stdout -> shell variable

Use backticks or $() in bash:
         echo  "f1.h  f2.h  f3.h"  >  file.lst
         filelist=`cat  file.lst`
         cat  $filelist
         cat  `cat  file.lst`
         cat  $(cat  file.lst)
Shell arithmetic (expr in Bourne, $(()) in bash):
result=`expr  3  +  8  \*  2`
result=$((expr  3  +  8  *  2))