Capture pane output

capture-pane -p prints a pane’s visible screen. Sending a command and reading its result are separate operations: the pane’s shell may still be processing input when the first capture runs.

Read what’s on screen¶

This complete shell program starts a private tmux server, sends a command, and captures until the expected line appears. It removes the server on exit and fails after 100 unsuccessful checks with 50-millisecond pauses.

Save it as capture.sh and run sh capture.sh, or paste the whole block into a POSIX shell. It requires tmux 3.2 or newer and sleep with fractional seconds.

(
    set -eu
    directory=$(mktemp -d "${TMPDIR:-/tmp}/libtmux-capture.XXXXXX")
    socket="$directory/tmux.sock"

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

    tmux -S "$socket" -f /dev/null new-session -d -s capture \
        -e ENV=/dev/null 'sh'
    tmux -S "$socket" send-keys -t capture:0.0 -l \
        "printf '\\nlibtmux capture ready\\n'"
    tmux -S "$socket" send-keys -t capture:0.0 Enter

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

Cleanup also runs if startup fails after creating the server. If tmux cannot be stopped, the script reports the error and keeps its socket directory so you can inspect or stop that server.

Wait for output or completion¶

The leading newline puts the output on a fresh screen row even if the shell’s first prompt arrives late. grep -Fx matches the complete line, so the echoed command cannot satisfy the check. The short pause limits polling; the captured output determines when the program finishes.

Capture reads screen state, so it can miss output that has scrolled away. Capturing output covers history and streaming; Sending keys explains input and completion.

Use a language library¶

Complete programs with imports, setup, and cleanup:

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

The tmux manual source describes capture-pane, send-keys, and kill-server.