Send Foreground job to background redirecting output

Shell Programming and Scripting
1

I have many CPU intensive processes running and sometimes I run them in the foreground so that I can see what the output is.

I want to send that foreground process to the background, but also have it direct the output to a logfile.

I know to send something to the bg I do

Ctrl-z on the FG job and then

 bg %1

is there a way to send the output of that process to the background...

for instance (and this does not work)

 bg %1 > somelogfile.log &

thanks

2

The shell does redirection before it creates a process, not after, so you're out of luck there. You can't redirect a running process unless you're using something like screen which uses socket or tty tricks to do so.

3

Just a small thought...

Assume a process(test.sh) is running with a pid xxx

If we give the SIGSTP signal, say

kill -17 xxx

, it will signal the process

[1] + Stopped (SIGSTOP)        sh test.sh

now, why cant we give bg which will change it to running in bg?

4

@corona688 ok so it creates the output at the initial process call. what if i were to start it in the background with a logfile...is there a way to bring it into the FG?

@dennis.jacob - yea it can be sent into the background by just doing bg, but then I can't log the output in a logfile

5

I repeat: The shell cannot redirect output for a process that already exists. You can't just reach in and change a completely different process' file table for it.

The shell has to do all redirection for a new process before the new process is run -- it does this by creating a subshell, which does all redirection, then replaces itself with the new program but keeping I/O redirections intact. Once that's done, the new process controls its own output like any other program.

If you want to view the logfile as its written to, try 'tail -f logfile'. This will watch logfile for new lines and print them to standard output.

There's also the screen terminal/utility/shell/thing, which uses network sockets to do something similar to you want. It keeps sessions that you can connect and disconnect to without killing or halting them.

6

ok. just thought id see if the opposite was possible. the tail -f logfile approach seems to be the best option here. thanks big guy

7

This will send the standard output of bg to somelogfile.log, not %1.