In your first
diff output (so called "normall diff") the meaning is as follows
< - denotes lines in file1.txt
> - denotes lines in file2.txt
5a5 denote line numbers affected and which actions were performed.
d stands for deletion,
a stands for adding (and
c stands for changing). the number on the left of the character is the line number in file1.txt, the number on the right is the line number in file2.txt. So
3d2 tells you that the 3rd line in file1.txt was deleted and has the line number 2 in file2.txt (or better to say that after deletion the line counter went back to line number 2).
5a5 tells you that the we started from line number 5 in file1.txt (which was actually empty after we deleted a line in previous action), added the line and this added line is the number 5 in file2.txt.
The output of
diff -u command is formatted a bit differently (so called "unified diff" format). Here
diff shows us a single piece of the text, instead of two separate texts. In the line
@@ -1,5 +1,5 @@ the part
-1,5 relates to file1.txt and the part
+1,5 to file2.txt. They tell us that
diff will show a piece of text, which is 5 lines long starting from line number 1 in file1.txt. And the same about the file2.txt -
diff shows us 5 lines starting from line 1.
As I have already said, the lines from both files are shown together
this is the original text
happy hacking !
+GNU is not UNIX
- denotes the lines, which were deleted from file1.txt and
+ denotes the lines, which were added.