How to make diff print a full file diff when a file does not exist?

Question

I'm trying to compare files on the root file system with a backup, and I'd like the comparison to work a bit like git or svn diff when a file has been added or removed - That is, display the full file diff. diff unfortunately just prints a No such file or directory message, which is not very useful. I couldn't find an option for this.

The simplest workaround I could find is to parse the diff output and diff -u path <(printf '') or diff -u <(printf '') path depending on which path exists. That's a bit of a kludge - Probably 10 lines of code for what could be a simple diff option.

Another workaround is

diff -u <([ -e "$path1" ] && cat "$path1") <([ -e "$path2" ] && cat "$path2")

, but that loses the file names in the diff.

Based on Craig Sanders's answer and the previous workaround, I ended up with this:

diff -u \
 "$([ -e "$path1" ] && echo "$path1" || echo /dev/null)" \
 "$([ -e "$path2" ] && echo "$path2" || echo /dev/null)"

Edit: Turns out there is an option for it:

-N, --new-file
 treat absent files as empty

In other words:

diff -Nu "$path1" "$path2"

• Mat: What OS/version of diff are you working with?
• l0b0: Ubuntu 12.04, diff (GNU diffutils) 3.2

Answer 1

With at least some versions of GNU diffutils' diff, you can use the -N flag to do that directly (although the documentation appears to say that only works for directory diffs).

$ ls
foo
$ cat foo
foo
$ diff -u foo bar
diff: bar: No such file or directory
$ diff -uN bar foo
--- bar 1970年01月01日 01:00:00.000000000 +0100
+++ foo 2012年09月04日 12:45:34.000000000 +0200
@@ -0,0 +1 @@
+foo
$ diff -uN foo bar
--- foo 2012年09月04日 12:45:34.000000000 +0200
+++ bar 1970年01月01日 01:00:00.000000000 +0100
@@ -1 +0,0 @@
-foo
$ diff -v
diff (GNU diffutils) 2.8.1
Copyright (C) 2002 Free Software Foundation, Inc.

• cas: the man page for GNU diff 3.2 says only: -N, --new-file treat absent files as empty
• Mat: The version I have on hand (2.8.1) says "In directory comparison, if a file is found in only one directory, treat it as present but empty in the other directory.", so that's why I put the disclaimer. Good to know that's now documented for non-directory diffs now.

Answer 2

have you tried diff -u /dev/null filename ?

that worked for me when i just tried it.

alternatively:

touch missing
diff -u missing filename
rm missing

Also, if you diff directories (e.g. diff -x /path/to/backup -uN / /path/to/backup) then diff will recursively show full diffs of all files, including missing files. You need the -x, --exclude=PAT option because /path/to/backup is a subdir of /

(thanks to Mat for reminding me of the -N option)

• cas: thanks for the accept but i think Mat's answer is better. diff -N seems to do exactly what you want without stuffing around with tempfiles or /dev/null, and it gives you the recursive two-directory diff that i edited out of my answer because it doesn't work properly without -N