Arrow keys in `less`

Question

In the manpage of less the up- and down-arrow keys aren't mentioned in the context of normal navigation, yet on Debian Linux (8.2) they obviously work to scroll up and down. Why is this? A lesskey file is mentioned in the manpage, maybe it is defined there? I can't find this file anywhere, does it exist in the standard installation of Debian?

Answer 1

The manual page, e.g, as you might find on OSX or Unix commands is incomplete:

• Among other things, less initializes itself using the termcap strings for the cursor keys, home/page keys, pageup/pagedown keys.
• In most configurations, it is also possible to override the termcap settings with environment variables beginning LESS_TERMCAP_, e.g., LESS_TERMCAP_ku to override the ku (cursor-up) string.

Looking at the source code, there's a table cmdtbl in decode.c which gives initial key-bindings, e.g.,

    'k',0,                          A_B_LINE,
    'y',0,                          A_B_LINE,
    CONTROL('Y'),0,                 A_B_LINE,
    SK(SK_CONTROL_K),0,             A_B_LINE,
    CONTROL('P'),0,                 A_B_LINE,
    SK(SK_UP_ARROW),0,              A_B_LINE,

You may be able to see the pattern here: they all go back one line. But I do not see any of that in the manual page.

Further reading:

• Colors in Man Pages, which mentions the environment variables.
• lesskey(1) manual page with some discussion of key-bindings