atomiechen / reredirect

Tool to dynamicly redirect outputs of a running process

reredirect - A tool to dynamically redirect outputs of a running program

reredirect is a utility for taking an existing running program and attaching its outputs (standard output and error output) to files or another process.

Using reredirect, you can log output of a already launched process, redirect debug output of a background process to /dev/null or to a pager as if you launched it with > or |.

Installation

Installation uses common rules for C projects. Compile with:

make

Then, install with:

sudo make install

Usage

Simple usage is:

reredirect -m FILE PID

It will redirect outputs of PID to FILE. It is also possible to redirect standard output and error output in different files:

reredirect -o FILE1 -e FILE2 PID

-m option is just a shortcut to -o FILE -e FILE.

After launched, reredirect, give you command to restore state of PID. It will looks like:

reredirect -N -O 5 -E 3 5453

-O and -E act as -o and -e but with already opened file descriptors in PID. They only used to restore previous state of PID.

Without -N, reredirect keep previous outputs opened which allow you to restore them. This will produce file descriptor leak if you are you call reredirect multiple times. You can use -N to close and forget previous outputs.

Redirect to your terminal or a command

This package also provide an utility called relink that allow to redirect outputs to current terminal. When relink exit (with Ctrl+C for exemple) original state is restored and command is detached.

For exemple:

relink 5453
relink 5453 | grep usefull_line

relink maintains stderr from original command to stderr. So you can do things like:

relink 5453 > /dev/null

Internally, relink is just a small shell script that create necessary context and call reredirect as necessary. It use "named pipes". Using "named pipes", you can redirect output of your target to another command (as a normal pipe):

First create a named pipe:

mkfifo /tmp/myfifo

Run reredirect to redirect your target to /tmp/myfifo:

reredirect -m /tmp/myfifo PID

Launch a command on this named pipe:

less < /tmp/myfifo
tee my_log < /tmp/myfifo
cat -n < /tmp/myfifo

Note that relink only redirect outputs. The target process keep its original terminal. So if you type Ctrl+Z or CTRL+C, they are not sent to target process. If you want to do that, you check reptyr command from Nelson Elhage.

Trick with Makefile

Sometime, I work with complex projects and I want to log subparts of compilation output in different files. I use this trick:

target:
	@FIFO=$$(mktemp -u); mkfifo $$FIFO; tee my_file.log < $$FIFO & ./redirect -m $$FIFO $$PPID > ./restore_$$PPID.cmd
	@echo Call sub makefile here
	@sh ./restore_$$PPID.cmd
	@echo No more in log file

Portability

reredirect is Linux-only. It uses ptrace to attach to the target and control it at the syscall level, so it is highly dependent on Linux's particular syscall API, syscalls, and terminal ioctl()s. A port to Solaris or BSD may be technically feasible, but would probably require significant re-architecting to abstract out the platform-specific bits.

reredirect 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

redirect 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.

How does it work?

Reredirect act as a debugger to take control of running process (it use ptrace syscall). Once it took control of runnign process, it use classical calls to dup, and dup2 to change targets of file descriptors 1 and 2.

Basicly, to redirect to file, this pseudo code is executed:

orig_fd = 1;
save_fd = dup(1);
new_fd = open(file, O_WRONLY | O_CREAT, 0666);
dup2(new_fd, orig_fd);
close(new_fd);

and to restore state:

ret = dup2(save_fd, orig_fd);
close(save_fd);

Credits

reredirect was mainly written by Jérôme Pouiller [e-mail]. You can contact hom for any questions or bug reports.

reredirect (and especially all ptrace layer) is based on reptyr programm. reptyr was written by Nelson Elhage [e-mail].

URL

http://github.com/jerome-pouiller/reredirect