Sending keys

send-keys -l types literal characters. Without -l, tmux recognizes key names such as Enter, C-c, and Up. Typing the word Enter and pressing Enter are separate operations.

Literal text, key names, and whether Enter follows¶

This complete script types the word Enter into a pane running cat, then presses the Enter key. Neither command starts an interactive tmux client. Save it as send.sh and run it in a POSIX shell with tmux 3.2a or newer and fractional sleep support.

#!/bin/sh
set -eu
directory=$(mktemp -d "${TMPDIR:-/tmp}/libtmux-guide.XXXXXX")
socket="$directory/tmux.sock"

cleanup() {
    status=$?
    trap - 0 HUP INT TERM
    if [ -S "$socket" ] && ! tmux -S "$socket" kill-server; then
        printf 'Cannot stop tmux; kept %s\n' "$directory" >&2
        exit 1
    fi
    rm -rf "$directory" || status=$?
    exit "$status"
}
trap cleanup 0
trap 'exit 1' HUP INT TERM

tmux -S "$socket" -f /dev/null new-session -d -s input 'cat'
tmux -S "$socket" send-keys -t input:0.0 -l 'Enter'
tmux -S "$socket" send-keys -t input:0.0 Enter

attempt=0
while [ "$attempt" -lt 100 ]; do
    screen=$(tmux -S "$socket" capture-pane -p -t input:0.0)
    if printf '%s\n' "$screen" | grep -Fqx 'Enter'; then
        printf '%s\n' "$screen"
        exit 0
    fi
    attempt=$((attempt + 1))
    sleep 0.05
done
printf '%s\n' 'Timed out waiting for typed input.' >&2
exit 1

Run the saved script:

$ sh send.sh

The screen contains Enter: the terminal echoes the input, and cat writes it back after the newline. The polling loop waits for visible text and fails after 100 unsuccessful checks. Cleanup stops only the private server.

Literal input disables tmux’s key-name lookup. The application still interprets those characters. In a shell pane, that includes shell quoting, expansions and commands; literal mode does not make shell input safe to compose from arbitrary text.

The race you can’t see from the call site¶

Completing send-keys means tmux accepted the input. It does not establish that the application read it or finished a command. Terminal echo can appear before the application processes a line.

For a shell command, wait for its distinct output or a completion signal before using the result. Capture pane output matches a complete output line so the echoed command cannot satisfy the check. Capturing output explains screen and history capture.

Use a language library¶

Each port’s complete capture program sends a command, waits for its output and cleans up. Use the port dropdown for its input APIs, or open the program:

Python · TypeScript · Go · Rust · Java · Kotlin · Scala · C# · F# · C++ · Swift · Ruby · Lua

tmux reference¶

The send-keys reference describes literal input, key names, and each supported flag. Select your installed tmux version on that page.

The tmux manual documents these commands and their flags.