One Polymorphic Executable, Multiple Commands

Let me present a very interesting concept: shipping a single executable that will give the system multiple commands. We are going to UNIX this topic very much!

Inspiration

Of course, I met this concept in a video , which explains it, and shows the most famous example: BusyBox. It is a single binary that provides POSIX commands like ls, cp, mv, gg (one is not real) etc. It has two advantages:

• It has low footprint.
• If it is used on Linux, you don’t have to call it GNU/Linux.

I was quite happy because I already know Mr. Non-Idle Square-Body from Alpine Linux, the OS for embedded systems, containers and frugal people.

The most sensible reaction after getting inspired, to create something like this. Since I’m learning the one and holy C programming language, this was the perfect opportunity.

Mechanism

How this is coming to fruition is very simple, and only needs two things.

In the source code, we have to check what name the program was called with. In C, and in other languages as well, this means inspecting the zeroth argument. But that begs the question, how can one executable have multiple names? Of course, with symlinks!

In the case of BusyBox, the executable is called busybox, and there is symlink for every utility it provides.

mv -> /bin/busybox
netstat -> /bin/busybox
nice -> /bin/busybox
pidof -> /bin/busybox
ping -> /bin/busybox

SysPerc

With all this knowledge, I started to build my own polymorphic executable, called SysPerc. It is supposed to provide system information in percentages, like CPU usage, memory usage, general mood and so on.

Two example of a command call that it provides:

batp -r
memp -r

These two commands will yield the battery percentage and memory usage percentage, respectively, as rounded numbers.

The simplified source for deciding which command to run is the following:

int main(int argc, char *argv[])
{
    Options options = get_options(argc, argv);
    return bus(argv[0], options, argc, argv);
}

int bus(char *command, Options options, int argc, char *argv[])
{
    if (!strcmp(command, "memp")) {
        return memp(options);
    } else if (!strcmp(command, "batp")) {
        return batp(argc, argv, options);
    }

    fprintf(stderr, "Unknown command in sysperc executeable: %s\n", command);
    return EXIT_FAILURE;
}

Yes, this is a “command bus”, you cannot stop me! And there is nothing in the world that a condition cannot solve.

As a very perceptive reader, you may have noticed that the commands use the same options. Since we have one source, option parsing is also unified. Because options affect how results are printed, that is unified as well.

This was the most fun I have had with my mini projects in recent times. For more explanation, check my attempt at C programming: SysPerc .

And as always, keep computering™!