Building simple unix shell problems - shell

Question

I am new with unix and I've got an assignemnt on college to build a simple shell in c with built in cd and kill command..
This is my code which is not working..tbh I dont understand it the best so Im not suprised it is not working.. can you help me with it? Also have no idea how I would implement kill command. thank you!
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define BUF_SIZE 1024
const int ARGSIZE = 20;
void execute(char*args[])
{
int pid, status;
pid = fork();
if(pid<0)
{
perror("Error forking!");
}
else if(pid > 0)
{
while(wait(&status) != pid)
continue;
}
else if(pid == 0)
{
if (execvp(args[0], args) == -1)
{
perror("Error");
}
}
}
void cd(char*directory)
{
int ret = 0;
if(directory == '\0')
directory = getenv("HOME");
ret = chdir(directory);
if(ret != 0)
fprintf(stderr,"Failed to enter directory: %s\n",directory);
else
printf("%s\n",directory);
}
int main()
{
char line[BUF_SIZE];
char *args[ARGSIZE];
int argIndex = 0;
while(1){
printf("> ");
fgets(line, BUF_SIZE, stdin);
char *token;
token = strtok(line," ");
while(token!=NULL)
{
args[argIndex]=token;
token = strtok(NULL," ");
argIndex++;
}
args[argIndex]=NULL;
if(strcmp(args[0], "quit") == 0 || strcmp(args[0], "exit") == 0)
break;
if(line== "\n")
printf("> ");
else if ((strcmp(args[0], "cd") == 0))
cd(args[1]);
else
execute(args);
}
return 0;
}

Answer 1

You were on the right track. There were a few subtle issues where you were not accounting for the trailing '\n' that would remain in line as the last character following whatever was entered at the prompt. Including " \n" in the delimiters used to tokenize the input with strtok will remove it, allowing valid strcmp comparisons with the final token (e.g. that is why quit and exit would not quit the application).
Other than than, there were several additional things you could do a little different/better, you could handle directories entered as e.g. '~/somedir', and similar additional checks that could be employed. I have notated most below as comments to the code.
Look over the changes below and let me know if you have any questions. There are always additional checks that can be added, etc.., but on balance your approach to the problem was pretty good. (note: some of the changes made were non-substantive, e.g. "shell> " as the prompt, instead of "> ". Just handle any of those as you wish.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
enum {ARGSIZE = 20, BUF_SIZE = 1024};
void execute (char **args);
void cd (char *directory);
int killpid (char *pitstr, int sig);
int main (void)
{
char line[BUF_SIZE] = {0};
char *args[ARGSIZE] = {NULL};
char *token;
int i, argIndex = 0;
while (1) {
argIndex = 0; /* reinitialize variables */
for (i = 0; i < ARGSIZE; i++)
args[i] = NULL;
printf ("shell> "); /* prompt */
if (fgets (line, BUF_SIZE, stdin) == NULL) {
printf ("EOF received\n");
return 0;
}
if (*line == '\n') /* Enter alone */
continue;
token = strtok (line, " \n"); /* add \n to delimiters */
while (token != NULL) {
args[argIndex] = token;
token = strtok (NULL, " \n");
argIndex++;
}
if (!argIndex) continue; /* validate at least 1 arg */
if (strcmp (args[0], "quit") == 0 || strcmp (args[0], "exit") == 0)
break;
/* handle 'cd' or 'kill' separately */
if ((strcmp (args[0], "cd") == 0))
cd (args[1]);
else if ((strcmp (args[0], "kill") == 0)) {
if (args[1]) killpid (args[1], SIGTERM);
}
else
execute (args);
}
return 0;
}
void execute (char **args)
{
int pid, status;
pid = fork ();
if (pid < 0) {
perror ("Error forking!");
return;
}
else if (pid > 0) {
while (wait (&status) != pid)
continue;
}
else if (pid == 0) {
if (execvp (args[0], args) == -1) {
perror ("Error");
}
_exit (EXIT_FAILURE);
}
}
void cd (char *directory)
{
char dir[BUF_SIZE] = {0};
if (!directory) { /* handle 'cd' */
directory = getenv ("HOME");
if (chdir (directory))
fprintf (stderr, "Failed to enter directory: %s\n", directory);
else
printf ("%s\n", directory);
return;
}
if (*directory == '~') { /* handle cd ~/stuff */
strcpy (dir, getenv ("HOME"));
strcat (dir, "/");
strcat (dir, directory + 2);
if (chdir (dir))
fprintf (stderr, "Failed to enter directory: %s\n", dir);
else
printf ("%s\n", dir);
return;
}
if (chdir (directory)) /* handle given directory */
fprintf (stderr, "Failed to enter directory: %s\n", directory);
else
printf ("%s\n", directory);
}
int killpid (char *pidstr, int sig)
{
pid_t pid = (pid_t)atoi (pidstr);
if (pid < 1) {
fprintf (stderr, "warning: requested pid < 1, ignoring\n");
return (int)pid;
}
printf (" killing pid '%d' with signal '%d'\n", (int)pid, sig);
// return kill (pid, sig);
return 0;
}
Sample Usage/Output
$ ./bin/ushell
shell> cd
/home/david
shell> cd ~/tmp
/home/david/tmp
shell> kill 18004
killing pid '18004' with signal '15'
shell>
shell> quit