Shell Programming and Scripting

# $1=lo[file1]
# $2=hi[file1]
# $3=lo[file2]
# $4=hi[file2]
overlap(){
[ $1 -le $4 -a $4 -le $2 -o $3 -le $2 -a $2 -le $4 ]
}

overlap  22 25  24 25  && echo overlap

{
read line1
echo "$line1"
read line2 <&3
echo "$line2"
} <input1 3<input2

F1:   |-----|
F2:      |-----|

F1:   |-----|
F2:     |--|

F1:   |-----|
F2: |-----|

F1:   |-----|
F2:  |---------|

#!/bin/sh
#
set -f # no globbing
#
#debug=echo
debug=:
#
# $1=lo[file1]
# $2=hi[file1]
# $3=lo[file2]
# $4=hi[file2]
overlap(){
[ $1 -le $4 -a $4 -le $2 -o $3 -le $2 -a $2 -le $4 ]
}
#
{
readfrom=1
while :
do
  if [ $readfrom -eq 1 ]
  then
    read line || {
      readfrom=2
      read line <&3 || break
    }
  else
    read line <&3 || {
      readfrom=1
      read line || break
    }
  fi
  set -- $line
  if [ -n "$hip" ]
  then
    if overlap $lop $hip $2 $3
    then
      echo "$line"
      lop=$2; hip=$3
    else
$debug no overlap $lop,$hip $2,$3
      if [ -n "$saved" ]
      then
$debug restore $save
        saved=""
        set -- $save
        overlap $lop $hip $2 $3 || echo ""
        echo "$save"
        lop=$2; hip=$3
      else
        if [ $readfrom -eq 1 ]
        then
          readfrom=2
        else
          readfrom=1
        fi
$debug change to input$readfrom
      fi
$debug save $line
      save=$line
      saved=1
    fi
  else
    echo "$line"
    lop=$2; hip=$3
  fi
done
if [ -n "$saved" ]
then
$debug restore $save
  set -- $save
  overlap $lop $hip $2 $3 || echo ""
  echo "$save"
fi
} <input1 3<input2