Simple diff command output

Question

I've seen many examples of the use of the diff command, but none detailing its basic use. Here is the content of the two files I would like to use:

cat -A file1.txt
a$
b$
c$
d$

cat -A file2.txt
b$
c$
d$
e$

If I use the diff command like so:

diff file1.txt file2.txt

I get:

1d0
< a
4a4
> e

What I would like to know is what do they mean by lines 1 and 0 in 1d0, and 4 and 4 in 4a4, in addition to the less than and greater than signs. More generally, how come there is a less than sign before the a as opposed to a greater than sign? What's the difference?

Answer 1

From the diff manual:

‘lar’
Add the lines in range r of the second file after line l of the first file. For example, ‘8a12,15’ means append lines 12–15 of file 2 after line 8 of file 1; or, if changing file 2 into file 1, delete lines 12–15 of file 2.

‘fct’
Replace the lines in range f of the first file with lines in range t of the second file. This is like a combined add and delete, but more compact. For example, ‘5,7c8,10’ means change lines 5–7 of file 1 to read as lines 8–10 of file 2; or, if changing file 2 into file 1, change lines 8–10 of file 2 to read as lines 5–7 of file 1.

‘rdl’
Delete the lines in range r from the first file; line l is where they would have appeared in the second file had they not been deleted. For example, ‘5,7d3’ means delete lines 5–7 of file 1; or, if changing file 2 into file 1, append lines 5–7 of file 1 after line 3 of file 2.

The > and < make sense if you look at the side-by-side output format:

‘<’
The files differ and only the first file contains the line.

‘>’
The files differ and only the second file contains the line.

Example outputs from the manual:

• side-by-side:

  The Way that can be told of is n   <
  The name that can be named is no   <
  The Nameless is the origin of He        The Nameless is the origin of He
  The Named is the mother of all t   |    The named is the mother of all t
                                     >
  Therefore let there always be no        Therefore let there always be no
    so we may see their subtlety,           so we may see their subtlety,
  And let there always be being,          And let there always be being,
    so we may see their outcome.            so we may see their outcome.
  The two are the same,                   The two are the same,
  But after they are produced,            But after they are produced,
    they have different names.              they have different names.
                                     >    They both may be called deep and
                                     >    Deeper and more profound,
                                     >    The door of all subtleties!
  

• normal:

  1,2d0
  < The Way that can be told of is not the eternal Way;
  < The name that can be named is not the eternal name.
  4c2,3
  < The Named is the mother of all things.
  ---
  > The named is the mother of all things.
  > 
  11a11,13
  > They both may be called deep and profound.
  > Deeper and more profound,
  > The door of all subtleties!
  

Answer 2

The diff command is explained in detail here. You'll find what you're looking for near the top of the page under the heading "How diff Works."

Specifically, 1d0 means, you must delete one line from the first file so that they are synchronized up to line zero. This is not the first line of the file, it is basically saying. If you make that deletion, both files are starting at the empty point. The next line of output in file 1 is the first line of out put in both files (i.e. the next line should be line 1).

You might be better off running diff -c file1.txt file2.txt if you want something that is easier to read.