Difference between exec, execvp, execl, execv?

Question

I am writing a code that represent a new shell to Linux. One of the commands I want to support is running a process for example if I get the following line

command [arguments]

Then I want to run command as a process until it finishes running the process.

To do so I know I need to use fork() in order to get the child process and gets it's PID, my problem is that I don't know what is the difference between :

exec, execvp, execl, execv ... and I don't know which one to use and why.

My current code :

void External_Process(char *arguments[MAX_ARG], char* command)
{
    int pID;
    switch(pID = fork())
    {
    case -1:
        perror("fork failed");
        break;
    case 0 :
        setpgrp();

        //execv(command, arguments);
        //execvp(command, arguments);
        //execl("/bin/bash", "/bin/bash","-c",command,NULL);

        printf("smash error: > bad command %s\n" , command);
        exit(-1) ;

        break;

    default:
        return ;
    }
}

Thank you !

Answer 1

Summary: In your case I would recommend to use execvp.

To find out the differences between the exec* functions you should read the documentation:
https://linux.die.net/man/3/exec
https://linux.die.net/man/2/execve

The difference between execl* and execv* is the argument passing. execl* require a list of arguments while execv* require a vector of arguments.
A list of arguments is useful if you know all the arguments at compile time. In your case the arguments will be entered by the user and you have to construct a vector of arguments at run time, so you should use one of the execv* functions.

The functions with suffix p use the PATH environment variable to find the program (e.g. "ls"), without this you have to specify the full path (either absolute or relative to the current directory, e.g. "/bin/ls"). Using PATH is what shells normally do, so this seems to be the correct choice for you.

The functions with suffix e allow to specify the environment for the process. For simplicity I would not use this in your case.

This leads to the conclusion: execvp

Of course you could also use system (instead of fork/exec*/wait*) as mentioned in vladxjohn's answer, but in this case you would merely use a shell to interpret your command instead of implementing a basic shell.