What are steps that a UNIX shell follows while processing a command?

Question

I am using below script to validate IPV6 address but this script also pass invalid IP such as empty string, name etc.

if [[ "$IPv6ADDR"=~"^:?([a-fA-F0-9]{1,4}(:|.)?){0,8}(:|::)?([a-fA-F0-9]{1,4}(:|.)?){0,8}$" ]]; then
  echo "valid ipv6 addr"
  break;
else
  echo "Invalid IP Address"
  break;
fi

Can someone identify what's wrong in the regex, Please?

Answer 1

The sequence of executing commands by shell are as follows:

Parsing: The command is broke into words, by using the spaces as delimiters unless quoted. The tabs are replaced by a single space.

Evaluation of a variable: The words with $ as preceding character will be evaluted unless quoted.

Command Substitution: The commands surrounded by backquotes will be executed by the shell. Later these will be replaced into standard output of the command into command line.

Wild card interpretation: Finally, the shell identifies all the wild cards(?,*) in the command line and replaces the wild card with sorted list of file names which maches the pattern.

PATH evaluation: Ultimately shell looks for the PATH variable for the purpose of determining the sequence of directories to search for the files or commands.