Reference

Keys

Every key and command of the viewer, as F1 lists them inside aless.

The keys are jless's where jless has the key, and a count before a key repeats it (3j). Press F1 or type :help inside aless for this list; the same text is plain text here.

Moving #

j k / ↓ ↑ / C-n C-p / Enter Backspace   down / up one row (count allowed)
h / ←     collapse an expanded container, else go to the parent
l / →     expand a collapsed container, else step to the first child
H         go to the parent without collapsing
J K       next / previous sibling
w b       forward / back to the next change in depth
0 ^ $     first / last sibling
g G / Home End    first / last row;  Ng NG  go to row N
C-f C-b / PgDn PgUp   a page down / up
C-d C-u   half a page down / up (a count sets the distance)
C-e C-y   scroll one row without leaving the focus behind
zz zt zb  focused row to the centre / top / bottom
. , ;     scroll a long value right / left / to its end and back
< >       less / more indentation

Folding #

Space     toggle the focused container
c C       collapse the focused node and its siblings (C: deeply)
e E       expand the focused node and its siblings (E: deeply)
m         switch between data mode and line mode
%         in line mode, jump between a container's brackets

Search #

regular expressions; smart case; add /s to force case; [ ] { } are literal

/pat ?pat   search forward / backward       n N   next / previous match
* #         search for the focused key forward / backward

Copy and print #

y copies, p prints to the screen

yy pp   pretty-printed value     yv pv   one-line value    ys ps   string contents
yk pk   key                      yp pP   path .a[0].b      yb pb   ["a"][0]["b"]
yq pq   jq path

Explorer #

aless DIR, or aless alone for the current directory

the directory tree uses the same keys: l / Space expand a directory (its
contents are listed as you go), h collapses, / searches the listed names
Enter     open the file under the cursor in a new tab (toggle a directory)
-         go up: the parent directory becomes the root
:cd DIR   change the root     :explore [DIR]   open another directory tab
:set hidden | nohidden | hidden!   show dot-files (also --hidden)
yp        copy the entry's filesystem path

Errors #

A file that does not parse shows the parser's report: the message, the
source lines around the error with a caret under it, and the grammar's
hint. While a watched file is broken its last good document stays on
screen with the report docked below it, until the file parses again.
! / :error   the active tab's report, full size (after a failed
             :format, that attempt's report); h/l pan a long line

Tabs, files and watching #

Tab / Shift-Tab   next / previous tab      :tab N   go to tab N
:open PATH [FORMAT]   open a file (or a directory) in a new tab (:e is an alias)
q / :q    close the tab (the last one closed quits)   :qa / :quit   quit
W / :watch on|off   toggle reloading the tab when its file changes
r / :reload         reload now (in the program pane, the program)
s / :source         show the raw source (the focused node's line centred)
:format FORMAT      parse the tab's text as another format
:depth N            fold everything below depth N
:w[!] FILE          write the document as JSON
:set number | nonumber | relativenumber | norelativenumber | so=N | indent=N
:N or :line N       go to row N
F1 / :help          this help

Panes #

aless --panes out[,program], or :vsplit

C-w                 move the focus to the next pane; the keys work there
s                   show the focused pane's text, or its tree again; in the
                    text, : and r work too, and any other key but the
                    scrolling ones shows the tree
q                   close an output or program pane (the input pane stays)
:vsplit / :split    side by side / stacked, opening the output beside the input
:arrange            switch between side by side and stacked
:pane out|program|close   open the output or the program pane, or close one
:only               keep the input pane alone
The output is the document as --render or --alchemy writes it, read
back as a tree; it is written again when the tab reloads or changes.

A watched tab reloads when its file changes and keeps your place: the
focused node is found again by path, or by its nearest surviving
ancestor and then the node closest to its old source line; folds that
still exist are kept, and the focus stays on the same screen row.
Formats: json jsonl jsonic jsonc json5 yaml toml ini csv tsv xml zon
markdown feed text, and the NAME of any --grammar (by extension or whole
file name; --kind or :open ... FORMAT to force).