Expand lesson on aliases and .bashrc files

This PR creates a draft lesson for how to customize the bash
environment with aliases and .bashrc, .bash_profile, and .bash_logout
files.  I also added a discussion on customizing bash prompts.  I
tried to keep this lesson relatively concise.  This lesson will
probably need some checking and a test run to make sure everything
works alright.

_episodes/07-aliases.md

@@ -1,18 +1,137 @@
 ---
-title: Aliases and the .bashrc
-teching: 5
+title: Aliases and bash customization
+teaching: 10 minutes
 exercises: 0
 questions:
-- "How to customize shell environment"
+- How do I customize my bash environment?
 objectives:
-- "FIXME"
+- Create aliases.
+- Add customizations to the `.bashrc` and `.bash_profile` files.
+- Change the prompt in a bash environment.
 keypoints:
-- "FIXME"
----
+- Aliases are used to create shortcuts or abbreviations
+- The `.bashrc` and `.bash_profile` files allow us to customize our 
+  bash environment.
+- The `PS1` system variable can be changed to customize your bash
+  prompt.
 
-## The .bash_profile, .bash_logout, and .bashrc files
+---
 
+Bash allows us to customize our environments to fill our own
+particular needs.
 
 ## Aliases
 
-FIXME
+Sometimes we need to use long commands that have to be typed over and
+over again.  Fortunately, the `alias` command allows us to create
+shortcuts for these long commands.
+
+As an example, let's create aliases for going up one, two, or three
+directories.
+
+~~~
+alias up='cd ..'
+alias upup='cd ../..'
+alias upupup='cd ../../..'
+~~~
+{: .bash}
+
+Let's try these commands out.
+
+~~~
+cd /usr/local/bin
+upup
+pwd
+~~~
+{: .bash}
+
+~~~
+/usr
+~~~
+{ .output}
+
+We can also remove a shortcut with `unalias`.
+
+~~~
+unalias upupup
+~~~
+{: .bash}
+
+If we create one of these aliases in a bash session, they will only
+last until the end of that session. Fortunately, bash allows us to
+specify customizations that will work whenever we begin a new bash
+session.
+
+## Bash customization files
+
+Bash environments can be customized by adding commands to the
+`.bashrc`, `.bash_profile`, and `.bash_logout` files in our home
+directory.  The `.bashrc` file is executed whenever entering
+interactive non-login shells whereas `.bash_profile` is executed for
+login shells.  If the `.bash_logout` file exists, then it will be run
+after exiting a shell session.
+
+Let's add the above commands to our `.bashrc` file.
+
+~~~
+echo "alias up='cd ..'" >> ~/.bashrc
+tail -n 1 ~/.bashrc
+~~~
+{: .bash}
+
+~~~
+alias up='cd ..'
+~~~
+{: .output}
+
+We can execute the commands in `.bashrc` using `source`
+
+~~~
+source ~/.bashrc
+cd /usr/local/bin
+up
+pwd
+~~~
+{: .bash}
+
+~~~
+/usr/local
+~~~
+{: .local}
+
+Having to add customizations to two files can be cumbersome.  It we
+would like to always use the customizations in our `.bashrc` file,
+then we can add the following lines to our `.bash_profile` file.
+
+~~~
+if [ -f $HOME/.bashrc ]; then
+        source $HOME/.bashrc
+fi
+~~~
+{: .bash}
+
+## Customizing your prompt
+
+We can also customize our bash prompt by setting the `PS1` system
+variable. To set our prompt to be `$ `, then we can run the command
+
+~~~
+export PS1="$ "
+~~~
+{: .bash}
+
+To set the prompt to `$ ` for all bash sessions, add this line to the
+end of `.bashrc`.
+
+Further [bash prompt
+customizations](https://www.howtogeek.com/307701/how-to-customize-and-colorize-your-bash-prompt)
+are possible.  To have our prompt be `username@hostname[directory]: `,
+we would set
+
+~~~
+export PS1="\u@\h[\W]: "
+~~~
+{: .bash}
+
+where `\u` represents username, `\h` represents hostname, and `\W`
+represents the current directory.