如何从写入过程外部读取打开的文件描述符

Question

如何打开文件描述符并将其回显到从进程写入的终端？

我有一个备份程序，即双重性，它将其日志写入由--log-fd=16参数指定的文件描述符。

当然，如果我运行lsof -p <duplicity PID>，我会看到：

python2 9224 myuser    0r      CHR                1,3      0t0         6 /dev/null
python2 9224 myuser    1w      CHR                1,3      0t0         6 /dev/null
python2 9224 myuser    2w      CHR                1,3      0t0         6 /dev/null
python2 9224 myuser    3u  a_inode               0,11        0      7005 [eventfd]
python2 9224 myuser    4u     unix 0x0000000000000000      0t0    158199 type=STREAM
python2 9224 myuser    5u  a_inode               0,11        0      7005 [eventfd]
python2 9224 myuser    6u  a_inode               0,11        0      7005 [eventfd]
python2 9224 myuser    7r      DIR                8,3     4096  22414346 <some random file being accessed during the backup>
python2 9224 myuser    8r      CHR                1,9      0t0        11 /dev/urandom
python2 9224 myuser   15r     FIFO               0,10      0t0    157054 pipe
python2 9224 myuser   16w     FIFO               0,10      0t0    157054 pipe

但是，如果我试图在Python中打开文件描述符，就会得到一个错误：

>>> import os
>>> os.fdopen(16)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 9] Bad file descriptor

为什么会这样呢？如何读取文件描述符？

Answer 1

我相信--log=fd选项是针对复杂的管道的，在这里您希望将stderr和stdout从日志中分离出来。

这个对这个问题的回答给出了一个例子。下面是一个简单的例子：

#!/bin/sh
# Generate output on three different fds
echo hello >&3
echo world >&2
echo today >&1

当像这样被处决时，

./foo 2> 2.log 3> 3.log 1> 1.log

结果在

$ cat 1.log 2.log 3.log
today
world
hello

Answer 2

使用strace (跟踪系统调用和信号)。

用法：

sudo strace -p <PID of writing process> -s 9999 -e write=<corresponding FD>

从手册页：

       -p pid      Attach to the process with the process ID pid and begin tracing.  The trace may be terminated
                   at any time by a keyboard interrupt signal (CTRL-C).  strace will respond by detaching itself
                   from  the  traced process(es) leaving it (them) to continue running.  Multiple -p options can
                   be used to attach to many processes in addition to command (which is optional if at least one
                   -p option is given).  -p "`pidof PROG`" syntax is supported.
    
       -s strsize  Specify the maximum string size to print (the default is 32).  Note that  filenames  are  not
                   considered strings and are always printed in full.
    
       -e read=set
              Perform a full hexadecimal and ASCII dump of all the data read from file descriptors listed in the
              specified set.  For example,  to  see  all  input  activity  on  file  descriptors  3  and  5  use
              -e read=3,5.   Note  that  this  is independent from the normal tracing of the read(2) system call
              which is controlled by the option -e trace=read.

       -e write=set
              Perform a full hexadecimal and ASCII dump of all the data written to file  descriptors  listed  in
              the  specified  set.   For  example,  to  see  all output activity on file descriptors 3 and 5 use
              -e write=3,5.  Note that this is independent from the normal tracing of the write(2)  system  call
              which is controlled by the option -e trace=write.

参考文献：https://man7.org/linux/man-pages/man1/strace.1.html