[SOLVED] How do I implement a CD builtin to a C Shell?

Kslkgh · 07-31-2015, 02:09 AM

I have written (with the help of the community at LQ) a C Shell, named vsh. I now need to implement the CD command. I have tried various implementations, but nothing I have tried works. My code is as follows:

Code:

/**
	@file vsh.c
	
	@author George Gibson
	
	@date Wednesday 29th July 2015
	
	@brief vsh Shell Version 1.0 BETA
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/wait.h>

#define LINESIZE 1024
#define RL_BUFSIZE 1024
#define TOK_BUFSIZE 64
#define TOK_DELIM " \t\r\n\a"

char **split_line(char *line)
{
    int bufsize = TOK_BUFSIZE, position = 0;
    char **tokens = malloc(bufsize * sizeof(char*));
    char *token, **tokens_backup;

    if (!tokens) {
        fprintf(stderr, "vsh: allocation error\n");
        exit(EXIT_FAILURE);
    }
		
    token = strtok(line, TOK_DELIM);
    while (token != NULL) {
        tokens[position] = token;
        position++;
	
        if (position >= bufsize) {
            bufsize += TOK_BUFSIZE;
            tokens_backup = tokens;
            tokens = realloc(tokens, bufsize * sizeof(char*));
            if (!tokens) {
		        free(tokens_backup);
                fprintf(stderr, "vsh: allocation error\n");
                exit(EXIT_FAILURE);
            }
        }
	
        token = strtok(NULL, TOK_DELIM);
    }
    tokens[position] = NULL;
    
	return tokens;
}

int main(int argc, char **argv)
{
	int finish = 0;
	char *user = getenv("USER");
	char hostname[1024];
	char *pwd = getenv("PWD");
    char line[LINESIZE] = {0};
    char lineCopy[LINESIZE] = {0};
    char *command = NULL;
    char **args;
    
    // Print out a welcome message
    printf("Welcome to vsh\n");
	
	// Print out the message of the day
    system("/bin/cat /etc/motd");
    
    printf("\n");
	
	gethostname(hostname, 1024);
	
    while (!finish)
    {
		// Update 'pwd' variable
		pwd = getenv("PWD");
		// Print out prompt
		printf("[");
        printf(user);
        printf("@");
        printf(hostname);
        printf("] ");
        printf(pwd);
        printf(" $ ");
        fflush(stdout);
        
        if(NULL == fgets(line, sizeof line, stdin))
        {
			// If NULL, leave vsh
            finish = 1;
            printf("\nLeaving vsh\n");
        }
        else
        {
			// If there is something there...
            printf("The command read was s", line);
            printf("\n");
            char *newLine = strchr(line, '\n');
            
            if(newLine != NULL)
            {
                *newLine = '\0';
                strcpy(lineCopy, line);
            }
            
            command = strchr(line, ' ');
            
            if(command != NULL)
            {
                *command++ = '\0';
                printf("Command= _%s_\n\n", line);                
            }
            
            args = split_line(line);
            
            if(strcmp(line, "") == 0) {
				fprintf(stderr, "vsh: Expected command\n");
			}
            else if(strcmp(line, "cd") == 0)
            {
				printf("cd not available in BETA versions of vsh");
			}
            else if(strcmp(line,"programmer") == 0)
            {
                printf("The programmer of vsh is George Gibson\n");
            }
            else if(strcmp(line,"ver") == 0)
            {
                printf("The current version is 1.0\n");
            }
            else if(strcmp(line, "help") == 0)
            {
				printf("Help for vsh\n\nType a command to run it. The builtin commands are : cd, programmer, ver, help and exit.Their functions are as follows:\n\ncd: Change Directory. Changes the current working directory.\n\nprogrammer: Display the programmers of vsh.\n\nver: Display the version of vsh you are running.\n\nhelp: Launches this program.\n\nexit: Leave vsh Shell.\n\nThese are the only builtins in vsh, but note that if you define a command in /bin with one of these names, your program will not run in vsh unless you use it's explicit name (/bin/yourprogram) rather than \"yourprogram\". vsh 1.0 written and programmed by George Gibson.\n");
			}
            else if(strcmp(line,"exit") == 0)
			{
				finish = 1;
				printf("\nLeaving vsh...\n");
				getchar();
			}
            else
            {
				// Run the command
                pid_t pid;
				int err;

