Shell arithmetic: referencing a variable without the leading $

Question

I was surprised to read this in the Bash manual, describing Arithmetic Evaluation:

Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax.

I tried it out, and indeed, var behaves the same as $var:

❯ var=9
❯ echo $(( var + 5 ))
14
❯ (( var < 10 )) && echo 'single digits!'
single digits!

Is it considered bad practice to do so? Is it portable to other shells, or is this a Bashism?

Answer 1

It is not considered bad practice to not use $ for variables inside of arithmetic evaluation. As far as portability, ((…)) in general is not POSIX, so it will not work in shells such as dash. It will work in zsh and ksh. On the other hand, $((…)) is in POSIX and works in all modern sh-style shells.