Unix

Context

This is an older paper for Unix. Therefore, some implementations and designs might be different from what we know for modern unix today.
Unix has a narrower set of goals compared to Multics, which tried to invent and experiment on a lot of new things.

Goal

Unifying Abstraction → try to make as many things as possible into files
- Easy to use/Familiar abstraction
- Single (simple) protection model for everything
- Impose more work, but less restriction on programmer
- Modularization
- Streaming I/O interface

Note

What's good about I/O interface is that it works better with devices compared to memory-mapped file access.

Implementation

File System

Unix File System

Unix only has a single hierarchical file system (starting from root). You can attach a file system to this main namespace with mount. Also, everything is a file in the file system (even directories).

// TODO: Insert Diagram

Hard Link

Actual pointer to the file

Soft Link

A file contains the pathname

set user id

When set user id is called in unix on a file, the protection check is not done on the user of the process, but the owner of the process. (e.g. running os programs stored in the /bin/ directory)

In this way, we can do the right augmentation. Unlike Multics, Unix implements a protected subsystem by the level of granularity of processes instead of individual procedures.

We make sure that the protection system is still working by ensuring that the user does not decide which instructions get executed. It can only decide to run the program, but not what the program does. Essentially, this is like gates (entry points) in Multics.

Sudo

sudo is changing your user id as if you are running as a root user.

Shell Code in Unix

while(1){
    read command; // wait for user input (i.e. enters blocked state until user input)
    if((childPid = fork()) == 0){
        open(file_we_need_to_write);
        close(original_parent_fd);
        reassign_stdin_stdout(); // and other things such as piping
        exec(command);
    }
    join(childPid) // This is called if shell is not run in the background  
}

How can you implement redirection, reassign stdin, and stdout if you don't have fork() and exec?