lg

GNOME Shell integrated debugger and inspector

TLDR

Open the Looking Glass debugger from the GNOME Shell run dialog

$ > Alt+F2, type "lg", press Enter

Enable Looking Glass on systems where it is hidden

$ gsettings set org.gnome.shell development-tools true

Re-disable Looking Glass

$ gsettings set org.gnome.shell development-tools false

Reference the most recent evaluator result inside Looking Glass

$ > it

Recall an earlier numbered result inside Looking Glass

$ > r([number])

Slow GNOME Shell animations for debugging

$ > St.set_slow_down_factor([factor])

SYNOPSIS

lg (typed into the GNOME Shell run dialog)

Looking Glass is the integrated debugger, inspector, and JavaScript REPL of GNOME Shell. It is opened from inside a running GNOME session by pressing Alt+F2 and entering the keyword lg. It is intended for debugging the shell itself and writing or troubleshooting GNOME Shell extensions.The interface offers four panes. The Evaluator is an interactive JavaScript prompt with full access to the GNOME Shell process, including the St, Clutter, Meta, and GLib APIs. The Windows pane lists open windows. The Extensions pane shows installed shell extensions and links to their source. The Errors pane displays logged errors (in newer releases these are typically routed to journalctl).A picker tool in the top-left lets the user click a UI element on screen to retrieve the corresponding actor in the evaluator for inspection.Press Esc in the evaluator pane to dismiss Looking Glass.

CONFIGURATION

Looking Glass is exposed to GNOME Shell through the org.gnome.shell GSettings schema. Set development-tools to true to enable it on distributions where it is hidden by default. Command history is persisted to dconf.

EVALUATOR HELPERS

Reference the most recently evaluated expression.

r(_n_)

Reference the result with index n.

St.set_slow_down_factor(_f_)

Multiply animation durations by f (>1 makes them slower); useful when debugging transitions.

CAVEATS

Not a stand-alone CLI tool: lg is a keyword interpreted by GNOME Shell's run dialog, not a binary in PATH. Available only inside a running GNOME Shell session (X11 or Wayland). It runs JavaScript directly inside the shell process, so a buggy snippet can freeze or crash the desktop session.

HISTORY

Looking Glass was introduced with GNOME Shell 3.0 in April 2011 as an introspection tool inspired by Firebug, providing extension authors and shell developers with a live JavaScript console into a running session. It has remained part of GNOME Shell across all subsequent releases.

lg