How do I pipe colored diff output to less?

Most diff implementations cannot output colors, you need another program, such as colordiff for that. Newer GNU diffutils, however, do have a --color option, so if you're on Linux and have a recent version (after v3.4 I think), then you can do this natively with diff. If your diff doesn't support --color, read on.

Colors in the terminal are printed via ANSI escape codes which less does not interpret by default. To get less to correctly show colors, you need the -r, or even better, -R switch:

colordiff -- "$file1" "$file2" | less -R 

From man less:

 -R or --RAW-CONTROL-CHARS Like -r, but only ANSI "color" escape sequences are output in "raw" form. Unlike -r, the screen appearance is maintained correctly in most cases. ANSI "color" escape sequences are sequences of the form: ESC [ ... m where the "..." is zero or more color specification characters For the purpose of keeping track of screen appearance, ANSI color escape sequences are assumed to not move the cursor. You can make less think that characters other than "m" can end ANSI color escape sequences by setting the environment variable LESSAN‐ SIENDCHARS to the list of characters which can end a color escape sequence. And you can make less think that characters other than the standard ones may appear between the ESC and the m by setting the environment variable LESSANSIMIDCHARS to the list of characters which can appear. 

Alternatively, you can use more which will display colors correctly by default.

If you cannot install external programs, you should be able to get the same output using a more manual approach:

diff a b | perl -lpe 'if(/^</){$_ = "\e[1;31m$_\e[0m"} elsif(/^>/){$_ = "\e[1;34m$_\e[0m"}' 

The other answers here might be out of date. As of coreutils 3.5 diff can indeed produce colored output which is turned off by default when the stdout is not a console.

From the man page:

--color[=WHEN]
colorize the output; WHEN can be never, always, or auto (the default)

To force color output when stdout is a pipe diff --color=always -- "$file1" "$file2" | less -R should work.

To pipe colored diff to less:

diff $file1 $file2 | colordiff | less -r 

To make it more readable, by limiting it to a single screen:

diff -uw $file1 $file2 | colordiff | less -r 

And, to cause less not to display if there is only one screens worth of content:

diff -uw $file1 $file2 | tee /dev/stderr | colordiff | less -r -F 

The -F causes less to close immediately if there is less than one screens worht of content, the pipe to stderr is because when less closes you lose the output - by piping to stderr, it gets output even if less does not display.

An alternative (and, I think, better) way, is to just use -X to prevent less clearing the screen:

diff -uw $file1 $file2 | colordiff | less -r -X -F 

This works well for me, but might be specific to bash. colordiff is not a built-in, but is easily installed.

I'd use riff:

diff "$A" "$B" | riff 

Or just this, which will implicitly invoke diff under the hood:

riff "$A" "$B" 

Riff not only tells you which lines changed, but also what parts of the lines that changed (see screenshot below).

Riff defaults to paging the output the same way that git does, so you don't need to worry about pager integration.

On top of that, Riff integrates with git so you can get this output from git diff and its friends as well.

Get it here: https://github.com/walles/riff/releases/

Disclaimer: I wrote riff myself so of course I'm recommending it :).