Redirect output in shell? - c

Question

I have written a function for the purposes of redirecting output from some command to a file. Something like this
ls > ls.txt
Here is the function that does output redirection:
int redirect_output(char* cmdline, char **output_filename) {
int i;
char* args[MAX_ARGS];
// int nargs = get_args(cmdline, args);
for(i = 0; args[i] != NULL; i++) {
// Look for the >
if(!strcmp(args[i], ">")) {
// Get the filename
if(args[i+1] != NULL) {
*output_filename = args[i+1];
} else {
return -1; //syntax error
}
return 1; //there is an >
}
}
return 0; //no redirect
}
The function successfully redirects output from some command to a file, but for some reason it causes other commands to stop working. So for example, my program with redirect_output works for something like ls but it does not work for something like cat ls.txt, if I do not call the function it works for any command. And, by not work it mean it gets stuck. Here are the other important functions.
int get_args(char* cmdline, char* args[])
{
int i = 0;
/* if no args */
if((args[0] = strtok(cmdline, "\n\t ")) == NULL)
return 0;
while((args[++i] = strtok(NULL, "\n\t ")) != NULL) {
if(i >= MAX_ARGS) {
printf("Too many arguments!\n");
exit(1);
}
}
/* the last one is always NULL */
return i;
}
void execute(int output, char *outputefile, char* cmdline)
{
int pid, async;
char* args[MAX_ARGS];
int nargs = get_args(cmdline, args);
if(nargs <= 0) return;
if(!strcmp(args[0], "quit") || !strcmp(args[0], "exit")) {
exit(0);
}
/* check if async call */
if(!strcmp(args[nargs-1], "&")) {
async = 1;
args[--nargs] = 0;
}
else async = 0;
pid = fork();
if(pid == 0) { /* child process */
//if(output)
// freopen(outputefile, "w+", stdout);
execvp(args[0], args);
/* return only when exec fails */
perror("exec failed");
exit(-1);
} else if(pid > 0) { /* parent process */
if(!async) waitpid(pid, NULL, 0);
else printf("this is an async call\n");
} else { /* error occurred */
perror("fork failed");
exit(1);
}
}
And here is my main:
int main (int argc, char* argv [])
{
char cmdline[BUFSIZ];
char *output_filename;
char **args;
int output;
for(;;) {
printf("COP4338$ ");
if(fgets(cmdline, BUFSIZ, stdin) == NULL) {
perror("fgets failed");
exit(1);
}
output = redirect_output(cmdline, &output_filename);
switch(0) {
case -1:
printf("Syntax error!\n");
break;
case 0:
break;
case 1:
printf("Redirecting output to: %s\n", output_filename);
break;
}
execute (output, output_filename, cmdline);
}
return 0;
}
Any help will be much appreciated! Thanks!