Segmentation fault C in writing a shell program - c

Question

My code worked fine until I add the extra stuffs into it, like identifying and deal with cd, >, <, >> and |. Could you please check my code and see where did the error happens?
Btw the requirement of the assignment is only up to 1 pipe. And I think the problem starts somewhere from the for loop, cause I put printf right after it to check if it would print args[k], and it did but then the bug popped up and the program stopped.
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <string.h>
#include <fcntl.h>
int main()
{
const int MAX_ARGS = 10;
char *token[MAX_ARGS + 1]; /*assume max number of token <=10*/
char *temp;
char line[256], command[MAX_ARGS];
char prompt[] = "sh2 % ";
pid_t pid;
int i=0, j,k, status;
printf("%s", prompt);
while (fgets(line, sizeof line, stdin) != NULL)
{
line[strlen(line)-1] = '\0'; /*get rid of \n*/
token[0] = strtok(line," ");
strcpy(command,token[0]);
temp = strtok(NULL," ");
while (temp != NULL)
{
i = i+1;
token[i] = temp;
temp = strtok(NULL," ");
}
char *args[i+2];
for (j = 0; j < (i+1); j++)
{
args[j] = token[j];
}
args[i+1] = NULL;
if (!strcmp("exit",command))
exit(0);
if (!strcmp("cd", command))
{
int success;
if (success = chdir(args[1]) <0)
{
printf("Failed to change dir.\n");
}
}
else
{
int piping = 0;
int fd;
for (k = 0; k < sizeof args; k++)
{
if (!strcmp(">",args[k]))
{
fd = open(args[k+1],O_WRONLY | O_TRUNC | O_CREAT, S_IRUSR | S_IWUSR | S_IXUSR);
if (fd <0) { printf("Open file failed.\n");}
else
{
dup2(fd, 1);
args[k] = '\0';
fflush(stdout);
close(fd);
}
}
if (!strcmp("<", args[k]))
{
fd = open(args[k+1], O_RDONLY);
if (fd <0) {printf("Open file failed.\n");}
else
{
dup2(fd, 0);
args[k] = '\0';
close(fd);
}
}
if (!strcmp(">>", args[k]))
{
fd = open(args[k+1], O_APPEND | O_CREAT, S_IRUSR | S_IWUSR | S_IXUSR);
if (fd <0) {printf("Open file failed");}
else
{
dup2(fd,1);
args[k] = '\0';
fflush(stdout);
close(fd);
}
}
if (!strcmp("|", args[k]))
{
piping = 1;
}
} /*end for*/
if (!(piping))
{
pid = fork();
if (pid <0) {}
else if (pid == 0)
{
if ( (status = execvp(command, args)) < 0 )
{
printf("Command not found\n");
}
}
else /*parent*/
{
wait(&status);
} /*end parent*/
}/*end if not pipe*/
else /*its pipe*/
{
int pfd[2];
pipe(pfd);
fd = fork();
if (fd < 0) {}
else if (fd ==0)
{
close(pfd[1]);
dup2(pfd[0],0);
close(pfd[0]);
execvp(args[2],args[2]);
}
else /*parent*/
{
close(pfd[0]);
dup2(pfd[1],1);
close(pfd[1]);
execvp(args[0],args[0]);
}
}/*end pipe*/
} /*end outter else*/
printf("%s", prompt);
}/*end while*/
return 0;
}

Answer 1

for (k = 0; k < sizeof args; k++)
This is not how you iterate through args: this will go far beyond the end of the array. You want something like:
num = sizeof(args) / sizeof(*args);
for (k = 0; k < num; k++)
Alternatively, since you set the last element as NULL, you could do
for (char **arg = args; *arg; arg++)
Also note that you iterate with k until the end of the array and then use k + 1, which is very likely to cause problems.