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 uses common rules for C projects. Compile with:
make
Then, install with:
sudo make install
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.
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
command from Nelson Elhage.
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
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.
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.
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);
reredirect was mainly written by Jérôme Pouiller [email protected]. 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 [email protected].