Add a snippet to suggest using the ? operator for environmental varia…

…bles (#1142)

* Add a snippet to suggest using the ? operator for environmental variables

* Add "default" and "in" examples for envvars

book/environment.md

@@ -80,6 +80,38 @@ Individual environment variables are fields of a record that is stored in the `$
 BAR
 ```
 
+Sometimes, you may want to access an environmental variable which might be unset. Consider using the [question mark operator](variables_and_subexpressions.md#variable-paths) to avoid an error:
+```nu
+> $env.FOO | describe
+Error: nu::shell::column_not_found
+
+  × Cannot find column
+   ╭─[entry #1:1:1]
+ 1 │ $env.FOO
+   · ──┬─ ─┬─
+   ·   │   ╰── cannot find column 'FOO'
+   ·   ╰── value originates here
+   ╰────
+
+> $env.FOO? | describe
+nothing
+
+> $env.FOO? | default "BAR"
+BAR
+```
+
+Alternatively, you can check for the presence of an environmental variable with `in`:
+
+```
+> $env.FOO
+BAR
+
+> if "FOO" in $env {
+>     echo $env.FOO
+> }
+BAR
+```
+
 ## Scoping
 
 When you set an environment variable, it will be available only in the current scope (the block you're in and any block inside of it).