Features›SGR (Text Styling)›Underline (SGR 4)

Underline (SGR 4)

Category: sgr · Baseline: core · Tags: ECMA-48 Standard, VT100 · Specification ↗

ESC [ 4 m

SGR 4 draws a single underline beneath text. The sequence is ESC [ 4 m. One of the original VT100 attributes, universally supported across terminals. Reset with SGR 24.

How this is testedautomated
Send \x1b[4mX, verify the cell has a truthy underline attribute.

The same probe runs against headless backends (via Termless) and real terminal apps (via a daemon launched in each terminal). This lets us distinguish parser correctness from rendering correctness.

Analysis2026-04-06

Supported by all 12 tested terminals — universal adoption. Part of the Core TUI baseline.

Supported by 14 of 14 backends (100%)

Terminal Applications

Terminal	Version	Support	Notes
iTerm2	3.6.9	✓ yes
Ghostty	1.3.1	✓ yes
VS Code		✓ yes
Warp		✓ yes
Cursor		✓ yes
Terminal.app		✓ yes

Headless Backends

Parser correctness only — a ✓ means the parser accepts the sequence.

Backend	Version	Support	Notes
Alacritty	0.26.0	✓ yes
Kitty	0.40.0	✓ yes
vt100.js	0.2.1	✓ yes
vterm	0.2.0	✓ yes
WezTerm	0.1.0-fork.5	✓ yes
xterm.js	5.5.0	✓ yes

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