File permissions

The file system has the concept of permissions, which describe what you (or other users) are allowed to do with a file.

This is a complex topic, which is core to ideas of computer security. But for the superbasics you really just need to know that permissions exist and that they typically take into account things like:

• what your relationship is to the file (for example, did you create it?)
• what kind of user you are (are you an administrator?)
• the specific settings on the file
• settings on each of the directories in the file’s path

On Unix

On Unix systems, every file has three basic permissions:

There are three different user classes to which those permissions apply, which depend on how you (or, more precisely, your user account) are related to the file:

Whenever you try to access a file, the operating system determines your class, checks the permissions for that class, and allows or denies the operation you are attempting.

On the college Linux systems, this mechanism is what prevents you accessing other students’ files.

Incidentally, you might not need execute permission on a Python program (for example, example.py) in order to run it, because the program you’re really executing there is the Python interpreter… So in that case you probably just need read permission to access the Python source code, but execute permission on the Python3 binaries.

If you want to see the permissions on your files, do:

ls -l

The -l option is “long format”. It will list the files (in the current directory, if you don’t specify a directory as an argument) with extra information including the permissions, and they will look something like this:

-rwxr-xr--

The first character (-) indicates if the file is a special type, such as a directory or symlink. In this example it is neither (if it was, it would be d or l). Being a directory is not really a permission, but it’s relevant because the meaning of rwx is slightly different for directories. For example, the x permission on a directory indicates whether or not a user is allowed to cd (change directory) into it.

The next three (rwx) are the permissions for the owner. In this example, the owner has all three permissions, so can do anything to this file (including deleting it).

The next three (r-x) are the permissions for users who are not the owner, but are in the same group as the file. In this case, such users can read and execute the file, but cannot change it: there’s a - where the w (write) permission should be.

The last three (r--) are the permissions for any other users. They can only read the file, but not change it or execute it.

For full details, read the manual for the ls command.