Cat command didn't work correctly with execvp - c

Question

1) I can't use some commands in this code, such as : cat somefile.txt > somefile2.txt
also I can't use : cat somefile.txt | less
2) When I use commands like : ( cd ../) (cd ./Desktop) and then I want to exit the program, I need to execute exit commands more than one time: ie "if I use 3 cd command, I will need 3 exit commands to end the program"
#define MAX_ARGS 5
// Global Declarations
// Mini Functions
void remove_new_line_char(char line[])
{
int i=0;
while(line[i]!= '\n')
i++;
line[i] = '\0';
}
// Grand Functions
int read_line(char line[])
{
fgets(line, 10000, stdin); // File Get String
remove_new_line_char(line); // Remove New Line Charactere
if (strlen(line) > 512)
{
fprintf(stderr,"The Command exceeded available line length\n");
return 0;
}
if (strcmp(line, "exit") == 0)
exit(0);
return 1;
}
int parse_line(char* args[], char line[])
{
int i=0;
args[i] = strtok(line, " ");
if(args[i] == NULL)
{
printf("Command Line is Empty!\n");
return -1;
}
while (args[i] != NULL)
{
int flag = 0;
if(strcmp(args[i],"&") == 0)
flag = 1;
i++;
args[i] = strtok(NULL, " "); // NULL maintains a static pointer to the previously passed string.
if (args[i] == NULL && flag == 1)
{
args[i-1] = NULL; // Remove & From Argument List and Set Background Flag.
return 1;
}
}
return 0;
}
// Main
int main()
{
char* args[MAX_ARGS]; // Array of Strings
char line[10000]; // String
while(1)
{
printf("Shell> ");
if(read_line(line) == 1) // No Errors
{
int background = parse_line(args, line);
if(background != -1) // Command Line isn't Empty
{
// Fork and Execute
pid_t child_pid = fork();
if(child_pid == 0) // Child
{
if (strcmp(args[0], "cd") == 0 && args[1]!= NULL && args[2] == NULL) // Special Handling For CD
{
//printf("%s\n",args[2]);
int check = chdir(args[1]);
if(check == -1)
fprintf(stderr, "Invalid Directory\n");
}
// Handle if args[1]== NULL, Don't even execvp()
else // Other Functions
{
execvp(args[0], args); // args[0] is actually the command.
fprintf(stderr,"an error occured in execution\n%s\n",strerror(errno));
//fprintf(stderr,"Invalid Instruction\n");
}
}
else // Parent
{
if(background == 0)
waitpid(child_pid, 0);
wait(1000);
}
}
}
}
return 0;
}
I suspect that I can't use any command that has characters like: > < |
Thanks in advance

Answer 1

1) i can't use some commands in this code as : cat somefile.txt > somefile2.txt also i can't use : cat somefile.txt | less
In the standard shell, the > and | are operators interpreted by the shell, not arguments to the command. Since in this case the shell is your program itself, if you must support those operators then you'll need to implement the appropriate redirections yourself. Refer to open(), pipe(), and dup2(), and for the pipe case you'll also need judicious application of close().
2)when i use commands like : ( cd ../) (cd ./Desktop) and then i want to exit the program, i need to execute exit command more than one time "if i use 3 cd command i will need 3 exit command to end the program"
In the special case of the cd command, you fork and then change directory in the child process, but the child does not terminate or exec another process. That leaves you with two copies of your shell running. You need to exit both before control returns to whatever process launched your program. Possibly in that case you want to instead execute chdir without forking (or waiting for a child).