Thread: Qustion about writing a shell

Hello! This is my first post and I have a question about some code I am working with. This is working code for a basic Linux shell. It works fine for running external commands,but I am wondering how I can add built in functions,such as cd and the like. Any help is greatly appreciated.

Code:

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<signal.h>
#include<string.h>
#include<sys/wait.h>
#include<sys/types.h>
#include<ctype.h>
#include<errno.h>
#include<sys/stat.h>
#include<fcntl.h>

#define NUM1 10
#define NUM2 100
#define NUM3 256

extern int errno;

typedef void(*sighandler_t)(int);

static char buf[NUM3];
static char *my_argv[NUM2],*my_envp[NUM2];
static char *search_path[NUM1];

void show_prompt() 
{
	getcwd(buf,sizeof buf);	
	printf("%s:* ",buf);
}

void handle_signal(int signo) /
{
	printf("\n");
	show_prompt();
	fflush(stdout);
}

void fill_argv(char *tmp_argv) 
	char *foo = tmp_argv;
	int index = 0;
	char ret[100];
	bzero(ret,100);
	while(*foo != '\0')
	{
		if(index == 10)
			break;
		if(*foo == ' ')
		{
		if(my_argv[index] == NULL)
		my_argv[index]=(char*)malloc(sizeof(char)*strlen(ret)+1);
		else
		{
			bzero(my_argv[index],strlen(my_argv[index]));
		}
		strncpy(my_argv[index],ret,strlen(ret));
		strncat(my_argv[index],"\0",1);
		bzero(ret,100);
		index++;
		}
		else
		{
			strncat(ret,foo,1);
		}
		foo++;
	}
	my_argv[index]=(char*)malloc(sizeof(char)*strlen(ret)+1);
	strncpy(my_argv[index],ret,strlen(ret));
	strncat(my_argv[index],"\0",1);

}

void copy_envp(char **envp)
{
	int index = 0;
	for(;envp[index]!=NULL;index++)
	{
	my_envp[index]=(char*)malloc(sizeof(char)*(strlen(envp[index])+1));
	memcpy(my_envp[index],envp[index],strlen(envp[index]));
	}
}

void get_path_string(char **tmp_envp,char *bin_path)
{
	int count = 0;
	char *tmp;
	while(1)
	{
		tmp = strstr(tmp_envp[count], "PATH");
		if(tmp == NULL)
		{
			count++;
		}
		else
		{
			break;
		}
	}
	strncpy(bin_path,tmp,strlen(tmp));
}

void insert_path_str_to_search(char *path_str)
{
	int index = 0;
	char *tmp = path_str;
	char ret[100];
	
	while(*tmp != '=')
		tmp++;
	tmp++;

	while(*tmp != '\0')
	{
		if(*tmp==':')
		{
			strncat(ret,"/",1);
			search_path[index]=(char*)malloc(sizeof(char)*(strlen(ret)+1));
			strncat(search_path[index],ret,strlen(ret));
			strncat(search_path[index],"\0",1);
			index++;
			bzero(ret,100);
		}
		else
		{
			strncat(ret,tmp,1);
		}
		tmp++;
	}
}

int attach_path(char *cmd)
{
	char ret[100];
	int index;
	int fd;
	bzero(ret,100);

	for(index=0;search_path[index]!=NULL;index++)
	{
		strcpy(ret,search_path[index]);
		strncat(ret,cmd,strlen(cmd));
		if((fd=open(ret,O_RDONLY))>0)
		{
			strncpy(cmd,ret,strlen(ret));
			close(fd);
			return(0);
		}
	}
	return(0);
}

void call_execve(char *cmd)
{
	int i;
	printf("\n");
	
	if(fork() == 0)
	{
		i = execve(cmd,my_argv,my_envp);
		if(i<0)
		{
			printf("%s:command not found\n",cmd);
			exit(1);
		}
	}
	else
	{
		wait(NULL);
	}
}

void free_argv()
{
	int index;
	
	for(index=0;my_argv[index]!=NULL;index++)
	{
		bzero(my_argv[index],strlen(my_argv[index])+1);
		my_argv[index]=NULL;
		free(my_argv[index]);
	}
}


int main(int argc, char *argv[], char *envp[])
{
	char c;
	int i,fd;
	char *tmp=(char*)malloc(sizeof(char)*100);
	char *path_str=(char*)malloc(sizeof(char)*256);
	char *cmd=(char*)malloc(sizeof(char)*100);
	signal(SIGINT,SIG_IGN);
	signal(SIGINT,handle_signal);
	
	copy_envp(envp);
	get_path_string(my_envp,path_str);
	insert_path_str_to_search(path_str);

	if(fork() == 0)
	{
		execve("/usr/bin/clear",argv,my_envp);
		exit(1);
	}
	else
	{
		wait(NULL);
	}
	show_prompt();
	fflush(stdout);

	while(c!=EOF)
	{
		c=getchar();
		switch(c){
			case '\n':if(tmp[0]=='\0'){
				show_prompt();
			}else{
				fill_argv(tmp);
				strncpy(cmd,my_argv[0],strlen(my_argv[0]));
				strncat(cmd,"\0",1);
				if(index(cmd,'/')==NULL){
				  if(attach_path(cmd)==0){
				    call_execve(cmd);
				   }else{
				    printf("%s:command not found\n",cmd);
				   }
				}else{
				  if((fd=open(cmd,O_RDONLY))>0){
				    close(fd);
				    call_execve(cmd);
				  
				}else{
					printf("%s:command not found\n",cmd);
					}
				}
			free_argv();
			printf("\n");
			show_prompt();
			bzero(cmd,100);
			}
		bzero(tmp,100);
		break;
		default:strncat(tmp,&c,1);
		}
	}   
	
	free(tmp);
	free(path_str);
	for(i=0;my_envp[i]!=NULL;i++)
		free(my_envp[i]);
	for(i=0;i<10;i++)
		free(search_path[i]);
	printf("\n");
	return(0);
}

Here's what I was thinking. The function fill_argv() takes the user input and uses it to fill the internal my_argv[] array, as opposed to using main's argv[] array. Is it possible to test argv[0] to find out if that value matches the name of an interally defined function and execute it,and if the test fails,then the shell knows it is executing an external command. That is what I have been looking at doing,just not quite sure how to do it.

