Compare two files strictly line-by-line, without insertions or deletions

To understand the report, remember that diff is prescriptive, describing what changes need to be made to the first file (file1) to make it the same as the second file (file2).

Specifically, the d in 1d0 means delete and the a in 2a2 means add.

Thus:

• 1d0 means line 1 must be deleted in file1 (apples). 0 in 1d0 means line 0 is where they would have appeared in the second file (file2) had they not been deleted. That means when changing file2 to file1 (backwards) append line 1 of file1 after line 0 of file2.
• 2a2 means append the second line (oranges) from file2 to the now second line of file1 (after deleting the first line in file1, oranges switched to line 1)

• diff -u

may be what you need for your assignment.

To take your example and using diff -u:

michael@x071:[/home/michael]diff -u file?
--- file1       2016-11-24 07:48:41 +0000
+++ file2       2016-11-24 07:48:57 +0000
@@ -1,3 +1,3 @@
 orange
 apple
-peach
+strawberry

A word of advice - RTM - or - Read The Manual. There are often other options. FYI: the historic options of diff (and diff3 when comparing three files) were to assist with creating "program inout" that would change file1 into file2 (or file2 back into file1). This has been the base of all "version control" software.

The diff options I remember from long ago:

• -e : Produces output in a form suitable for use with the ed editor to convert File1 to File2.
• -f : Produces output in a form not suitable for use with the ed editor, showing the modifications necessary to convert File1 to File2 in the reverse order of that produced under the -e flag.
• -n : Produces output similar to that of the -e flag, but in the opposite order and with a count of changed lines on each insert or delete command. This is the form used by the revision control system (RCS).

The last option I will highlight is a "new" one - relatively speaking. (also several years old but was often not in POSIX implementations). Rather than creating output suitable for 'ed' of 'RCS', this is suitable for patch:

• -u : Produces a diff command comparison with three lines of unified context. The output is similar to that of the -c flag, except that the context lines are not repeated; instead, the context, deleted, and added lines are shown together, interleaved.

IMHO: the key value of diff -c is as an improvement over the command cmp - when you want to know more than ONLY if two files differ, or not. I had never paid attention (maybe it is a "new" option as well) - but shall think about it when my question is a recursive search for files that differ between two directory trees.