Shell Programming and Scripting

awk 'FNR==NR{                   #### FNR and NR are the awk's built-in variables, so FNR will be representing the current Input_file's line number same as NR, only deference between them is FNR will be RESET on each Input_file's starting and NR will keep increasing till the last Input_file is completely read. So FNR==NR condition will be TRUE when 1st Input_file1 will be read  not the second one.
A[$0];                          #### Creating an array named A whose index is $0(complete line) of Input_file2.
next}                           #### next is awk's built-in keyword which will be used to skip all further statements. So here it will skip all next mentioned statements.
((substr($0,6,3)) in A)         #### substr is again awk's built in keyword which is mostly used to get specific string out of a line, it's syntax is substr(line/variable's value, starting position number, Number of characters you need to get from the starting position mentioned). So here as per your requirement we have to get 6,7,8 characters in Input_file2 so mentioning ($0,6,3) means complete line's 6th character to 3 characters take and then check if they are present in array A(which we created while Input_file2 was getting read).
{                               #### If that substring is present in array A then execute following statements.
print $0 > "output_file1.txt";  #### print the complete line($0) to file named "output_file1.txt" as per your requirement.
delete A[substr($0,6,3)];       #### Deleting the array's element to remove duplication here.
next}                           #### Using next keyword here to skip all further statements here.
!((substr($0,6,3)) in A)        #### Now checking substring for characters 6,7,8 which are NOT present in array A(means to get lines which are present in Input_file and not in Input_file2)
{print > "output_file2.txt"}    #### printing those lines into file named "output_file2.txt" as per your requirements.
'   Input_file2   Input_file1   #### Mentioning Input_file1 and Input_file2 here.