Ubuntu – How does “less” switch to the text, then back to the prompt

command linegitless

When I run less on some file, the prompt goes away, and I get the contents of the file on screen. Then when I press q, it goes back to the prompt. I think less does this by saving the current terminal buffer, opening a new buffer, outputting the file into the new one, then when closed, discarding the new buffer, and restoring the old, but I don't know what this process is called or how it does this.

I looked in man less but didn't find anything. I tried googling it, but couldn't find anything useful, except that the screen command can do the same thing with shell sessions.

Ultimately, I'm asking because I want git diff to use a new buffer instead of printing inline. I've already set GIT_PAGER=less and git --paginate diff, but they didn't change anything. And I have a workaround, but it's a bit long: git diff --color=always | less -R

Update: After some research, it looks like tput smcup and tput rmcup are high-level commands to do the same thing, probably using the termcap package.

Best Answer

All of less’s screen management functions are in its screen.c module. At startup, unless it’s in “no init” (-X or --no-init) mode, it outputs its sc_init string, which is termcap’s ti string (“terminal initialisation”, which gets the terminal ready for full-screen, cursor-controlling operation). At exit, it outputs its sc_deinit string, which is termcap’s te string.

The behaviour you’re seeing is the default behaviour with git, if less isn’t configured (i.e. the LESS variable is unset); when that’s the case, git runs less with the FRX options, which cause less to respectively quit if the content fits on a single screen, display colours, and skip initialising the terminal.

To get the behaviour you’re after, set LESS to -R, or set the core.pager git setting to something like less -+X (see man git-config for details).

Related Solutions

Ubuntu – How to make `less` output colors

There are two problems here:

Commands like ls —which auto-detect the colour support— don't find support from pipes
less is set to just display colour codes by default.

Both can be overcome but it's a bit clunky:

ls --color=always | less -R

This isn't ls specific. Many commands that support colour also have an override argument.

A slightly more in-depth answer is that ls is checking whether or not its STDOUT belongs to a real terminal or not. When you pipe things around, the STDOUT is set to the STDIN of the next command.

You can see this at work in the ls source code. It's using the isatty command (a core POSIX interface) to work out what the situation is:

Are colours on by default:

    print_with_color = (i == color_always
                        || (i == color_if_tty
                            && isatty (STDOUT_FILENO)));

Do we try to output in multiple columns:

if (format == long_format)
  format = (isatty (STDOUT_FILENO) ? many_per_line : one_per_line);

//...

if (isatty (STDOUT_FILENO))
  {
    format = many_per_line;
    set_quoting_style (NULL, shell_escape_quoting_style);
    qmark_funny_chars = true;
  }
else
  {
    format = one_per_line;
    qmark_funny_chars = false;
  }

grep does a very similar thing, unless explicitly overridden, it'll detect colour support, with isatty:

color_option = isatty (STDOUT_FILENO) && should_colorize ();

Ubuntu – How to get the long ‘less’ prompt with sudo

You should keep this environmental variable when running sudo. Edit the file /etc/sudoers and add

Defaults        env_keep += "LESS"

Best Answer

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Ubuntu – How to make `less` output colors

Ubuntu – How to get the long ‘less’ prompt with sudo

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