Creating A Shell: Difference between revisions

Based on the explanations above, here is the main function for a simple shell:

 

void main(int argc, char* argv[]) // edit as appropriate for your kernel

{

}

}

 

=Implementation=

The implementation of the functions referred to in the above code snippet depends heavily on the design of your operating system, but here are some typical examples:

 

static void output_prompt()

{

// TODO: this function will allocate a char buffer containing the line read

}

 

=TODO=

First we need to make a file, shell.c, this file will contain all our functions and shell managers. The first function we need to write is our initialization function then our actual shell function.

 

/* shell.c */

void init_shell()

{

}

 

Now you need to add "init_shell()" to your setup routine in your kernel.c file or whatever you named it, followed by a while loop running shell until the boolean "exit_status" is true.

Ok, now we need to write a few things. The first is command table, or a way to store our commands. I do this with a structure as follows:

 

/* shell.h */

#define MAX_COMMANDS 100

void *function;

} command_table_t;

 

Now we have a way to store commands, but how do we use this? Well lets add a few variables to shell.c:

 

/* shell.c */

command_table_t CommandTable[MAX_COMMANDS];

 

This goes right above the function definitions. The first is a way to keep track of how many commands there are. The second is our command table. Ok, so now we have a "Command Table", but how to we access it? Well in order to add a command we need a function to do it. Add this below the other functions:

Ok, now we are getting somewhere. We can add a command. But what about getting command line input? Thats up to our "shell()" function. But before we code that, lets add another variable: "char* input_string". Now for the shell function:

 

/* shell.c */

void shell()

void (*command_run)(void);

}

 

Ok, now we can get a string. But what about finding what command the user typed? Well we rely on old trusty "findCommand()" function for that one:

Now we need to add a little something right below "gets(input_string)", but above "void (*command_function)(void)":

 

/* shell.c */

int i = findCommand(input_string);

return;

 

Now in our "init_shell()" function we need to add a few commands:

 

/* shell.c */

add_new_command("help", "You code this one.", help_command);

add_new_command("", "", empty_command);

 

Now in our shell.h file:

 

extern void add_new_command();

extern void help_command();

extern void empty_command();

 

Now you need to code a void help_command function and a empty_command that manages null input, all it needs is a definition:

 

void empty_command()

{

}

 

An idea is to loop through all the command table and print the command name and description - for the help command.