Appearance
Underline (SGR 4)
ESC [ 4 mSGR 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.
Underline was one of the original hardware-friendly emphasis attributes: it worked on monochrome terminals where color was unavailable. Modern terminals still treat SGR 4 as the compatibility floor for diagnostics, links, and emphasis. Richer underline styles build on top of this convention, but SGR 4 remains the fallback every app can expect.How this is testedautomated
Send
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.
Analysis2026-05-17
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 | ||
| Kitty | 0.46.2 | ✓ yes | |
| Cursor | ✓ yes | ||
| Terminal.app | ✓ yes |