IL — Insert Line (CSI Pn L)

Used in

Real-world tools that emit this sequence — anchors the bytes to commands you've already used.

• vim, neovim — `O` (open line above) primitive—every `O` / `o` / `:put!` boils down to `\x1b[1L` at the cursor row inside a DECSTBM-bracketed region — the editor reuses the terminal's native insert-line rather than redrawing every row below the cursor
• less, more — scroll-up by N—scrolling backward through a long file emits `\x1b[<N>L` at row 1 inside the page region — the bottom N lines fall off, and the freshly-revealed top lines are then written into the blank slots
• tmux — pane resize / split-window redraw—growing a pane vertically uses IL inside the new pane's DECSTBM region to make room without forcing the surrounding panes through a full repaint — `tmux send-keys` benchmarks show ~10× fewer bytes vs. full-screen rewrite
• ncurses — `insertln()` / `winsertln()`—the libncurses API `insertln(WIN)` maps directly to `il1` (no Pn) or `il` (with Pn) terminfo capabilities — every TUI built on ncurses (htop, vifm, mc, weechat) inherits IL the moment the underlying terminal exports the cap
• git diff --color-moved=zebra inside `less -R`—the zebra stripe colouring on moved-block diff hunks works because `less` lays them out with IL + DL pairs inside a DECSTBM region — interleaving the alternating background colours without smearing the unchanged context

Related pitfalls

Known footguns this sequence creates in the wild. Each links to the full symptom + cause + fix.

• ⚠IL inside a decstbm scroll region pushes lines off the bottom — they're gone, not stashed·#alt-screen-newline
• ⚠\x1b[L (omitted Pn) defaults to 1, not 0 — same default rule as every other Pn-defaulting CSI·#csi-default-params

Frequently asked

Short answers to the questions developers actually search for this sequence.

Does \x1b[L insert a line or scroll the screen?

Insert. IL (\x1b[Pn L) opens Pn blank lines *at the cursor row* and pushes subsequent rows down within the active scroll region — no scrolling of the top of the region, no content loss above the cursor. Lines that get pushed past the bottom of the scroll region (set by decstbm) are discarded; if you haven't set a scroll region, the bottom lines fall off the visible screen. This is the primitive vim / neovim use for the O and o commands; rolling your own with \n re-implements the bug ed(1) ships with.

How does IL interact with the scroll region (DECSTBM)?

IL only takes effect when the cursor sits *inside* the active scroll region. If the cursor is above the top margin or below the bottom margin, IL is a no-op on most terminals (xterm, kitty, alacritty, wezterm, ghostty) — your draw silently fails to open lines. Inside the region, lines push down toward the bottom margin and any rows past the margin are discarded. The portable pattern is: set DECSTBM (\x1b[<top>;<bottom>r) → move the cursor inside with CUP → emit IL → reset DECSTBM with \x1b[r (defaults to full screen) when done. See pitfall alt-screen-newline for a related cursor-row off-by-one trap.