Double parenthesis with and without dollar

Question

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

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

Also, is ${...} the same as {...}?

More generally what does the dollar sign stand for? Thank you.

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.

Answer 2

Adding to the answer above:

1. [..] is used in conditions or logical expressions. Example:

$ VAR=2 $ if [ $VAR -eq 2 ]  > then > echo 'yes' > fi yes 

2. [[...]] offers extended functionality to single square brackets. Particularly, it is useful for =~ operator (used in regular expressions). Example:

$ VAR='some string' $ if [[ $VAR =~ [a-z] ]]; then > echo 'is alphabetic' > fi is alphabetic 

Reference:

https://linuxconfig.org/bash-scripting-parenthesis-explained