Merge pull request #234 from ryneches/gh-pages

describe ${foo} vs. $foo in for loops

_episodes/04-redirection.md

@@ -339,7 +339,33 @@ When the shell sees the keyword `for`, it knows to repeat a command (or group of
 Each time the loop runs (called an iteration), an item in the list is assigned in sequence to the **variable**, and 
 the commands inside the loop are executed, before moving on to  the next item in the list. Inside the loop, we call for 
 the variable's value by putting `$` in front of it. The `$` tells the shell interpreter to treat the **variable**
-as a variable name and substitute its value in its place, rather than treat it as text or an external command. 
+as a variable name and substitute its value in its place, rather than treat it as text or an external command. In shell programming, this is usually called "expanding" the variable.
+
+Sometimes, we want to expand a variable without any whitespace to its right.
+Suppose we have a variable named `foo` that contains the text `abc`, and would
+like to expand `foo` to create the text `abcEFG`.
+
+~~~
+$ foo=abc
+$ echo foo is $foo
+foo is abc
+$ echo foo is $fooEFG      # doesn't work
+foo is
+~~~
+{: .bash}
+
+The interpreter is trying to expand a variable named `fooEFG`, which (probably)
+doesn't exist. We can avoid this problem by enclosing the variable name in 
+braces (`{` and `}`, sometimes called "squiggle braces").
+
+~~~
+$ foo=abc
+$ echo foo is $foo
+foo is abc
+$ echo foo is ${foo}EFG      # now it works!
+foo is abcEFG
+~~~
+{: .bash}
 
 Let's write a for loop to show us the first two lines of the fastq files we downloaded earlier. You will notice shell prompt changes from `$` to `>` and back again as we were typing in our loop. The second prompt, `>`, is different to remind us that we haven’t finished typing a complete command yet. A semicolon, `;`, can be used to separate two commands written on a single line.