reptyr

by nelhage

nelhage / reptyr

Reparent a running program to a new terminal

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reptyr - A tool for "re-ptying" programs.

reptyr is a utility for taking an existing running program and attaching it to a new terminal. Started a long-running process over ssh, but have to leave and don't want to interrupt it? Just start a screen, use reptyr to grab it, and then kill the ssh session and head on home.

USAGE

reptyr PID

"reptyr PID" will grab the process with id PID and attach it to your current terminal.

After attaching, the process will take input from and write output to the new terminal, including ^C and ^Z. (Unfortunately, if you background it, you will still have to run "bg" or "fg" in the old terminal. This is likely impossible to fix in a reasonable way without patching your shell.)

Typical usage pattern

  • Start a long running process, e.g.
    top
  • Background the process with CTRL-Z
  • Resume the process in the background:
    bg
  • Display your running background jobs with
    jobs -l
    , this should look like this:
    • [1]+  4711 Stopped (signal)        top
    • (The
      -l
      in
      jobs -l
      makes sure you'll get the PID)
  • Disown the jobs from the current parent with
    disown top
    . After that,
    jobs
    will not show the job any more, but
    ps -a
    will.
  • Start your terminal multiplexer of choice, e.g.
    tmux
  • Reattach to the backgrounded process:
    reptyr 4711
  • Detach your terminal multiplexer (e.g. CTRL-A D) and close ssh
  • Reconnect ssh, attach to your multiplexer (e.g.
    tmux attach
    ), rejoice!

"But wait, isn't this just screenify?"

There's a shell script called "screenify" that's been going around the internet for nigh on 10 years now that uses gdb to (supposedly) accomplish the same thing. The difference is that reptyr works much, much, better.

If you attach a "less" using screenify, it will still take input from the old terminal. If you attach an ncurses program using screenify, and resize the window, your program won't notice. If you attach a process with screenify, ^C in the new terminal won't work.

reptyr fixes all of these problems, and is the only such tool I know of that does so. See below for some more details on how it accomplishes this.

PORTABILITY

reptyr supports Linux and FreeBSD. Not all functionality is currently available on FreeBSD. (Notably, FreeBSD doesn't support

reptyr -T
at this time.

reptyr
uses ptrace to attach to the target and control it at the syscall level, so it is highly dependent on details of the syscall API, available syscalls, and terminal ioctl()s. A port to other operating systems may be technically feasible, but requires significant low-level knowledge of the relevant platform, and may entail significant refactors.

reptyr works on i386, x86_64, and ARM. Ports to other architectures should be straightforward, and should in most cases be as simple as adding an arch/ARCH.h file and adding a clause to the ifdef ladder in ptrace.c.

ptrace_scope on Ubuntu Maverick and up

reptyr
depends on the
ptrace
system call to attach to the remote program. On Ubuntu Maverick and higher, this ability is disabled by default for security reasons. You can enable it temporarily by doing
# echo 0 > /proc/sys/kernel/yama/ptrace_scope

as root, or permanently by editing the file /etc/sysctl.d/10-ptrace.conf, which also contains more information about exactly what this setting accomplishes.

reptyr -l

As a bonus feature, if you run "reptyr -l", reptyr will create a new pseudo-terminal pair with nothing attached to the slave end, and print its name out.

If you are debugging a program in gdb, you can pass that name to "set inferior-pty". Because there is no existing program listening to that tty, this will work much better than passing an existing shell's terminal.

How does it work?

The main thing that reptyr does that no one else does is that it actually changes the controlling terminal of the process you are attaching. I wrote a blog post explaining just what the shenanigans involved are.

PRONUNCIATION

I pronounce it like "repeater", but since that's easily ambiguous, "re-P-T-Y-er" is also acceptable.

CREDITS

reptyr was written by Nelson Elhage [email protected]. Contact him with any questions or bug reports.

URL

http://github.com/nelhage/reptyr

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