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  • Thank you very much and it was very helpful. By the way, how would you know that regular expression? such as =~ ^[[:alpha:]_][[:alnum:]_]*$ ]] Commented May 30, 2018 at 20:33
  • Please don't rush to accept this answer, particularly since I avoided using grep! =~ accepts regular expressions, so I just did it in-shell. To learn more, start with conditional expressions and man -s7 regex for a list of character classes. See the regular-expression tag on this site for more, such as unix.stackexchange.com/questions/119905/… Commented May 30, 2018 at 20:37
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    Can use extended pattern instead of regex: [[ $1 == [[:alpha:]_]*([[:alnum:]_]) ]] Commented May 30, 2018 at 20:40