'The Linux Command Line' added (#2245)

free-programming-books.md

@@ -793,6 +793,7 @@
 * [Linux Shell Scripting Tutorial - A Beginner's Handbook](http://www.freeos.com/guides/lsst/)
 * [Slackbook](http://slackbook.org)
 * [The Bash Academy](http://www.bash.academy)
+* [The Linux Command Line](http://linuxcommand.org/tlcl.php) - William E. Shotts, Jr. (PDF)
 * [Writing Shell Scripts](http://linuxcommand.org/lc3_writing_shell_scripts.php) - William E. Shotts, Jr.