Lesson 7: Input and Output

1. File Descriptors
   Every process is given three open file upon its execution. These open files are inherited from the shell
   1. stdin - Standard Input (0) defaults to the user's keyboard
   2. stdout - Standard Output (1) defaults to the user's terminal
   3. stderr - Standard Error (2) defaults to the user's terminal
2. File Redirection
   The input and output of a program can be redirected from and to other files:
   1. < filename   Input will now come from filename rather than the keyboard.
   2. > filename   Output will now go to filename instead of the terminal.
   3. 2> filename   Error messages will now go to filename instead of the terminal.
   4. >> filename   Output will now be appended to filename.
3. Pipelines
   Commands may be chained together in susch a way that the stdout of one command is "piped" into the stdin of a second process.
   1. Filters - A program that both reads from stdin and writes to stdout.
   2. Tees - A filter program that reads stdin and writes it to stdout and the file specified as the argument. For example, the following command sends a sorted list of the current users logged on to the system to the screen, and saves an unsorted list to the file users.
      who | tee users | sort
      
4. Miscellaneous Commands
   1. find
   2. grep
   3. sort
   4. spell
   5. wc
5. Tasks to perform with pipelines:
   1. Count how many files are on the system
   2. Count how many users are logged onto the system
   3. Sort and count the misspelled words in a series of documents
   4. Search for a particular string in the output of a command