case - string testing structure.

case that perform an if, else if, ..., else test sequence on 
a provided string.

case uses the file-name wild-card metacharacters to describe a
match template to compare a provide string against. If matched, a block
of specified actions are then executed.

case general structure

case "$var" in
  str_template-1 )
    action_cmd[s]
  ;;

  str_template-2 )
    action_cmd[s]
  ;;

esac


case - keyword starting case structure.

"$var" - variable containing string to be tested. 
   Usually quoted so a null string does not crash case statement.

in - keyword closes the line specifying the target string.

str_template-1 - string pattern to match against. 
   Match is exact.
   Wild cards and alternative matches used to create a more general
     match pattern.

) - right parenthesis indicates close of a match statement.

action_cmd[s] - action[s] to take 
  if variable contents matches string pattern.

  This can be a whole block of bash code including other case statements
  Or even invoke other programs.

;; - double semi-colon, indicates end of particular action block.

esac - keyword that terminates the case structure.

case features and properties

Multiple str_template / action blocks may be specified.

Each block is exclusive.

Each block can contain any amount of code include other case statements.

Once a block is matched/executed, the case exits out the bottom.

Place specific tests first. 
  Match value starting with B (capitalized)
    action 1

  Match value starting with any alpha character
    action 2 

  Any values starting with B will be matched first,.

  ******

  Match value starting with any alpha character
    action 2 

  Match value starting with B (capitalized)
    action 1
  
  In this case, the second match will never occur.


Template match patterns and wild-cards

• literal - exact match. When using any of the meta-characters as literals, use \ to escape.
  "" can be use for null string.
  yes ) will match only yes and nothing else.

• * - wild card match very similar to file-name wild-card.
  It may appear anywhere in the match patttern.
  b* ) - matches any string starting with b
  *and* ) - matches any string containing the character sequence and anywhere in it.

• [list] - represents 1 character of any value listed in [].
  Use multiple brackets for multiple characters.
  [Yy][Ee][Ss] - matches yes in any mix of upper and lower case.

• [[:symbolic:]] - symbolic references recognized.

• str1|str2 - pipe, |, represents alternative string patterns.

• ? - represents 1 character of any value.
  [yY]?? - matches any 3 character string starting with either y or Y

• * ) - by itself represents 0 or more characters of any value. Basically, an else statement. Will always be matched if match not already found.

These can be combined in any order or repetition. Just remember 1st match is
only match, so work from specific to general.