Add navigation to the end of each file

lessons/shell/creating-things.md

@@ -21,3 +21,7 @@ described in this section.
 | Ctrl-e           | place cursor at end of line
 | Ctrl-k           | clear text to end of line
 | Ctrl-l           | clear text in terminal
+
+[Introduction to the shell](./index.md) |
+Previous: [Files and directories](./files-and-directories.md) |
+Next: [Pipes and filters](./pipes-and-filters.md)

lessons/shell/files-and-directories.md

@@ -25,3 +25,8 @@ described in this section.
 | dirs              | print directory stack
 | tree              | view directory tree *(nonstandard)*
 | tab               | complete command
+
+
+[Introduction to the shell](./index.md) |
+Previous: [index](./index.md) |
+Next: [Creating things](./creating-things.md)

lessons/shell/finding-things.md

@@ -20,3 +20,7 @@ and special characters described in this section.
 | echo             | print variables to terminal
 | which            | show location of command
 | alias            | make a shortcut for a command
+
+[Introduction to the shell](./index.md) |
+Previous: [Pipes and filters](./pipes-and-filters.md)
+Next: [Getting things from elsewhere](./getting-things.md)

lessons/shell/getting-things.md

@@ -16,3 +16,6 @@ The table below summarizes the commands described in this section.
 | zip      | package and compress/uncompress files
 | gzip     | package and compress/uncompress files
 | tar      | package and compress/uncompress files
+
+[Introduction to the shell](./index.md) |
+Previous: [Finding things](./finding-things.md)

lessons/shell/pipes-and-filters.md

@@ -22,3 +22,7 @@ described in this section.
 | ?                 | match single characters
 | sort              | sorts input
 | sed               | stream editor
+
+[Introduction to the shell](./index.md) |
+Previous: [Creating things](./creating-things.md) |
+Next: [Finding things](./finding-things.md)