I have literally rebelled against using diff and git diff forever. Thought it all looked intimidating a long time ago, and learned to cope without them. By cope, I mean, I’ve survived diff’n files the brutish, caveman way line-by-line. The reality is, before this past year, it wasn’t even a huge concern: I worked on a few, mostly one-off coding/research efforts at a time. But nowadays? I’m constantly switching between projects at work, and working on new ones that are extremely similar to old ones. This means, I have a lot more need for organization, intelligent reuse of code, and the ability to quickly determine changes I made to projects I worked on last week, or last month, or just 1 of the 3 I worked on yesterday.

You can say I’ve had my “aha” moment: “There should be a tool for this! Say what? There already is?”

Toy Example

touch old.txt
echo 'Greetings, Earthling!' > old.txt
touch new.txt
echo '<beginGreeting>' > new.txt
echo 'greetings, eARTHLING!' >> new.txt

Straight-Up Diff

diff old.txt new.txt
  1c1,2
  < Greetings, Earthling!
  ---
  > <beginGreeting>
  > greetings, eARTHLING!

From what I’ve read, the resulting diff output is considered to be machine-readable, and that the more human-friendly stuff needs the -u flag. But I gotta say: I find it pretty readable.

First thing to understand is that the diff output provides the steps necessary for transforming old.txt into new.txt. At the top of an operation, we see some shorthand: 1c1,2 just says that line 1 in old.txt needs to be changed to lines 1,2 in new.txt. The lines underneath tells us what operations to perform to get from old.txt to new.txt:

• first, we are shown an operation on old.txt: “<” means remove
  • we remove line 1 from old.txt
• the operations below the dashed line (—) tell us the two lines we need to then add to arrive at new.txt

Ignore Case

diff -i old.txt new.txt
  0a1
  > <beginGreeting>

If we do not care about uppercase or lower case, then the instructions are much simpler: after line 0 in old.txt add 1 line (“") to get to a state equivalent to new.txt (ignoring any differences in case).

Some Other Flags

The -w flag tells diff to ignore whitespace, which seems pretty dang useful.

Unified Diff

This one is popular because it is similar to the output of git diff, so even if you like the straight-up diff output above, you might as well get more comfy w/ unified diff.

diff -u old.txt new.txt
  --- old.txt	2018-02-16 15:29:22.000000000 -0500
  +++ new.txt	2018-02-16 15:29:23.000000000 -0500
  @@ -1 +1,2 @@
  -Greetings, Earthling!
  +<beginGreeting>
  +greetings, eARTHLING!

Ignore Case

diff -u old.txt new.txt
  --- old.txt	2018-02-16 15:29:22.000000000 -0500
  +++ new.txt	2018-02-16 15:29:23.000000000 -0500
  @@ -1 +1,2 @@
  +<beginGreeting>
   Greetings, Earthling!

Git Diff

This is not for comparing files, but for comparing versions of a file housed in different git commits, the working directory, and/or the staging area.

Diff against previous commit

git diff HEAD

Diff All Unstaged Changes in Working Directory with Staged Files

git diff

Diff All Staged Files in Working Directory with Last Commit

git diff --staged

Diff Files in Different Commits

git diff SHA1 SHA2

VimDiff

This is a visual diff tool in Vim:

vimdiff file.txt similarFile.txt

Importantly, you can use vimdiff as a visual difftool in Git:

git config --global diff.tool vimdiff
git difftool HEAD

Some Refs

• Github/Git Cheatsheet