title: How to generate for loop number sequence by using variable names in bash?
body: <p>Could anybody explain how can I use variable names in bash for loop to generate a sequence of numbers? </p>
  
  <pre><code>for year in {1990..1997}
  do
    echo ${year}
  done
  </code></pre>
  
  <p>Results:</p>
  
  <p>1990
  1991
  1992
  1993
  1994
  1995
  1996
  1997</p>
  
  <p>However</p>
  
  <pre><code>year_start=1990
  year_end=1997
  for year in {${year_start}..${year_end}}
  do
    echo ${year}
  done
  </code></pre>
  
  <p>Results:</p>
  
  <p>{1990..1997}</p>
  
  <p>How can I make the second case work? otherwise I have to write tedious while loops.
  Thanks very much.</p>