Command Substitution
       Command  substitution  allows the output of a command to replace the command name.  There are two
       forms:

              $(command)
       or
              ‘command‘

       Bash performs the expansion by executing command and replacing the command substitution with  the
       standard  output  of  the command, with any trailing newlines deleted.  Embedded newlines are not
       deleted, but they may be removed during word splitting.  The command substitution $(cat file) can
       be replaced by the equivalent but faster $(< file).

       When  the old-style backquote form of substitution is used, backslash retains its literal meaning
       except when followed by $, ‘, or \.  The first backquote not preceded by a  backslash  terminates
       the command substitution.  When using the $(command) form, all characters between the parentheses
       make up the command; none are treated specially.

       Command substitutions may be nested.  To nest when using the backquoted form,  escape  the  inner
       backquotes with backslashes.

       If  the  substitution appears within double quotes, word splitting and pathname expansion are not
       performed on the results.