				pid=fork();
				
				if(pid == -1) {
					perror("vsh");
				}
				else if(pid == 0) {
					execvp(args[0], args);
				    perror("vsh");
				}
				else {
					waitpid(pid, &err, 0);
				}
            }
        }
    }
    
    return 0;
}

ntubski · 07-31-2015, 02:56 AM

Have you tried calling chdir()?

Kslkgh · 07-31-2015, 03:29 AM

Yes, but it never works properly.

ntubski · 07-31-2015, 03:42 AM

Quote:

Originally Posted by Kslkgh

Yes, but it never works properly.

"never works properly" or other variations of "it doesn't work" are not descriptive enough for anyone to be able to help you.

Kslkgh · 07-31-2015, 06:10 AM

I call it in the following manner:

Code:

directory = args[1];
chdir(directory);

It compiles fine, but nothing happens when I run the cd command with or without an input. It reads the command, but does not change the working directory.

ntubski · 07-31-2015, 06:19 AM

That's still not really enough context, but I guess that you either you misparsed args[1] and ignored the error returned by chdir, or you called chdir in a subprocess (i.e. after forking); chdir only affects the current process.

Kslkgh · 07-31-2015, 06:33 AM

If there is not enough to go on, here is my complete code:

Code:

/**
	@file vsh.c
	
	@author George Gibson
	
	@date Wednesday 29th July 2015
	
	@brief vsh Shell Version 1.0 BETA
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/wait.h>

#define LINESIZE 1024
#define RL_BUFSIZE 1024
#define TOK_BUFSIZE 64
#define TOK_DELIM " \t\r\n\a"

char **split_line(char *line)
{
    int bufsize = TOK_BUFSIZE, position = 0;
    char **tokens = malloc(bufsize * sizeof(char*));
    char *token, **tokens_backup;

    if (!tokens) {
        fprintf(stderr, "vsh: allocation error\n");
        exit(EXIT_FAILURE);
    }
		
    token = strtok(line, TOK_DELIM);
    while (token != NULL) {
        tokens[position] = token;
        position++;
	
        if (position >= bufsize) {
            bufsize += TOK_BUFSIZE;
            tokens_backup = tokens;
            tokens = realloc(tokens, bufsize * sizeof(char*));
            if (!tokens) {
		        free(tokens_backup);
                fprintf(stderr, "vsh: allocation error\n");
                exit(EXIT_FAILURE);
            }
        }
	
        token = strtok(NULL, TOK_DELIM);
    }
    tokens[position] = NULL;
    
	return tokens;
}

int main(int argc, char **argv)
{
	int finish = 0;
	int errno;
	char *user = getenv("USER");
	char hostname[1024];
	char *pwd = getenv("PWD");
    char line[LINESIZE] = {0};
    char lineCopy[LINESIZE] = {0};
    char *command = NULL;
    char **args;
    char *directory;
    
    // Print out a welcome message
    printf("Welcome to vsh\n");
	
	// Print out the message of the day
    system("/bin/cat /etc/motd");
    
    printf("\n");
	
	gethostname(hostname, 1024);
	
    while (!finish)
    {
		// Update 'pwd' variable
		pwd = getenv("PWD");
		// Print out prompt
		printf("[");
        printf(user);
        printf("@");
        printf(hostname);
        printf("] ");
        printf(pwd);
        printf(" $ ");
        fflush(stdout);
        
        if(NULL == fgets(line, sizeof line, stdin))
        {
			// If NULL, leave vsh
            finish = 1;
            printf("\nLeaving vsh\n");
        }
        else
        {
			// If there is something there...
            printf("The command read was s", line);
            printf("\n");
            char *newLine = strchr(line, '\n');
            
