Difference between ~ and ! in unix shell scripting [closed]

Question

Which operator should be used between ~ and ! while checking a regular expression in IF condition in shell scripting??

Will there be any difference between the two cases shared below ??

case 1:

if [[ $file_date != ^[0-9]{4}-[0-9]{2}-[0-9]{2}$ ]]; 
then    
    echo -e "The file_date is not in the required format"; 
else
    echo "doing good";
fi

case 2:

if [[ $file_date =~ ^[0-9]{4}-[0-9]{2}-[0-9]{2}$ ]]; 
then    
    echo -e "The file_date is not in the required format"; 
else
    echo "doing good";
fi

Answer 1

Comparing != and =~ is like comparing apples to oranges.

!= roughly means "not equal", while =~ means "matches". You can read more about this operators under [[ expression ]] in man bash. Here's an excerpt of the most relevant sections:

      When the == and != operators are used, the string to the  right  of
      the  operator  is considered a pattern and matched according to the
      rules described below under Pattern Matching,  as  if  the  extglob
      shell option were enabled.  The = operator is equivalent to ==.  If
      the shell option nocasematch is enabled,  the  match  is  performed
      without  regard  to  the case of alphabetic characters.  The return
      value is 0 if the string matches (==) or does not  match  (!=)  the
      pattern, and 1 otherwise.  Any part of the pattern may be quoted to
      force the quoted portion to be matched as a string.

      An additional binary operator, =~,  is  available,  with  the  same
      precedence  as == and !=.  When it is used, the string to the right
      of the operator is considered an extended  regular  expression  and
      matched accordingly (as in regex(3)).  The return value is 0 if the
      string matches the  pattern,  and  1  otherwise.   If  the  regular
      expression is syntactically incorrect, the conditional expression's
      return value is 2.