Thanks guys. I'll try tackling it from that angle, and if I really ball things up,I'll let you know I have an idea what to do, I just need to figure out how the actual implememtation will work.

Thanks all. Between your advice and some various code examples,I'm getting a handle of what to do. I'm going to bury my head in some code,but I'll try to poke it up and look around again soon.

Thanks again all. I got it. Now I can work on expanding it. I'll keep you posted.

Here is the current version of the code, for those who are curious. It's not what you would call feature rich,but it works. Any comments,suggestions, or constructive criticisms are welcome. Thanks again for the help and suggestions that got me this far.

Code:

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<signal.h>
#include<string.h>
#include<sys/wait.h>
#include<sys/types.h>
#include<ctype.h>
#include<errno.h>
#include<sys/stat.h>
#include<fcntl.h>



extern int errno;

typedef void(*sighandler_t)(int);

static char buf[100];
static char *my_argv[100],*my_envp[100];

void show_prompt()
{
	getcwd(buf,sizeof buf);	
	printf("%s* ",buf);
}
	
void handle_signal(int signo)
{
	printf("\n");
	show_prompt();
	fflush(stdout);
}

int cd()
{
	if(strchr(my_argv[1],'~'))
		chdir(getenv("HOME"));
	else
		chdir(my_argv[1]);
	return(0);
}

int run_pipe()
{
	puts("Sorry,this shell does not support pipes.");
	return(0);
}

int redirect()
{
	puts("Sorry,this shell does not support redirection.");
	return(0);
}

void fill_argv(char *tmp_argv)
{
	char *tmp = tmp_argv;
	int index = 0;
	char ret[100];
	bzero(ret,100);
	while(*tmp != '\0')
	{
		if(index == 30)
			break;
		if(*tmp == ' ')
		{
		if(my_argv[index] == NULL)
		my_argv[index]=(char*)malloc(sizeof(char)*strlen(ret)+1);
		else
		{
			bzero(my_argv[index],strlen(my_argv[index]));
		}
		strncpy(my_argv[index],ret,strlen(ret));
		strncat(my_argv[index],"\0",1);
		bzero(ret,100);
		index++;
		}
		else
		{
			strncat(ret,tmp,1);
		}
		tmp++;
	}
	my_argv[index]=(char*)malloc(sizeof(char)*strlen(ret)+1);
	strncpy(my_argv[index],ret,strlen(ret));
	strncat(my_argv[index],"\0",1);

}

void copy_envp(char **envp)
{
	int index = 0;
	for(;envp[index]!=NULL;index++)
	{
	my_envp[index]=(char*)malloc(sizeof(char)*(strlen(envp[index])+1));
	memcpy(my_envp[index],envp[index],strlen(envp[index]));
	}
}

void call_execvp()
{
	int i;
	
	if(fork() == 0)
	{
		i = execvp(my_argv[0],&my_argv[0]);
		if(i<0)
		{
			printf("%s:command not found\n",my_argv[0]);
			exit(1);
		}
	}
	else
	{
		wait(NULL);
	}
}

void free_argv()
{
	int index;
	
	for(index=0;my_argv[index]!=NULL;index++)
	{
		bzero(my_argv[index],strlen(my_argv[index])+1);
		my_argv[index]=NULL;
		free(my_argv[index]);
	}
}


int main(int argc, char *argv[], char *envp[])
{
	char c;
	int i;
	char *tmp=(char*)malloc(sizeof(char)*100);

	signal(SIGINT,SIG_IGN);
	signal(SIGINT,handle_signal);
	
	copy_envp(envp);

	show_prompt();
	fflush(stdout);

	while(c!=EOF)
	{
		c=getchar();
		switch(c){
			case '\n':if(tmp[0]=='\0'){
				show_prompt();
			}else{
				fill_argv(tmp);
				if(strchr(tmp,'|'))
				{
					run_pipe();
				}
				else if(strchr(tmp,'>') || strchr(tmp,'<'))
				{
					redirect();
				}
				else if(strcmp(my_argv[0],"cd")==0)
				{
					/*chdir(my_argv[1]);*/
					cd();
				}
				else if(strcmp(my_argv[0],"version") == 0)
				{
					puts("Version 0.5");
				}
				else if(strcmp(my_argv[0],"exit")==0)
				{
					exit(0);
				}
				else
				{
					call_execvp();
				}
			free_argv();
			show_prompt();
			}
		bzero(tmp,100);
		break;
		default:strncat(tmp,&c,1);
		}
	}   
	
	free(tmp);
	for(i=0;my_envp[i]!=NULL;i++)
		free(my_envp[i]);
	printf("\n");
	return(0);
}

Thanks. Moving getchar() inside the while statement works fine,but changing c to int produces a compiler warning, so I'll leave that as is for now.