Why does parameter expansion happen before brace expansion in this example?

Question

From https://unix.stackexchange.com/a/7739/674, why does parameter expansion happen before brace expansion in the following example?

eval rm foo.{$ext0..$extN}

Answer 1

It's doing a no-op brace expansion first (expanding to what you started with), then parameter expansion, then brace expansion within "eval":

# No-op brace expansion example:
echo foo.{not..understood}
# See: foo.{not..understood}

ext0=10
extN=20

# Parameter expansion
echo foo.{$ext0..$extN}
# See: foo.{10..20}

# Final brace expansion
eval echo foo.{$ext0..$extN}
# See: foo.10 foo.11 foo.12 foo.13 foo.14 foo.15 foo.16 foo.17 foo.18 foo.19 foo.20

# Now if you want to have fun with it (each eval unwraps one level of escapes)
eval eval eval eval echo foo.{\\\\\\\$ext0..\\\\\\\$extN}
# See: foo.10 foo.11 foo.12 foo.13 foo.14 foo.15 foo.16 foo.17 foo.18 foo.19 foo.20