diff and grep. Let's do the same for man.

Our man pages don't contain many useful colors (just blue links),
moreover, many people have groff SGR disabled, so they don't see any
colors with man pages.

We can set LESS_TERMCAP variables to render bold and underlined text
with colors in the pager; a common trick[1].

Bold is rendered as red, underlined as blue, and standout (prompt and
highlighted search) as inverse cyan.

Obviously this only works when the less pager is used.

If the user already has LESS_TERMCAP variables set in his/her
environment, those are respected and nothing changes.

A new color configuration is added: `color.man` for the people that want
to turn this feature off, otherwise `color.ui` is respected.
Additionally, if color.pager is not enabled, this is disregarded.

Normally check_auto_color() would check the value of `color.pager`, but
in this particular case it's not git the one executing the pager, but
man. Therefore we need to check pager_use_color ourselves.

Also--unlike other color.* configurations--color.man=always does not
make any sense here; `git help` is always run for a tty (it would be very
strange for a user to do `git help $page > output`, but in fact, that
works anyway, we don't even need to check if stdout is a tty, but just
to be consistent we do). So it's simply a boolean in our case.

So, in order for this change to have any effect:

 1. The user must use less
 2. Not have the same LESS_TERMCAP variables set
 3. Have color.ui enabled
 4. Not have color.pager disabled
 5. Not have color.man disabled
 7. Not have git with stdout directed to a file

Fortunately the vast majority of our users meet all of the above, and
anybody who doesn't would not be affected negatively (plus very likely
comprises a very tiny minority).

[1] https://unix.stackexchange.com/questions/119/colors-in-man-pages/147

Suggested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Phillip Wood <phillip.wood123@gmail.com>
Comments-by: Jeff King <peff@peff.net>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
---

I returned back to the LESS_TERMCAP variables because -D options only
exists for non-dos systems since 2021.

I tested this with a version of less that is 10 years old, and everything
works as expected.

Also, I noticed less already chooses nice colors for standout with
--use-color, and the prompt color is cyan, which is the fist one
visible, so I changed standout to cyan.

Range-diff against v5:
1:  64c93d501f ! 1:  3a3e2837ad help: colorize man pages
    @@ Commit message
         moreover, many people have groff SGR disabled, so they don't see any
         colors with man pages.
     
    -    We can set the LESS variable to render bold, underlined, and standout
    -    text with colors in the less pager.
    +    We can set LESS_TERMCAP variables to render bold and underlined text
    +    with colors in the pager; a common trick[1].
     
         Bold is rendered as red, underlined as blue, and standout (prompt and
    -    highlighted search) as inverse magenta.
    +    highlighted search) as inverse cyan.
     
         Obviously this only works when the less pager is used.
     
    -    If the user has already set the LESS variable in his/her environment,
    -    that is respected, and nothing changes. The same if any LESS_TERMCAP_*
    -    variables are set.
    +    If the user already has LESS_TERMCAP variables set in his/her
    +    environment, those are respected and nothing changes.
     
         A new color configuration is added: `color.man` for the people that want
         to turn this feature off, otherwise `color.ui` is respected.
    @@ Commit message
         So, in order for this change to have any effect:
     
          1. The user must use less
    -     2. Not have the LESS variable set
    +     2. Not have the same LESS_TERMCAP variables set
          3. Have color.ui enabled
          4. Not have color.pager disabled
          5. Not have color.man disabled
    @@ Commit message
         anybody who doesn't would not be affected negatively (plus very likely
         comprises a very tiny minority).
     
    +    [1] https://unix.stackexchange.com/questions/119/colors-in-man-pages/147
    +
         Suggested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
         Phillip Wood <phillip.wood123@gmail.com>
         Comments-by: Jeff King <peff@peff.net>
    @@ builtin/help.c: static void exec_man_konqueror(const char *path, const char *pag
     +	if (!man_color || !want_color(GIT_COLOR_UNKNOWN) || !pager_use_color)
     +		return;
     +
    -+	/* User already has configured less colors */
    -+	if (getenv("LESS_TERMCAP_md") ||
    -+		getenv("LESS_TERMCAP_us") ||
    -+		getenv("LESS_TERMCAP_so")) {
    -+		return;
    -+	}
    -+
     +	/* Disable groff colors */
     +	setenv("GROFF_NO_SGR", "1", 0);
     +
    -+	/* Add red to bold, blue to underline, and magenta to standout */
    -+	/* No visual information is lost */
    -+	setenv("LESS", "Dd+r$Du+b$Ds+m", 0);
    ++	/* Bold */
    ++	setenv("LESS_TERMCAP_md", GIT_COLOR_BOLD_RED, 0);
    ++	setenv("LESS_TERMCAP_me", GIT_COLOR_RESET, 0);
    ++
    ++	/* Underline */
    ++	setenv("LESS_TERMCAP_us", GIT_COLOR_BLUE GIT_COLOR_UNDERLINE, 0);
    ++	setenv("LESS_TERMCAP_ue", GIT_COLOR_RESET, 0);
    ++
    ++	/* Standout */
    ++	setenv("LESS_TERMCAP_so", GIT_COLOR_CYAN GIT_COLOR_REVERSE, 0);
    ++	setenv("LESS_TERMCAP_se", GIT_COLOR_RESET, 0);
     +}
     +
      static void exec_man_man(const char *path, const char *page)
    @@ builtin/help.c: static int git_help_config(const char *var, const char *value, v
      }
      
      static struct cmdnames main_cmds, other_cmds;
    +
    + ## color.h ##
    +@@ color.h: struct strbuf;
    + #define GIT_COLOR_FAINT		"\033[2m"
    + #define GIT_COLOR_FAINT_ITALIC	"\033[2;3m"
    + #define GIT_COLOR_REVERSE	"\033[7m"
    ++#define GIT_COLOR_UNDERLINE	"\033[4m"
    + 
    + /* A special value meaning "no color selected" */
    + #define GIT_COLOR_NIL "NIL"

