How to generate for loop number sequence by using variable names in bash?

Could anybody explain how can I use variable names in bash for loop to generate a sequence of numbers?

for year in {1990..1997}
do
  echo ${year}
done

Results:

1990 1991 1992 1993 1994 1995 1996 1997

However

year_start=1990
year_end=1997
for year in {${year_start}..${year_end}}
do
  echo ${year}
done

Results:

{1990..1997}

How can I make the second case work? otherwise I have to write tedious while loops. Thanks very much.

Solution

You can use something like this

for (( year = ${year_start}; year <=${year_end}; year++ ))
do
  echo ${year}
done

seq command is outdated and best avoided.

And if you want real bad to use the following syntax,

for year in {${year_start}..${year_end}}

please modify it as follows:

for year in $(eval echo "{$year_start..$year_end}")

Personally, I prefer using for (( year = ${year_start}; year <=${year_end}; year++ )).