Git pager is less, but what is causing the output coloring?

Question

less itself isn't capable of doing syntax highlighting, according to this thread.

However, git diff nicely shows colored output in less, its default pager. When I redirect the output of git diff into a file, no color escape sequences are visible.

Does git diff know where it's being sent, and formats the output accordingly? How would one do that?

I just noticed that git colors the diff output (e.g. git diff), however, it doesn't know how to syntax highlighting in general. e.g.

git show 415fec6:log.tex

doesn't enable any TeX-like syntax.

Reading the git sources, I found the following hints

in diff.h:

int use_color;

I was previously referring to syntax highlighting, but that was not correct. What I mean is output coloring, see e.g.

example color output

Do you have any real syntax highlighting, or just the red and green color for removed and added lines? — Paŭlo Ebermann, Sep 05 '14 at 19:43
BTW: to get git colouring into less: `git diff --color=always | less -r` (or `less -R` for ANSI). BTW: for other commands that don't paginate by default (`git diff` does) you can switch it on: `git -p some_git_command` — hyperpallium, Aug 16 '19 at 06:30

ysdx · Accepted Answer · 2014-09-05T17:12:32.753

18

Git uses isatty() to check whether stdout is a tty: this is used to see if a pager must be used (pager.c) as well as colors (color.c).

edited Sep 05 '14 at 17:12

answered Sep 05 '14 at 13:33

ysdx

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score 14 · Answer 2 · answered Sep 05 '14 at 13:30

Running:

git diff --color=always > output

shows the color escapes.

Damn I found it in git's color.c file:

static int check_auto_color(void)                                           
{                                                                           
  if (color_stdout_is_tty < 0)                                              
    color_stdout_is_tty = isatty(1);                                        
  if (color_stdout_is_tty || (pager_in_use() && pager_use_color)) {         
    char *term = getenv("TERM");                                            
    if (term && strcmp(term, "dumb"))                                       
      return 1;                                                             
  }                                                                         
  return 0;                                                                 
}

score 3 · Answer 3 · edited Mar 20 '17 at 10:18

3

less can support colour with -r --raw-control-chars and -R --RAW-CONTROL-CHARS

You could use Pythons pygmentize to do the highlighting and pipe it in.

To combine it all configure a less filter as described on superuser

edited Mar 20 '17 at 10:18

Community

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answered Sep 05 '14 at 12:35

Matt

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the link is indeed interesting, thanks. Do you know if `git` is actually using pygmentize? Because it is not listed as a dependency. – Sebastian Sep 05 '14 at 12:39
btw, when I run `git diff` the child `less` process is not appended by any option (neither `-R` nor `-r`). However, I don't know if git calls it in a special way and thus the args are not visible in `htop`. – Sebastian Sep 05 '14 at 12:46
`git` outputs the colour escape code it's self. It's also written in `c` so adding Python would be a huge dependency. It might be able to control `less` via the environment – Matt Sep 05 '14 at 12:56
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maybe via the `LESS` env var? – Matt Sep 05 '14 at 13:02
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Yes, LESS=FRX is added to the environment (see [pager.c](https://github.com/git/git/blob/398dd4bd039680ba98497fbedffa415a43583c16/pager.c)). – ysdx Sep 05 '14 at 23:02

Git pager is less, but what is causing the output coloring?

3 Answers3