Double parenthesis with and without dollar

Question

Is $(...) the same as (...) in Bash?

Also, is $((...)) the same as ((...))?

Also, is ${...} the sa

Answer 1

• $(...) means execute the command in the parens in a subshell and return its stdout. Example:

  $ echo "The current date is $(date)"
  The current date is Mon Jul  6 14:27:59 PDT 2015
  

• (...) means run the commands listed in the parens in a subshell. Example:

  $ a=1; (a=2; echo "inside: a=$a"); echo "outside: a=$a"
  inside: a=2
  outside: a=1
  

• $((...)) means perform arithmetic and return the result of the calculation. Example:

  $ a=$((2+3)); echo "a=$a"
  a=5
  

• ((...)) means perform arithmetic, possibly changing the values of shell variables, but don't return its result. Example:

  $ ((a=2+3)); echo "a=$a"
  a=5
  

• ${...} means return the value of the shell variable named in the braces. Example:

  $ echo ${SHELL}
  /bin/bash
  

• {...} means execute the commands in the braces as a group. Example:

  $ false || { echo "We failed"; exit 1; }
  We failed
  

More generally what does the dollar sign stand for?

It means whatever it means in the given context.