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

Tool to dynamicly redirect outputs of a running process
https://github.com/jerome-pouiller/reredirect

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Tool to dynamicly redirect outputs of a running process

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README 

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

==========================================================================



`reredirect` is a utility for taking an existing running program and

attaching its output (standard output and error output) to files or

to another process.



Using `reredirect`, you can log output of an 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 output 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 being launched, reredirect give you the ability to restore state of PID.

It will look something like this : 



    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 output opened which allow you to

restore them. This will produce file descriptor leak if you call

`reredirect` multiple times. You should use `-N` to close and forget previous

output.



Redirect to your terminal or a command

--------------------------------------



This package also provide an utility called `relink` that allows to redirect

output to current terminal. When `relink` exits (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 creates necessary context

and call `reredirect` as necessary. It uses "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 redirects output. 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 should check the [`reptyr`](http://github.com/nelhage/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 acts as a debugger to take control of a running process (it uses ptrace 

syscall). Once it takes control of a running process, it uses 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 . You can

contact him for any questions or bug reports.



reredirect (and especially all ptrace layer) is based on reptyr programm. reptyr 

was written by Nelson Elhage .



URL

---

[http://github.com/jerome-pouiller/reredirect]()