execvp() creating a small unix shell based program - c

Question

I have made a small program that forks and executes another program. Basically it's supposed to work just like unix shell.
Here is my code:
int main(int argc, char *argv[]){
pid_t cpid;
char *shell[5];
shell[0]=argv[1];
shell[1]=argv[2];
shell[2]=argv[3];
shell[4]=NULL;
if(argc!=4){
printf("Program expects 4 arguments");
} else{
cpid=fork();
if(cpid==0){
execvp("/bin/sh",shell);
}//end child process
if (cpid != wait(NULL)) { /* parent code */
printf("Parent failed to wait");
return 1;
}
}//end else
}//end main
However, when I give the command
$ ./shell simple sml_prog1 A
it says sml_prog1 not found about 15 or 20 times.
The shell is supposed to run simple which takes sml_prog1 A as arguments.
The program does work on its own with the same arguments.
I changed the permissions of sml_prog1 to read/write/executable. Moreover sml_prog1 is a .txt file that contains data that the program simple uses

Answer 1

The main issue is how you're calling execvp. You don't want to execute /bin/sh, you want to execute the program the user passed in, i.e. argv[1].
Change the call to this, and add the following error checking:
execvp(shell[0],shell);
perror("exec failed"); // This line never gets called unless execvp fails
exit(1); // end the child process
Also, you never set shell[3] to anything. You probably want to set this to NULL instead of shell[4]:
shell[0]=argv[1];
shell[1]=argv[2];
shell[2]=argv[3];
shell[3]=NULL;

Answer 2

The simplest possible example of using execvp() I can think of is as follows:
#include <stdio.h>
#include <unistd.h>
int main(int argc, char **argv)
{
++argv;
if (*argv)
execvp(*argv, argv);
return 0;
}
Compile it with:
cc -Wextra -Wall some.c -o some
And run it like:
./some ls -la
Or to display that all the arguments are getting passed to execvp() more clearly:
./some ls -la -R /etc
To demonstrate it put to use with your exact scenario of invoking a shell compare the output of these two commands (be warned this it is pretty redundant to use /bin/sh directly when one is already using execvp()):
./some bash -c 'type history'
./some sh -c 'type history'

Answer 3

If you need to execute your program through /bin/sh, not directly, you must do it differently.
You must pass the program somehow this way:
char * shell[4];
shell[0] = "sh";
shell[1] = "-c";
shell[2] = "./simple sml_prog1 A";
shell[3] = NULL;
See ? with -c option, and the full program
So something like this should do the job for you:
char * shell[4];
shell[0] = "sh";
shell[1] = "-c";
char prog[100]; // be careful with this number
snprintf (prog, 100, "./%s %s %s",argv[1], argv[2], argv[3]);
shell[2] = prog;
shell[3] = NULL;