When I am writing my own interpreter...

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# 15  
Old 10-18-2007
Smilie Yeah sure... I might seem a little ambitious and I'm sure it'll take some time to build all that but I'll show it to you once I'm done with it... So do you have any suggestions to give me as far as my last post is concerned? I've just pasted the relevant matter here:

And while implementing pipes, when I issue a command such as "ls | grep .c" in my interpreter, after parsing it, what should I be doing? I read about file descriptors and am assuming the following has to be done:

1. Parse the command line
2. argv[0] contains ls, so fork a process and execute it but redirect the output to a file (I don't know how this can be done internally in the C Code. I know I need to use the execv command to execute but how will I redirect?)
3. In the next parse, I scan the "|" character and so I know that the user wants to pipe the output and at this stage I would fork another process with the argv[2] string i.e. "grep .c" (but this will be stored in argv[2] and argv[3]. How will I know that the second one has command line arguments too and how should I handle them?) and then direct the output of this to the stdout.
# 16  
Old 10-18-2007
You need to read in a logical line, so a "\" followed by a new line continues.

Then you need to do macro expansion, so look for all "$" and expand as per environment variable.

I would then look for all commands between back quotes, run a subshell to execute the contents and expand the command with the stdout.

Then I would split the result based on pipes to work out what the actual processes are with arguments.

Then I would pull out the <,>,<< and >> tokens and do the appropriate IO redirection

Then I would look for a "&" and flag if I found that to say don't wait for result.

Finally I would setup the pipe line and do the chain of forks and execs.

Quite where you choose to detect "cd", "if", "while", "do" etc is a good question.
# 17  
Old 10-18-2007
Thank you so much... Will get to work now that I've got some suggestions from you Smilie Will let you know as and when I'm done...
# 18  
Old 10-24-2007
Thanks to everyone here... I am slowly able to realize the shell... I'm actually stuck at piping... I am able to handle a single pipe but how do I solve the problem of multiple pipes? I know it can be solved using recursion but some pseudo algorithm will be excellent... I don't understand how to actually use recursion here... I'm currently doing something like a parent creates two children and the first one executes one command and pipes it onto the second child which displays the output...

And when I used valgrind, to my surprise I found 15 memory leaks from the piping function that I wrote and I don't understand what could've gone wrong... My pseudo code looks something like this:

	int fd[2]; /* provide file descriptor pointer array for pipe */
	pid_t pid1, pid2; /* process ids for each child */
	/* create pipe and check for an error */
	/* apply fork and check for error */
	    /* processing for child */
	    close (fd[1]); /* close output end, leaving input open */
	    /* set standard input to pipe */
	    if (fd[0] != STDIN_FILENO)
		if (dup2(fd[0], STDIN_FILENO) != STDIN_FILENO)
		    perror("dup2 error for standard input");
	   execlp the second function
	    //First child finished
	    /* processing for parent */
	    /* spawn second process */
	   /* apply fork again for second child*/
	  /* processing for child */
		close (fd[0]);
		/* set standard output to pipe */
		if (fd[1] != STDOUT_FILENO)
		    if (dup2(fd[1], STDOUT_FILENO) != STDOUT_FILENO)
			perror("dup2 error for standard output");
		    close(fd[1]); /* not needed after dup2 */
		execlp the first function
		/* print to the pipe, now standard output */
		/* processing continues for parent */
		waitpid (pid1, NULL, 0); /* wait for first child to finish */
		waitpid (pid2, NULL, 0); /* wait for second child to finish */

Am I doing something wrong?

Last edited by Legend986; 10-24-2007 at 03:45 AM..
# 19  
Old 10-24-2007
It should be simpler than the way you have it....

all that "|" means is give the write end to stdout of the left hand process and the read end to stdin for the right hand process.

but apart from > and <, the stdin/stdout/stderr should just be left alone.

To do ">" you just open a file and use it for stdout, end of story.

I personally would parse the line into a tree where each node is what I want to run in one process, each node would have pointers to where they get their stdin/stdout/stderr from.
# 20  
Old 10-24-2007
Hmm... you're method seems more logical... But I think I've done the same thing, the only difference being that I've written a few extra lines of code Smilie I know it was a mistake that was supposed to have been rectified in the beginning but I've already written my parser and it gives me something like:

argv[0] = ls
argv[1] = -la
argv[2] = |
argv[3] = wc
argv[4] = |
argv[5] = wc

for an input like "ls -la | wc | wc"

I am able to get it to work for "ls -la | wc" but the next thing is what is puzzling me... Is it possible using the architecture that I've written?

And yeah, thanks for the tip on redirection... Will attempt that tomorrow Smilie
# 21  
Old 10-24-2007
I think you need to turn the list you have into the chain of processes, where each process is one element in the chain, then you can associate the arguments with the appropriate process and then when the list is all assembled have a piece of code that goes through the whole list forking/pipe'ing and exec'ing as required.

Of course the whole linked list of processes becomes one job.
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