How to redirect output of an already running process

Question

Normally I would start a command like   longcommand  amp     I know you can redirect it by doing something like  longcommand  gt   dev null    for instance to get rid of the output or  longcommand 2 gt  amp 1  gt  output log   to capture output   But I sometimes forget  and was wondering if there is a way to capture or redirect after the fact   longcommand ctrl-z bg 2 gt  amp 1  gt   dev null   or something like that so I can continue using the terminal without messages popping up on the terminal

User · Answer

Screen If process is running in a screen session you can use screen s log command to log the output of that window to a file  Switch to the script s window  C-a H to log  Now you can     tail -f screenlog 2   grep whatever  From screen s man page   log  on off  Start stop writing output of the current window to a file  quot screenlog n quot  in the window s default directory  where n is the number of the current window  This filename can be changed with the  logfile  command  If no parameter is given  the state of logging is toggled  The session log is appended to the previous contents of the file if it already exists  The current contents and the contents of the scrollback history are not included in the session log  Default is  off    I m sure tmux has something similar as well

User · Answer

I collected some information on the internet and prepared the script that requires no external tool  See my response here  Hope it s helpful

User · Answer

You can also do it using reredirect  https   github com jerome-pouiller reredirect    The command bellow redirects the outputs  standard and error  of the process PID to FILE  reredirect -m FILE PID  The README of reredirect also explains other interesting features  how to restore the original state of the process  how to redirect to another command or to redirect only stdout or stderr  The tool also provides relink  a script allowing to redirect the outputs to the current terminal  relink PID relink PID   grep usefull content   reredirect seems to have same features than Dupx described in another answer but  it does not depend on Gdb

User · Answer

Dupx Dupx is a simple  nix utility to redirect standard output input error of an already running process  Motivation I ve often found myself in a situation where a process I started on a remote system via SSH takes much longer than I had anticipated  I need to break the SSH connection  but if I do so  the process will die if it tries to write something on stdout error of a broken pipe  I wish I could suspend the process with  Z and then do a bg  1  gt  tmp stdout 2 gt  tmp stderr   Unfortunately this will not work  in shells I know    http   www isi edu  yuri dupx

User · Answer

See Redirecting Output from a Running Process      Firstly I run the command cat  gt  foo1 in one session and test that data from stdin is copied to the file  Then in another session I redirect the output       Firstly find the PID of the process     ps aux   grep cat rjc 6760 0 0 0 0 1580 376 pts 5 S  15 31 0 00 cat       Now check the file handles it has open     ls -l  proc 6760 fd total 3 lrwx       1 rjc rjc 64 Feb 27 15 32 0 - gt   dev pts 5 l-wx       1 rjc rjc 64 Feb 27 15 32 1 - gt   tmp foo1 lrwx       1 rjc rjc 64 Feb 27 15 32 2 - gt   dev pts 5       Now run GDB     gdb -p 6760  bin cat GNU gdb 6 4 90-debian   license stuff snipped   Attaching to program   bin cat  process 6760   snip other stuff that s not interesting now    gdb  p close 1   1   0  gdb  p creat   tmp foo3   0600   2   1  gdb  q The program is running  Quit anyway  and detach it    y or n  y Detaching from program   bin cat  process 6760       The p command in GDB will print the value of an expression  an expression can be a function to call  it can be a system call    So I execute a close   system call and pass file handle 1  then I execute a creat   system call to open a new file  The result of the creat   was 1 which means that it replaced the previous file handle  If I wanted to use the same file for stdout and stderr or if I wanted to replace a file handle with some other number then I would need to call the dup2   system call to achieve that result       For this example I chose to use creat   instead of open   because there are fewer parameter  The C macros for the flags are not usable from GDB  it doesn   t use C headers  so I would have to read header files to discover this     it   s not that hard to do so but would take more time  Note that 0600 is the octal permission for the owner having read write access and the group and others having no access  It would also work to use 0 for that parameter and run chmod on the file later on       After that I verify the result   ls -l  proc 6760 fd  total 3 lrwx       1 rjc rjc 64 2008-02-27 15 32 0 - gt   dev pts 5 l-wx       1 rjc rjc 64 2008-02-27 15 32 1 - gt   tmp foo3  lt      lrwx       1 rjc rjc 64 2008-02-27 15 32 2 - gt   dev pts 5       Typing more data in to cat results in the file  tmp foo3 being appended to       If you want to close the original session you need to close all file handles for it  open a new device that can be the controlling tty  and then call setsid

[linux] How to redirect output of an already running process

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