Toy Shell 1

Table Of Contents

• Program Logic
• Reference Code

Main Program Logic

This is the basic layout of a toy shell.

Each of the following steps will be detailed below.

int main()
{
	bool keepLooping = true;
	while(keepLooping)
	{
		1 - GRAB THE COMMAND LINE
		2 - PARSE THE COMMAND LINE
		3 - ERROR CHECK COMMAND LINE

		if(/*command is exit*/)
		{
			keepLooping = false;
		}
		else
		{
			3 - FORK
			4 - PARENT PROCESS:
				4.1 - WAIT FOR CHILD
			5 - CHILD PROCESS:
				5.1 - EXECL
		}
	}
	return 0;
}

Reference Code

Grab The Command Line

The following code can be used to grab a line text from the command line.

The entire line of text upto the \n will be put into the buffer.

char buffer[256];
cin.getline(buffer, 256);

Parse The Command Line

The following code is an example of parsing a commmand line.

This code uses strtok_s to grab the elements from the buffer 1 at a time.

This code can be modified to form a set of arguements that can be passed to the execl function.

char* token;
char* nextToken;
token = strtok_s(buffer, " ", &nextToken);
while(token != NULL)
{
	token = strtok_s(NULL, " ", &nextToken);
}

Here is the same example with strtok

char* token;
token = strtok(buffer, " ");
while(token != NULL)
{
	token = strtok(NULL, " ");
}

This code loops through the command line and puts each successive element of the command line into token

As you grab the tokens one at a time you can decide what you want to do with them.

There is only 1 special token you have to handle. The > token.

> is the redirection token and your program should handle it the same way unix does. Redirect STDOUT to the specified file.

EXAMPLE:

cat someFile.txt > double.txt

Normally cat would print someFile.txt to the screen but the > token forces the output to a file called double.txt instead.

Fork => The Parent

Have the parent wait() on the child process.

Fork => The Child

Use your preferred version of execl to execute the command line.
Lab Page On Fork() and Execl()