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Say I start a process in the terminal and it sends output to standard error while it runs. I want to move the process into the background and also silence it at the same time.

Is there a way to do this without stopping the process and starting it again using & and > /dev/null 2>&1 ? I'm wondering if there is some command that performs bg and can change the output descriptors of the target process too.

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up vote 6 down vote accepted

Too late. After a process is started, shell has no more control on process file descriptors so you can not silence it by a shell command.

You can only try to kill a SIGHUP to the process. If your process handles it correctly, It should detach from controlling tty. Unluckily, many software do not handle it correctly and simply die.

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can't you do CTRL+Z to put the process in background? it doesn't silence the process, and if you log out, i think it kills the process too. – Roy Rico Oct 14 '11 at 18:52
@RoyRico, killing a SIGSTOP to a process (or as you say doing CTRL+Z) does not silence a process. It stops a process! – andcoz Oct 14 '11 at 22:38

The process that is run from a terminal has its stdin, stdout and stderr bound to the terminal and you cannot do anything about it without re-gaining control over that terminal... Normally. But there are some tricky tools that actually let you do it. Have a look at this example. And other programs like retty mentioned there.

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I disagree with andcoz that it is too late to silence the output.

Assuming the process is running in the foreground, use Ctrl-Z to suspend the process. Doing so will report the job number and the process ID. You can go ahead and move the process to the background using bg %1 (for job #1). Although this moves the process to the background, it doesn't silence the output. To silence the output you can use gdb to redirect all output to /dev/null.

This has been very nicely described previously at http://stackoverflow.com/questions/593724/redirect-stderr-stdout-of-a-process-after-its-been-started-using-command-lin/593764#593764.

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Yet another command-line tool is detach. The latest version supports writing pidfiles, using files for stdin, stdout and stderr, and running in the foreground. (The website of detach also discusses similar tools.)


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