Shell Programming and Scripting

field1  field2    field3
1        A           B
2        C           D
3        E           F

4       G           H
1       I           J
5       K           L

4      M           N
6      O           P

6    O    P
4    G    H
5    K    L
1    A    B
2    C    D
3    E    F

awk '{if (! a[$1] ) {a[$1]++;print}}' file*

# ./justdoit
4       G           H
6      O           P
1        A           B
5       K           L
2        C           D
3        E           F

 ## justdoit @ygemici ##
 
#!/bin/bash
for file in file3 file2 file1
 do
  while read -r l
   do
     isthis=$(echo $l | sed 's/^\(.\).*/\1/')
     status=write
     for i in ${arr[@]}
      do
       if [ "$i" = "$isthis" ] ; then
        status=notwrite
       fi
      done
     if [ "$status" != "notwrite" ] ; then
        if grep $isthis file1 ; then
           grep ${isthis} file1>/dev/null
           arr=("$arr $isthis" )
        elif grep ${isthis} file2 ; then
           grep ${isthis} file2>/dev/null
           arr=("$arr $isthis" )
        else
           grep ${isthis} file3
           arr=("$arr $isthis" )
        fi
     fi
   done <$file
 done

awk '!a[$1]++' file*