title: List the content of a directory specified by loop indices
body: <p>I'm working with numerical weather forecast  named as follow:</p>
  
  <pre><code>sub_gfsanl_4_2011MMDD-IIII-FFFF.grb2
  </code></pre>
  
  <p>-MM stands for month  from 01 to 12 </p>
  
  <p>-DD stands for days   from 01 to 31</p>
  
  <p>-IIII stands for initialisation time, the first and second digits are for hours the third and last are for minutes</p>
  
  <p>-FFFF stands for forecast hour , the first and second digits are for hours the third and last are for minutes</p>
  
  <p>In my directory I have several files for a given days of a given months. A day has 4 data, one at every six hours IIII=0000 ,0600,1200,1800.</p>
  
  <p>What I'm trying to do is to list all files of a given days, here the f90 code I wrote:</p>
  
  <pre><code>program test_ec
  
  implicit none
  
  !==variable declaration==
  
  integer :: mi,di,dil,mil
  character*3 :: temp
  
  !==Program instructions==
  
  mil=1
  write(temp,'(i2.2)') mil
  read(temp,'(i2.2)') mi
  !convert the month into a two digit value mi=01  
  
  
   ! change to directory where the data are stored
   CALL chdir('/media/Hello/ncfiles/GFS' )
  
  do dil=1,31
  
       !loop over days
       write(temp,'(i2.2)') dil
       read(temp,'(i2.2)') di
  
       ! converting day number into a two digit number, store this value into di. ex dil=9 then di=09
  
  CALL execute_command_line( 'ls sub_gfsanl_4_2011${mi}${di}*.nc &gt; yes.txt' )
  !list all files with the correct month and days and store it to yes.txt
  
  end do
  
  end program test_ec
  </code></pre>
  
  <p>For some reasons the <code>execute_command_line</code> doesn't seem to like the $ for variable...</p>