tmux — sessions that survive the disconnect

$ tmux new -d -s ops -x 140 -y 30 'watch -n 5 "df -h /"'
tmux split-window -h -t ops -d 'tail -f /var/log/syslog'
tmux split-window -v -t ops -d 'journalctl -f -n 5'
tmux split-window -v -t ops -d 'watch -n 5 "free -m"'
tmux select-layout -t ops tiled
tmux ls
tmux list-panes -a -F '#{pane_index}  #{pane_width}x#{pane_height}  at #{pane_left},#{pane_top}  #{pane_current_command}'

ops: 1 windows (created Tue Oct  6 09:07:47 2026)
0  69x14  at 0,0  watch
1  70x14  at 70,0  watch
2  69x15  at 0,15  journalctl
3  70x15  at 70,15  tail

The first five commands print nothing at all, which is normal and quite the point. -x/-y fix the geometry so a session created with no terminal still has a sane size (140x30 became four panes of 69x14 and 70x15). -h splits side by side, -v splits top and bottom, -d leaves focus on the pane you were already in, and select-layout tiled evens the mess out. Real output on tmux 3.4.

$ tmux capture-pane -pt ops:0.0
tmux list-panes -a

Every 5.0s: df -h /                     nuc: Tue Oct  6 09:07:50 2026

Filesystem                         Size  Used Avail Use% Mounted on
/dev/mapper/ubuntu--vg-ubuntu--lv  455G  409G   27G  94% /
ops:0.0: [69x14] [history 0/2000, 4999 bytes] %0 (active)
ops:0.1: [70x14] [history 0/2000, 5143 bytes] %3
ops:0.2: [69x15] [history 20/2000, 59203 bytes] %2
ops:0.3: [70x15] [history 28/2000, 68542 bytes] %1

-p prints the pane to stdout instead of dumping it into your own scrollback, -t names the target, and panes are addressed as session:window.pane. The default list-panes -a output shows the history buffer per pane: 2000 lines is the compiled-in history-limit, and both watch panes sit at 0/2000 because watch repaints in place instead of ever emitting a newline.

$ tmux new-window -t ops -n quick
tmux send-keys -t ops:quick 'uptime' Enter
sleep 1
tmux capture-pane -pt ops:quick

kmail@nuc:~/.hermes/cache/scratch$ uptime
 09:07:17 up 6 days, 20:24,  3 users,  load average: 0.75, 1.19, 1.30

Named keys (Enter, C-c, Escape, Up) are real key presses; add -l when your text merely looks like a key name. `tmux send-keys -t ops:0.2 C-c` is your Ctrl-C on a process you cannot see, and it is the fastest way to stop something that is about to fill a disk. The uptime and load figures are live values from this machine.

$ tmux new -d -s import './load_orders.sh'
tmux pipe-pane -t import 'cat >> /tmp/import.log'
sleep 12
ls -l /tmp/import.log
cat -v /tmp/import.log
tmux pipe-pane -t import        # no argument stops the pipe

-rw-rw-r-- 1 kmail kmail 80 Oct  6 09:03 /tmp/import.log
09:03:19  imported batch 002 (25 rows)^M
09:03:24  imported batch 003 (25 rows)^M

That is a real 12-second run against a script that prints one timestamped line every 5 seconds — pipe-pane only sees text a program writes as lines. Pointed at a full-screen redrawer instead (my first attempt used the df watch pane) it produced a 0-byte file over eleven seconds, because watch moves the cursor and repaints rather than printing. Strip the ^M with `tr -d '\r'` before feeding the log to anything: a pty ends its lines with CRLF.

$ tmux has-session -t ops 2>/dev/null && echo "ops already running - reusing it" || tmux new -d -s ops
tmux has-session -t nope 2>/dev/null; echo "has-session nope rc=$?"
tmux new -d -s job 'sleep 3; tmux wait-for -S sync-done'
time tmux wait-for sync-done

ops already running - reusing it
has-session nope rc=1

real	0m3.011s
user	0m0.001s
sys	0m0.002s

has-session exits 1 when the session is missing, which is exactly what a || guard wants — send its stderr ("can't find session: nope") to /dev/null. wait-for is the handshake: -S name sets it from inside the pane, plain `wait-for name` blocks in your calling script until it is set. Start the job, block until the pane signals, then send the report — no sleep-and-hope.