Brace expansion with a Bash variable - {0..$foo}

Question

WEEKS_TO_SAVE=4
mkdir -p weekly.{0..$WEEKS_TO_SAVE}

gives me a folder called weekly.{0..4}

Is there a secret to curly brace expansion while creating folders I'm missing?

Answer 1

bash does brace expansion before variable expansion, so you get weekly.{0..4}.
Because the result is predictable and safe(Don't trust user input), you can use eval in your case:

$ WEEKS_TO_SAVE=4
$ eval "mkdir -p weekly.{0..$((WEEKS_TO_SAVE))}"

note:

1. eval is evil
2. use eval carefully

Here, $((..)) is used to force the variable to be evaluated as an integer expression.

Answer 2

Curly braces don't support variables in BASH, you can do this:

 for (( c=0; c<=WEEKS_TO_SAVE; c++ ))
 do
    mkdir -p weekly.${c}
 done