How to see moved lines in a file comparison with vimdiff / diff

diff()vimvimdiff

Moved lines are frequently annotated as deletes and adds but they can be shown as moved-from and moved-to instead.

Like in this screenshot of Notepad++.

Notepad++ diff'ing moved lines

Best Answer

For its diff view, Vim relies on the output of the diff tool, which consists of line insertions (+) and deletions (-), but no moves. For convenience, Vim is able to highlight changes within a line, but it does not detect moves of (blocks of) lines, unlike Notepad++. You would need to implement this kind of feature yourself (e.g. using the sign column to show the moves).

The Test

Three files are involved in my test:
File test1:

    text

File test2:

    text
    #comment

File test3:

    changed text
    #comment

The commands:

$ # comparing files with comment-only changes
$ diff -u -I '#.*' test{1,2}
$ # comparing files with both comment and regular changes
$ diff -u -I '#.*' test{2,3}
--- test2       2011-07-20 16:38:59.717701430 +0200
+++ test3       2011-07-20 16:39:10.187701435 +0200
@@ -1,2 +1,2 @@
-text
+changed text
 #comment

The alternative way

Since there is no answer so far explaining how to use the -I option correctly, I'll provide an alternative which works in bash shells:

diff -u -B <(grep -vE '^\s*(#|$)' test1)  <(grep -vE '^\s*(#|$)' test2)

diff -u - unified diff
- -B - ignore blank lines
<(command) - a bash feature called process substitution which opens a file descriptor for the command, this removes the need for a temporary file
grep - command for printing lines (not) matching a pattern
- -v - show non-matching lines
- E - use extended regular expressions
- '^\s*(#|$)' - a regular expression matching comments and empty lines
  - ^ - match the beginning of a line
  - \s* - match whitespace (tabs and spaces) if any
  - (#|$) match a hash mark, or alternatively, the end of a line

Result of diff two files with switched lines says missing the same line twice

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)

Best Answer

Related Solutions

How to diff files ignoring comments (lines starting with #)

The Test

The alternative way

Result of diff two files with switched lines says missing the same line twice

Related Question