle Plain Text)
  1. // fork
  2.     times(&strt);
  3.     pid = fork();
  4.     switch(pid)
  5.     {
  6.         case -1:  
  7.             cout << "Fork Failed" << endl;
  8.             exit(-1);
  9.         case  0:
  10.             execvp(cmd[0], cmd);
  11.  
  12.             if(execvp(cmd[0], cmd) == -1)
  13.             {
  14.                 cout << "Command Not Found" << endl;
  15.                 exit(0);
  16.             }
  17.  
  18.         default:  
  19.  
  20.             wait(NULL);
  21.             times(&end);
  22.             cout << "Process took: " << (end.tms_utime-strt.tms_utime)/100;
  23.             cout << " seconds" << endl;   
  24.             cout << "Child has finished" << endl;
  25.     }
Reputation Points: 13
Solved Threads: 0
Junior Poster in Training
kharri5 is offline Offline
56 posts
since Jan 2005
Permalink
Jan 31st, 2006
3

Re: Making a UNIX Shell, so inexperienced at it

That looks like an appropriate change, though you don't need to check for execvp returning -1. If it returns, we already know something failed. It returns -1 from historic practice, but the return value has never been standardized. (Or so says the man page on my machine.)
Team Colleague
Reputation Points: 1135
Solved Threads: 173
Super Senior Demiposter
Rashakil Fol is offline Offline
2,479 posts
since Jun 2005
Permalink
Jan 31st, 2006
0

Re: Making a UNIX Shell, so inexperienced at it

Quote originally posted by Rashakil Fol ...
That looks like an appropriate change, though you don't need to check for execvp returning -1. If it returns, we already know something failed. It returns -1 from historic practice, but the return value has never been standardized. (Or so says the man page on my machine.)
You surely are a king among kings. I thank you for your advice and counsel on the matter.
Reputation Points: 13
Solved Threads: 0
Junior Poster in Training
kharri5 is offline Offline
56 posts
since Jan 2005

This thread is more than three months old

No one has posted to this discussion for at least three months. Please let old threads die and do not reply to them unless you feel you have something new and valuable to contribute that absolutely must be added to make the discussion complete. Otherwise, please start a new thread in this forum instead.
Message:
Previous Thread in C++ Forum Timeline: MFC problems
Next Thread in C++ Forum Timeline: hi





About Us | Contact Us | Advertise | Acceptable Use Policy
Forum Index | Build Custom RSS Feed


Follow us on Twitter


© 2011 DaniWeb® LLC