            if(newLine != NULL)
            {
                *newLine = '\0';
                strcpy(lineCopy, line);
            }
            
            command = strchr(line, ' ');
            
            if(command != NULL)
            {
                *command++ = '\0';
                printf("Command= _%s_\n\n", line);                
            }
            
            args = split_line(line);
            
            if(strcmp(line, "") == 0) {
				fprintf(stderr, "vsh: Expected command\n");
			}
            else if(strcmp(line, "cd") == 0)
            {
				directory = args[1];
				chdir(directory);
				if(chdir(directory) == -1)
				{
					fprintf(stderr, "vsh: Error changing directory\n");
				}
			}
            else if(strcmp(line,"programmer") == 0)
            {
                printf("The programmer of vsh is George Gibson\n");
            }
            else if(strcmp(line,"ver") == 0)
            {
                printf("The current version is 1.0\n");
            }
            else if(strcmp(line, "help") == 0)
            {
				printf("Help for vsh\n\nType a command to run it. The builtin commands are : cd, programmer, ver, help and exit.Their functions are as follows:\n\ncd: Change Directory. Changes the current working directory.\n\nprogrammer: Display the programmers of vsh.\n\nver: Display the version of vsh you are running.\n\nhelp: Launches this program.\n\nexit: Leave vsh Shell.\n\nThese are the only builtins in vsh, but note that if you define a command in /bin with one of these names, your program will not run in vsh unless you use it's explicit name (/bin/yourprogram) rather than \"yourprogram\". vsh 1.0 written and programmed by George Gibson.\n");
			}
            else if(strcmp(line,"exit") == 0)
			{
				finish = 1;
				printf("\nLeaving vsh...\n");
				getchar();
			}
            else
            {
				// Run the command
                pid_t pid;
				int err;

				pid=fork();
				
				if(pid == -1) {
					perror("vsh");
				}
				else if(pid == 0) {
					execvp(args[0], args);
				    perror("vsh");
				}
				else {
					waitpid(pid, &err, 0);
				}
            }
        }
    }
    
    return 0;
}

There appears to be an error, as I get "vsh: Error changing directory" when I try to run cd, even with no arguments.

ntubski · 07-31-2015, 06:39 AM

Quote:

There appears to be an error, as I get "vsh: Error changing directory" when I try to run cd, even with no arguments.

I suggest adding a perror("cd") to see exactly what the error is. Examining the value of args[1] using a debugger could also provide some insight.

Kslkgh · 07-31-2015, 06:47 AM

OK, taken your advice and added a perror("cd"), now I get the error 'cd: Bad address'. I've had this error before, but that was being executing in a different manner, and thus won't work here. If I try to do args[0], I get 'cd: No such file or directory'.

NevemTeve · 07-31-2015, 06:47 AM

Never ever ignore return values and error codes, ok?

Code:

rc= chdir (directory);
if (rc) {
    int ern= errno;

    fprintf (stderr, "*** Error in chdir('%s') errno=%d: %s\n",
        directory, ern, strerror (ern));
    }
}

ntubski · 07-31-2015, 07:18 AM

Quote:

Originally Posted by Kslkgh

OK, taken your advice and added a perror("cd"), now I get the error 'cd: Bad address'.

Ok, so that tells you args[1] is bad in some way. Try looking at it in the debugger.

Kslkgh · 07-31-2015, 07:45 AM

The variable errno is set to 0 and ern is set to "Success", but the directory doesn't change.

genss · 07-31-2015, 07:58 AM

use strace
it will help you a lot

also (unrelated to this) see termios, notably setting the terminal in to non-canonical mode
https://en.wikibooks.org/wiki/Serial...amming/termios

NevemTeve · 07-31-2015, 08:29 AM

Quote:

Originally Posted by Kslkgh

The variable errno is set to 0 and ern is set to "Success", but the directory doesn't change.

Exactly how do you know that?

Kslkgh · 07-31-2015, 09:40 AM

Quote:

Originally Posted by NevemTeve

Exactly how do you know that?

The debugger (and the fprintf() in your earlier post).