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getopts
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source: https://www.theunixschool.com/2012/08/getopts-how-to-pass-command-line-options-shell-script-Linux.html

there are three types of command line options

$ ls -l                   #no option
$ sort -n                 #no option
$ cut -d ","              #one argument follows
$ ls -l file1 file2 ...   #more than one 

Bash environmental variables
$OPTARG .. contains the argument value for a particular command line option
$OPTIND .. contains the index of the next command line option


getops optstring optvalue