How should I use git diff for long lines?

Or if you use less as default pager just type -S while viewing the diff to reenable wrapping in less.

The display of the output of git diff is handled by whatever pager you are using.

Commonly, under Linux, less would be used.

You can tell git to use a different pager by setting the GIT_PAGER environment variable. If you don't mind about paging (for example, your terminal allows you to scroll back) you might try explicitly setting GIT_PAGER to empty to stop it using a pager. Under Linux:

$ GIT_PAGER='' git diff

Without a pager, the lines will wrap.

If your terminal doesn't support coloured output, you can also turn this off using either the --no-color argument, or putting an entry in the color section of your git config file.

$ GIT_PAGER='' git diff --no-color

You can also use git config to setup pager to wrap.

$ git config core.pager 'less -r' 

Sets the pager setting for the current project.

$ git config --global core.pager 'less -r' 

Sets the pager globally for all projects

With full credit to Josh Diehl in a comment to this answer, I nevertheless feel like this ought to be an answer unto itself, so adding it:

One way to deal with seeing differences in long lines is to use a word-oriented diff. This can be done with:

git diff --word-diff

In this case, you'll get a significantly different diff output, that shows you specifically what has changed within a line.

For example, instead of getting something like this:

diff --git a/test-file.txt b/test-file.txt
index 19e6adf..eb6bb81 100644
--- a/test-file.txt
+++ b/test-file.txt
@@ -1 +1 @@
-this is a short line
+this is a slightly longer line

You might get something like this:

diff --git a/test-file.txt b/test-file.txt
index 19e6adf..eb6bb81 100644
--- a/test-file.txt
+++ b/test-file.txt
@@ -1 +1 @@
this is a [-short-]{+slightly longer+} line

Or, with colorization, instead of this:

You might get this:

Now, if you're comparing a really long line, you may still have issues with the pager situation you originally described, and which has been addressed, apparently to satisfaction, in other answers. Hopefully this gives you a new tool, though, to more easily identify what on the line has changed.

To use less as the pager and make line wrapping permanent you can simply enable the fold-long-lines option:

git config --global core.pager 'less -+S'

This way you do not have to type it while using less.

Cheers