# [Bash], <sup><s>25</s></sup> 22 bytes

<!-- language-all: lang-bash -->

    f()(
    echo $[1<<a++];f)

[Try it online!][TIO-lica0djl]

[Bash]: https://www.gnu.org/software/bash/
[TIO-lica0djl]: https://tio.run/##S0oszvj/P01DU4MrNTkjX0El2tDGJlFbO9Y6TfN/2n8A "Bash – Try It Online"   

_Saved 3 bytes thanks to [Digital Trauma](https://codegolf.stackexchange.com/users/11259/digital-trauma)_

Solution of [Digital Trauma](https://codegolf.stackexchange.com/users/11259/digital-trauma)'s:
# [Bash], 18 bytes

<!-- language-all: lang-bash -->

    echo $1
    $0 $[$1*2]

[Try it online!][TIO-lica3287]

[Bash]: https://www.gnu.org/software/bash/
[TIO-lica3287]: https://tio.run/##S0oszvifnFiiYGOjoO7q76auYKdQkF9upAcUTk3OyFdQMeRSMVBQiVYx1DKK/Q9UwZWckZufopCoXVQBU8mlpw9lKRj@BwA "Bash – Try It Online"