How is a program in a specific location?

Question

When writing a program, the program is sort of running in a particular directory which is the current working directory.

I'm trying to understand more about the idea of a cwd. How does a program know what its cwd is? Where is that information stored?

I know perfectly well how to use the os module in python, but I don't really understand what it means to have a cwd. Is it simply a data attribute, "this is where we are", that we can change arbitrarily? And we simply look for things and create things on that particular section of the HD? Or is some sort of pathway actually opening and closing actively when we change cwd, like a door getting shut and another being opened?

What happens on the computer when I change cwd in a program?

This may be language-agnostic, I am unsure.

Answer 1

The current working directory is (at least on most operating systems) an attribute of a process, so yes, it is more or less a simple attribute stating "this is where we are". As it's an attribute of a process, it is stored and managed by the OS kernel.

It can be changed arbitrarily by calling e.g. os.chdir in python, and a shell would similarly change its working directory each time you run the builtin cd command. And they both would normally call the same API of the operating system, e.g. chdir(). Changing the cwd is subject to filesystem permissions, so you can only change the working directory to a path that actually exists and you have permissions to.

The cwd may also be involved in file operations, as when a process opens a file path that is not an absolute path, the file name will be resolved relative to the cwd of the process.

On unix systems the cwd is inherited from the parent process, as such the cwd of a process you start from a shell will have its cwd to the directory you are in when you start the process (and not e.g. the directory of the executable you start).