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reptyr: Move a Running Process Into tmux After Forgetting to Start It

reptyr uses ptrace to re-attach a running process to a new terminal. Started a long job over SSH without tmux? reptyr moves the process into tmux so you can disconnect safely.

You SSH’d in and started a job that will run for hours. No tmux. No nohup. Now you need to leave, and closing the window kills the process. reptyr moves it into tmux so you can disconnect safely.

What reptyr Does

reptyr uses the ptrace syscall to re-attach a running process to a new terminal. It genuinely changes the process’s controlling terminal — not just redirecting I/O.

That distinction matters. Old gdb-based scripts like “screenify” can do something similar, but with three problems:

  • The old terminal still delivers input to the process
  • Window resize events don’t reach ncurses applications
  • Ctrl-C from the new terminal doesn’t work

reptyr fixes all three because it actually replaces the controlling terminal.

Installation

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# Ubuntu / Debian
sudo apt install reptyr

# Arch
sudo pacman -S reptyr

# From source
git clone https://github.com/nelhage/reptyr
cd reptyr && make && sudo make install

Basic Usage

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reptyr <PID>

The process attaches to the current terminal and receives its input, output, and signals (Ctrl-C, Ctrl-Z all work).

The Common Scenario: Forgot to Start tmux

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# 1. Find the process PID
jobs -l
# or
ps aux | grep my-script

# 2. If it's in the foreground, suspend it
Ctrl-Z

# 3. Move it to the background
bg

# 4. Detach it from the current shell (so the shell dying won't kill it)
disown

# 5. Start tmux
tmux new -s rescue

# 6. Pull the process into tmux
reptyr <PID>

# 7. Safe to disconnect now
Ctrl-B D  # detach tmux

When you reconnect:

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ssh yourserver
tmux attach -t rescue

The process is still running, output intact.

The ptrace_scope Problem (Ubuntu)

Ubuntu 10.10+ disables non-root ptrace by default. reptyr will fail with:

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Unable to attach to pid 12345: Operation not permitted

Temporarily allow it:

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echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope

Permanently:

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# Edit /etc/sysctl.d/10-ptrace.conf
kernel.yama.ptrace_scope = 0

Or just run sudo reptyr <PID>.

ptrace_scope values: 0 = allow all, 1 = parent-only (default), 2 = root only, 3 = disabled. Setting it to 1 is enough for reptyr to work if you own the process.

Advanced: reptyr -l

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reptyr -l
# Outputs: /dev/pts/7

Creates a detached pseudo-terminal pair not connected to any shell. Useful for gdb:

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(gdb) set inferior-pty /dev/pts/7

Cleaner than handing gdb your terminal directly.

When It Won’t Work

  • Daemon processes: already detached from any terminal — nothing to re-attach to
  • setuid binaries: ptrace can’t attach (security restriction)
  • Dead processes: obviously not salvageable

Summary

Forgetting tmux isn’t fatal. reptyr <PID> moves a process from a dying SSH session into tmux in one command. disown it first to cut the shell’s hold, then open tmux and pull it in. On Ubuntu, adjust ptrace_scope as needed.

References