Thread: Creation of a Command Language Interpreter

Hello fellow programmers,

I was recently given a task of creating a CLI in c under a unix based system. I have almost completed the task, but am currently stuck with 2 particular points, the logic of the task is as follows (direct reference to documentation):

main:
loop
get input line
if end of input exit
break line into words
found := false
if command is builtin
then
do_builtin( line)
found := true
else
found := find_and_execute( line)
end if
if not found report error
end loop

The marking of the task is as follows:
1. Read a full line of text from the user
2. Detect exit as first word
3. break line into words
4. Appropriate test of fork() return value
5. Correct process handles new task
6. Appropriate choice of exec() family member
7.Correct usage of exec() member
8. Correct use of wait()
9. Printing the exit status
10. Working cd implementation

I have completed points 1,2,3,4,6,8,9 and attempting point 10 currently.

I request assistance in undertaking points 5 and 7. I have identified that i would use the execv family member, but not am aware of how to use it in this situation.

The code of the execv family member should be located within the child process and if a keyword is entered that is identifed by unix (eg. ls = list directory), the process should carry out the task( ls = display output of list directory).

I am allowed to receive assistance, as long as reference is made regarding who i obtained assistance from. Unfortunatley as this is a current task i am reluctant to post up any code and am therefore requesting trust and reliance in the c programming community.

Please advise me in how i may be able to provide the code, without making it public, would people be willing to provide email addresses to which i can send the code?

Please also note that the program will only compile on a unix machine.

Thankyou

Sicilian

Not sure about the question as you'ave yourself solved item #5.
That is, a Unix utility like "ls" will be invoked in the child process.
Are you unsure about the arguments that are passed to execv()?

Sicilian_10

You need to add something similar to the below code, to your program, before you fork anything new.

cd cannot work as an external command because an external command creates a child process, and a process cannot affect the environment of its parent (shell in this case).

Code:

if (!strcmp (command, "cd"))
	    {
	      if (argv[1] == NULL)
		{
		  chdir ("/");
		}
	      else
		{
		  chdir (argv[1]);
		}
	      perror (command);
	    }

Oh I didn't read that you have already completed point 10 !

You mean by point number 7 that you didn't understand how to get an proper listing of files when you do ls through your program ?

Originally Posted by Sicilian_10

execv has made it more difficult.

execvp is more simpler than execl

See the following thread:
Making a C++ shell

not execvp its execv, there is a difference

Oh again i misread it
May be i am tired today !

Ok Try the following program now with input -a only.

I have fixed the command /usr/bin/who in the program.

Tell me if i have again misread something !

Code:

#include <unistd.h>     // getpid(), getcwd()
#include <sys/types.h>  // type definitions, e.g., pid_t
#include <sys/wait.h>   // wait()
#include <signal.h>     // signal name constants and kill()
#include <iostream>
#include <vector>
#include <string.h>
using namespace std;

int main()
{
	while ( true )
	{
      		// Show prompt.
		cout << get_current_dir_name () << "$ " ;
		char command[128];
		cin.getline( command, 128 );
      
		vector<char*> args;
		char* prog = strtok( command, " " );
		char* tmp = prog;
		while ( tmp != NULL )
		{
			args.push_back( tmp );
			tmp = strtok( NULL, " " );
		}
      
		char** argv = new char*[args.size()+1];
		for ( int k = 0; k < args.size(); k++ )
			argv[k] = args[k];
      
		argv[args.size()] = NULL;
      
		if ( strcmp( command, "exit" ) == 0 )
		{
			return 0;
		}
		else
		{
			pid_t kidpid = fork();

			if (kidpid < 0)
			{
				perror( "Internal error: cannot fork." );
				return -1;
			}
			else if (kidpid == 0)
			{
		  		// I am the child.					
				execv ("/usr/bin/who", argv);
		  
				  // The following lines should not happen (normally).
				perror( command );
				return -1;
			}
			else
			{
				  // I am the parent.  Wait for the child.
				if ( waitpid( kidpid, 0, 0 ) < 0 )
				{
					perror( "Internal error: cannot wait for child." );
					return -1;
				}
			}
		}
	}
  
	return 0;
}

The code that you have provided does not appear to work in the way intended for the program.

Thankyou for your attempt anyway

can you kindly explain what the code doesn't do that you want exactly !

The execv does not determine that a function keyword has been entered and as a result pass the task to the child to be carried out.

From my understanding the only thing the current code will do is identify who is connected (so to speak through the execv"/usr/bin/who, the arguments are not identified.