How to specify an optstring in the getopt function?

The getopt(3) manpage makes it pretty clear :

• the string itself is used for specifying the legal options that can appear on the commandline,
• if the option is followed by a :, then that option has a required parameter - not specifying it will cause the function to fail,
• if the option is followed by a ::, then that option has an optional parameter.

The options are one-letter identifiers. For example, specifying a string like aB:cD:: as the optstring will mean that your program takes options a, B with a required parameter, c, and D with an optional parameter.

If colon :is followed by a char or string means this option must require the argument and if there are no colon means no arguments

for more details do man 3 getopt or visit the link or manpage

It is just a string, and each character of this string represents an option. If this option requires an argument, you have to follow the option character by :.

For example, "cdf:g" accepts the options c, d, f, and g; f requires an additional argument.

An option in command line looks like -option, so you can use the options -c, -d, -f argument and -g.