Bash - Command Sub

gistfile1.txt

   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

       $(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.