 Documentation/config/color.txt |  5 +++++
 builtin/help.c                 | 32 +++++++++++++++++++++++++++++++-
 color.h                        |  1 +
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/Documentation/config/color.txt b/Documentation/config/color.txt
index d5daacb13a..11278b7f72 100644
--- a/Documentation/config/color.txt
+++ b/Documentation/config/color.txt
@@ -126,6 +126,11 @@ color.interactive.<slot>::
 	or `error`, for four distinct types of normal output from
 	interactive commands.
 
+color.man::
+	This flag can be used to disable the automatic colorizaton of man
+	pages when using the less pager. It's activated only when color.ui
+	allows it, and also when color.pager is on. (`true` by default).
+
 color.pager::
 	A boolean to enable/disable colored output when the pager is in
 	use (default is true).
diff --git a/builtin/help.c b/builtin/help.c
index bb339f0fc8..b6331afc2e 100644
--- a/builtin/help.c
+++ b/builtin/help.c
@@ -11,6 +11,7 @@
 #include "config-list.h"
 #include "help.h"
 #include "alias.h"
+#include "color.h"
 
 #ifndef DEFAULT_HELP_FORMAT
 #define DEFAULT_HELP_FORMAT "man"
@@ -43,6 +44,7 @@ static int verbose = 1;
 static unsigned int colopts;
 static enum help_format help_format = HELP_FORMAT_NONE;
 static int exclude_guides;
+static int man_color = 1;
 static struct option builtin_help_options[] = {
 	OPT_BOOL('a', "all", &show_all, N_("print all available commands")),
 	OPT_HIDDEN_BOOL(0, "exclude-guides", &exclude_guides, N_("exclude guides")),
@@ -253,10 +255,33 @@ static void exec_man_konqueror(const char *path, const char *page)
 	}
 }
 
+static void colorize_man(void)
+{
+	if (!man_color || !want_color(GIT_COLOR_UNKNOWN) || !pager_use_color)
+		return;
+
+	/* Disable groff colors */
+	setenv("GROFF_NO_SGR", "1", 0);
+
+	/* Bold */
+	setenv("LESS_TERMCAP_md", GIT_COLOR_BOLD_RED, 0);
+	setenv("LESS_TERMCAP_me", GIT_COLOR_RESET, 0);
+
+	/* Underline */
+	setenv("LESS_TERMCAP_us", GIT_COLOR_BLUE GIT_COLOR_UNDERLINE, 0);
+	setenv("LESS_TERMCAP_ue", GIT_COLOR_RESET, 0);
+
+	/* Standout */
+	setenv("LESS_TERMCAP_so", GIT_COLOR_CYAN GIT_COLOR_REVERSE, 0);
+	setenv("LESS_TERMCAP_se", GIT_COLOR_RESET, 0);
+}
+
 static void exec_man_man(const char *path, const char *page)
 {
 	if (!path)
 		path = "man";
+
+	colorize_man();
 	execlp(path, "man", page, (char *)NULL);
 	warning_errno(_("failed to exec '%s'"), path);
 }
@@ -264,6 +289,7 @@ static void exec_man_man(const char *path, const char *page)
 static void exec_man_cmd(const char *cmd, const char *page)
 {
 	struct strbuf shell_cmd = STRBUF_INIT;
+	colorize_man();
 	strbuf_addf(&shell_cmd, "%s %s", cmd, page);
 	execl(SHELL_PATH, SHELL_PATH, "-c", shell_cmd.buf, (char *)NULL);
 	warning(_("failed to exec '%s'"), cmd);
@@ -371,8 +397,12 @@ static int git_help_config(const char *var, const char *value, void *cb)
 	}
 	if (starts_with(var, "man."))
 		return add_man_viewer_info(var, value);
+	if (!strcmp(var, "color.man")) {
+		man_color = git_config_bool(var, value);
+		return 0;
+	}
 
-	return git_default_config(var, value, cb);
+	return git_color_default_config(var, value, cb);
 }
 
 static struct cmdnames main_cmds, other_cmds;
diff --git a/color.h b/color.h
index 98894d6a17..d012add4e8 100644
--- a/color.h
+++ b/color.h
@@ -51,6 +51,7 @@ struct strbuf;
 #define GIT_COLOR_FAINT		"\033[2m"
 #define GIT_COLOR_FAINT_ITALIC	"\033[2;3m"
 #define GIT_COLOR_REVERSE	"\033[7m"
+#define GIT_COLOR_UNDERLINE	"\033[4m"
 
 /* A special value meaning "no color selected" */
 #define GIT_COLOR_NIL "NIL"
