Bash script - variables in curly braces

Question

The following bash script

#!/bin/bash
startNumber=$(( 1 ))
endNumber=$(( $startNumber + 3 ))
#for number in {$startNumber..$endNumber}
for number in {1..4}
do
        echo $number
done
exit 0

gives the desired output

1
2
3
4

However, when I switch the uncommented and commented for loop, the output is

{1..4}

What am I doing wrong?

Answer 1

Variables won't expand inside brace expansion. You could do:

for ((number=startNumber; number<=endNumber; number++)); do
    echo "$number"
done

Also, there is no reason to use arithmetic expansion for startNumber you should simply do:
startNumber=1.

Additionally, you don't need to use $ to expand variables inside arithmetic expansion, so endNumber could be:
endNumber=$((startNumber+3))

Answer 2

As explained elsewhere the expansion won't work. Alternative way to get your sequence of numbers:

for number in $(seq $startNumber $endNumber)
do
  echo $number
done