ref 
-- <a href=https://github.com/dylanaraps/pure-sh-bible> pure sh bible </a> 
-- <a href=http://www.tldp.org/LDP/abs/html/> tldp.org </a>
-- <a href=https://www.shellscript.sh/> steve-parker.org sh </a>
-- <a href=http://tldp.org/LDP/abs/html/why-shell.html> why shell </a> 
-- <a href=http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18> opengroup </a>
-- <a href=http://www.tutorialspoint.com/unix/unix-shell.htm> tutorialspoint </a>
-- <a href=http://codewiki.wikidot.com/shell-script:if-else> wikidot </a>
<br><br>
rich --
<a href=http://www.etalabs.net/sh_tricks.html> rich's sh posix tricks </a> --
<a href=./rich.cp.note.txt> my note</a> --
<br><br>
grymoire --
<a href=https://www.grymoire.com/Unix/Sh.html> grymoire </a> --
<a href=./grymoire.cp.note.txt> my note </a>