Tmux Skill

Use tmux to interact with programs that need a real terminal — interactive REPLs, TUIs, editors, long-running processes you want to monitor, anything that expects a TTY or needs keystroke-level control.

Core Workflow

Every tmux interaction follows this pattern: create an isolated session, interact with it, and clean up. Here's the full flow:

# 1. Pick a unique socket value (avoid collisions with user's tmux or other agents)
# Once you've picked a `$SOCKET` value, reuse it for every tmux command in the task. Don't create a new socket per command.
SOCKET="tmux-agent-$(date +%s%N)"

# 2. Create an isolated session
#    -L $SOCKET  → use our own tmux server (never touch the user's)
#    -f <(:)     → skip loading user's tmux.conf (their config might remap keys,
#                  change the prefix, or set options that break our automation)
tmux -L $SOCKET -f <(:) new-session -d -s main -x $COLUMNS -y $LINES "bash"

# 3. Interact: send input, wait for output, capture it
tmux -L $SOCKET send-keys -t main "echo hello" C-m
sleep 0.5
tmux -L $SOCKET capture-pane -t main -p

# 4. Clean up when done (orphaned servers waste resources and can confuse future runs)
tmux -L $SOCKET kill-server

Once you've picked a $SOCKET value, reuse it for every tmux command in the task. Don't create a new socket per command.

Sending Input

tmux -L $SOCKET send-keys -t main "your command here" C-m

Special Keys

Key	What it sends
C-m	Enter
C-c	Ctrl+C (interrupt)
C-d	Ctrl+D (EOF)
Tab	Tab (autocomplete)
Escape	Escape
Space	Space (useful when building key sequences)
Up / Down	Arrow keys
BSpace	Backspace

Sending Literal Text

By default, send-keys interprets special sequences like C-m as keystrokes. If you need to send text that contains these patterns without interpretation, use the -l (literal) flag:

# This sends the literal string "C-m" rather than pressing Enter
tmux -L $SOCKET send-keys -t main -l "The sequence C-m means Enter"
# Then press Enter separately
tmux -L $SOCKET send-keys -t main C-m

This matters when sending code or text that contains characters tmux would otherwise interpret — semicolons, quotes, backslashes, or anything resembling a key binding.

Capturing Output

# Capture what's currently visible in the pane
tmux -L $SOCKET capture-pane -t main -p

# Capture with scrollback history (last 200 lines)
tmux -L $SOCKET capture-pane -t main -p -S -200

capture-pane -p only returns what fits in the visible pane (determined by the -x and -y you set at session creation). If a command produced more output than the pane height, the top lines scroll off. Use -S -<N> to reach into the scrollback buffer — set N high enough to capture everything you need.

Target Syntax

Use targets to address specific sessions, windows, or panes when you have more than one:

Target	Meaning
-t main	Session named "main" (default pane)
-t main:0	Window 0 in session "main"
-t main:1	Window 1
-t main:0.0	Pane 0 in window 0
-t main:0.1	Pane 1 in window 0

Waiting for Output

Interactive programs need time to process input and render output. There are two approaches:

Fixed Delays

Simple and usually sufficient. Use sleep between sending input and capturing:

tmux -L $SOCKET send-keys -t main "python3 -c 'print(42)'" C-m
sleep 0.5
tmux -L $SOCKET capture-pane -t main -p

Adjust the delay based on what you're waiting for:

• sleep 0.3 — fast commands (echo, simple REPL expressions)
• sleep 0.5-1 — typical interactive programs starting up
• sleep 2-5 — programs with slow startup (language servers, heavy apps)

Polling (More Robust)

For cases where timing is unpredictable, poll until the expected output appears. This avoids both waiting too long and capturing too early:

# Wait for a specific prompt or output pattern
for i in $(seq 1 20); do
  OUTPUT=$(tmux -L $SOCKET capture-pane -t main -p)
  if echo "$OUTPUT" | grep -q ">>>"; then
    break
  fi
  sleep 0.5
done

Polling is better than fixed delays when: a program has variable startup time, you're waiting for a specific prompt, or you need to be sure a command finished before sending the next one.

Error Handling

Session creation fails: This usually means the socket path is invalid or tmux isn't installed. Check which tmux first if there's any doubt.

Capture returns empty/stale output: The program hasn't rendered yet. Increase the delay or switch to polling.

Socket already exists: If you're reusing a socket name from a crashed previous run, kill-server it first:

tmux -L $SOCKET kill-server 2>/dev/null
tmux -L $SOCKET -f <(:) new-session -d -s main -x $COLUMNS -y $LINES "bash"

Program exited unexpectedly: The pane may have closed. Check if the session still exists:

tmux -L $SOCKET has-session -t main 2>/dev/null && echo "alive" || echo "dead"

Examples

Python REPL

SOCKET="tmux-agent-$(date +%s%N)"
tmux -L $SOCKET -f <(:) new-session -d -s main -x $COLUMNS -y $LINES "python3 -i"

# Wait for the >>> prompt
for i in $(seq 1 10); do
  OUTPUT=$(tmux -L $SOCKET capture-pane -t main -p)
  echo "$OUTPUT" | grep -q ">>>" && break
  sleep 0.5
done

tmux -L $SOCKET send-keys -t main "2 + 2" C-m
sleep 0.3
tmux -L $SOCKET capture-pane -t main -p

tmux -L $SOCKET send-keys -t main "exit()" C-m
tmux -L $SOCKET kill-server

Monitoring a Long-Running Process

SOCKET="tmux-agent-$(date +%s%N)"
tmux -L $SOCKET -f <(:) new-session -d -s main -x $COLUMNS -y $LINES "tail -f /var/log/syslog"
sleep 1

# Check output periodically — you can do other work between captures
tmux -L $SOCKET capture-pane -t main -p

# When done monitoring
tmux -L $SOCKET send-keys -t main C-c
sleep 0.3
tmux -L $SOCKET kill-server

Vim Automation

SOCKET="tmux-agent-$(date +%s%N)"
tmux -L $SOCKET -f <(:) new-session -d -s main -x $COLUMNS -y $LINES "vim /tmp/test.txt"
sleep 1  # vim takes a moment to start

# Enter insert mode, type text, save and quit
tmux -L $SOCKET send-keys -t main "i"
tmux -L $SOCKET send-keys -t main -l "Hello from tmux automation"
tmux -L $SOCKET send-keys -t main Escape
sleep 0.2
tmux -L $SOCKET send-keys -t main ":wq" C-m
sleep 0.3

tmux -L $SOCKET capture-pane -t main -p
tmux -L $SOCKET kill-server

Note: the Vim example uses send-keys -l for the text content so that any special characters in the text are sent literally rather than interpreted as tmux key bindings.