What exactly is current working directory?

Question

My book states:

Every program that runs on your computer has a current working directory, or cwd. Any filenames or paths that do not begin with the root folder are assumed to be under the current working directory

As I am on OSX, my root folder is /. When I type in os.getcwd() in my Python shell, I get /Users/apple/Documents. Why am I getting the Documents folder in my cwd? Is it saying that Python is using Documents folder? Isn't there any path heading to Python that begins with / (the root folder)? Also, does every program have a different cwd?

Answer 1

Every process has a current directory. When a process starts, it simply inherits the current directory from its parent process; and it's not, for example, set to the directory which contains the program you are running.

For a more detailed explanation, read on.

When disks became large enough that you did not want all your files in the same place, operating system vendors came up with a way to structure files in directories. So instead of saving everything in the same directory (or "folder" as beginners are now taught to call it) you could create new collections and other new collections inside of those (except in some early implementations directories could not contain other directories!)

Fundamentally, a directory is just a peculiar type of file, whose contents is a collection of other files, which can also include other directories.

On a primitive operating system, that was where the story ended. If you wanted to print a file called term_paper.txt which was in the directory spring_semester which in turn was in the directory 2021 which was in the directory studies in the directory mine, you would have to say

print mine/studies/2021/spring_semester/term_paper.txt

(except the command was probably something more arcane than print, and the directory separator might have been something crazy like square brackets and colons, or something;

lpr [mine:studies:2021:spring_semester]term_paper.txt

but this is unimportant for this exposition) and if you wanted to copy the file, you would have to spell out the whole enchilada twice:

copy mine/studies/2021/spring_semester/term_paper.txt mine/studies/2021/spring_semester/term_paper.backup

Then came the concept of a current working directory. What if you could say "from now on, until I say otherwise, all the files I am talking about will be in this particular directory". Thus was the cd command born (except on old systems like VMS it was called something clunkier, like SET DEFAULT).

cd mine/studies/2021/spring_semester
print term_paper.txt
copy term_paper.txt term_paper.backup

That's really all there is to it. When you cd (or, in Python, os.chdir()), you change your current working directory. It stays until you log out (or otherwise exit this process), or until you cd to a different working directory, or switch to a different process or window where you are running a separate command which has its own current working directory. Just like you can have your file browser (Explorer or Finder or Nautilus or whatever it's called) open with multiple windows in different directories, you can have multiple terminals open, and each one runs a shell which has its own independent current working directory.

So when you type pwd into a terminal (or cwd or whatever the command is called in your command language) the result will pretty much depend on what you happened to do in that window or process before, and probably depends on how you created that window or process. On many Unix-like systems, when you create a new terminal window with an associated shell process, it is originally opened in your home directory (/home/you on many Unix systems, /Users/you on a Mac, something more or less like C:\Users\you on recent Windows) though probably your terminal can be configured to open somewhere else (commonly Desktop or Documents inside your home directory on some ostensibly "modern" and "friendly" systems).

Many beginners have a vague and incomplete mental model of what happens when you run a program. Many will incessantly cd into whichever directory contains their script or program, and be genuinely scared and confused when you tell them that you don't have to. If frobozz is in /home/you/bin then you don't have to

cd /home/you/bin
./frobozz

because you can simply run it directly with

/home/you/bin/frobozz

and similarly if ls is in /bin you most definitely don't

cd /bin
./ls

just to get a directory listing.

Furthermore, like the ls (or on Windows, dir) example should readily convince you, any program you run will look in your current directory for files. Not the directory the program or script was saved in. Because if that were the case, ls could only produce a listing of the directory it's in (/bin) -- there is nothing special about the directory listing program, or the copy program, or the word processor program; they all, by design, look in the current working directory (though again, some GUI programs will start with e.g. your Documents directory as their current working directory, by design, at least if you don't tell them otherwise).

Many beginners write scripts which demand that the input and output files are in a particular directory inside a particular user's home directory, but this is just poor design; a well-written program will simply look in the current working directory for its input files unless instructed otherwise, and write output to the current directory (or perhaps create a new directory in the current directory for its output if it consists of multiple files).

Python, then, is no different from any other programs. If your current working directory is /Users/you/Documents when you run python then that directory is what os.getcwd() inside your Python script or interpreter will produce (unless you separately os.chdir() to a different directory during runtime; but again, this is probably unnecessary, and often a sign that a script was written by a beginner). And if your Python script accepts a file name parameter, it probably should simply get the operating system to open whatever the user passed in, which means relative file names are relative to the invoking user's current working directory.

python /home/you/bin/script.py file.txt

should simply open(sys.argv[1]) and fail with an error if file.txt does not exist in the current directory. Let's say that again; it doesn't look in /home/you/bin for file.txt -- unless of course that is also the current working directory of you, the invoking user, in which case of course you could simply write

python script.py file.txt

On a related note, many beginners needlessly try something like

with open(os.path.join(os.getcwd(), "input.txt")) as data:
    ...

which needlessly calls os.getcwd(). Why is it needless? If you have been following along, you know the answer already: the operating system will look for relative file names (like here, input.txt) in the current working directory anyway. So all you need is

with open("input.txt") as data:
    ...

One final remark. On Unix-like systems, all files are ultimately inside the root directory / which contains a number of other directories (and usually regular users are not allowed to write anything there, and system administrators with the privilege to do it typically don't want to). Every relative file name can be turned into an absolute file name by tracing the path from the root directory to the current directory. So if the file we want to access is in /home/you/Documents/file.txt it means that home is in the root directory, and contains you, which contains Documents, which contains file.txt. If your current working directory were /home you could refer to the same file by the relative path you/Documents/file.txt; and if your current directory was /home/you, the relative path to it would be Documents/file.txt (and if your current directory was /home/you/Music you could say ../Documents/file.txt but let's not take this example any further now).

Windows has a slightly different arrangement, with a number of drives with single-letter identifiers, each with its own root directory; so the root of the C: drive is C:\ and the root of the D: drive is D:\ etc. (and the directory separator is a backslash instead of a slash, although you can use a slash instead pretty much everywhere, which is often a good idea for preserving your sanity).