Posix

sh
https://sites.google.com/site/jdisnard/path-dupes remove duplicate paths. http://unix.stackexchange.com/questions/40749/remove-duplicate-path-entries-with-awk-command
 * https://stackoverflow.com/questions/793858/how-to-mkdir-only-if-a-dir-does-not-already-exist?rq=1 create directory.
 * https://stackoverflow.com/questions/8748831/when-do-we-need-curly-braces-in-variables-using-bash Curly braces.


 * http://www.tldp.org/LDP/abs/abs-guide.pdf 900 page guide
 * https://linuxacademy.com/blog/linux/conditions-in-bash-scripting-if-statements/ file conditions etc.
 * http://www.tutorialspoint.com/unix_commands/sh.htm
 * https://www.macs.hw.ac.uk/~hwloidl/Courses/LinuxIntro/x945.html
 * https://opensource.com/article/17/6/bash-parameter-expansion parameter expansion
 * Grep

Forums

 * http://www.unix.com/shell-programming-and-scripting/170808-bash-clearing-value-variable.html

https://www.dwheeler.com/essays/filenames-in-shell.html

Unix stackexchange
https://unix.stackexchange.com/questions/23271/pipes-vs-process-substitution?rq=1

tutorials

 * http://wiki.bash-hackers.org/scripting/tutoriallist?s[]=links
 * http://wiki.bash-hackers.org


 * http://sourcemage.org/Sorcery/Book shell style guide, a clone of Linuxfromscratch.com

links

 * LinuxNotes, ShellCommands Mesh networking shell commands