#!/bin/bash
usage ()
{
if [ $# -ne 2 ]; then
    echo "Usage: $0 file1 file2" 2>&1
    exit 1
    fi
}

# Default temporary directory
: ${TEMPDIR:=/tmp}

# Check argument count
usage "$@"

# Set up temporary files for sorting
file1=$TEMPDIR/$$.file1
file2=$TEMPDIR/$$.file2

# Sort
sort $1 > $file1
sort $2 > $file2

# Open $file1 and $file2 for reading. Use file descriptors 3 and 4.
exec 3<$file1
exec 4<$file2

# Read the first line from each file to figure out how to start.
read Line1 <&3
status1=$?
read Line2 <&4
status2=$?

# Strategy: while there is still input left in both files:
#   Output the line that should come first.
#   Read a new line from the file that line came from.
while [ $status1 -eq 0 -a $status2 -eq 0 ]
    do
        if [[ "$Line2" > "$Line1" ]]; then
            echo -e "1.\t$Line1"
            read -u3 Line1
            status1=$?
        else
            echo -e "2.\t$Line2"
            read -u4 Line2
            status2=$?
        fi
    done

# Now one of the files is at end-of-file.
# Read from each file until the end.
# First file1:
while [ $status1 -eq 0 ]
    do
        echo -e "1.\t$Line1"
        read Line1 <&3
        status1=$?
    done
# Next file2:
while [[ $status2 -eq 0 ]]
    do
        echo -e "2.\t$Line2"
        read Line2 <&4
        status2=$?
    done

# Close and remove both input files
exec 3<&- 4<&-
rm -f $file1 $file2
exit 0