What are processes vs programs?
  - A program in action
  - Processes cannot just be "created", must be cloned
    - Usually OS starts first process known as "init"
    - The OS keeps track of all parent, child, sibling relationships
    - Run pstree to get this info
  - What do they have associated with them?
    - pid -> global
    - file descriptors -> local
    - VM mapping
    - State -> Running, waiting, stopped, zombie
    - Scheduling
    - Processor "context"

How to clone processes? Look at fork examples:
  - Simple refresher
  - What is actually inherited?
    - new pid
    - file descriptors are the same
    - Context is the same
    - VM is copied

What is IPC?
  - We want to talk to another process, what do?
  - Kernel gives us a few options including:
    - FIFO, Pipe, Shared Memory, Signals
    - Sockets when we start networking

Look at fifo example
  - Stored on disk
  - Has to be opened at both ends simultaneously

Look at pipe example
  - Requires common parent process
  - Has atomicity guarantees
  - How might you implement bash pipe operator? "|"
  - redirection using dup2

Look at signals:
  - Software interrupts (as opposed to hardware ones)
  - Can be ignored, caught, or let the default action occur
  - Some cannot be changed or ignored (SIGKILL)
  - On exec() signals reset

Look at shared memory:
  - Draw diagram of shared memory
  - What else can mmap be used for?

What do you need to do for ex04?
  - Write a shell
  - Replaces stdin of command with fd from environment variable

How?
  - getenv("USTDIN")
  - command loop -> readline
  - fork
  - In child, open file from env var
  - Use dup2 to replace stdin
  - Use execl
  - In parent, wait for child