How can I log the stdout of a process started by s

Question

I am using an init script to run a simple process, which is started with:

start-stop-daemon --start --quiet --chuid $DAEMONUSER    \
    --make-pidfile --pidfile $PIDFILE --background       \
    --exec $DAEMON $DAEMON_ARGS

The process called $DAEMON usually prints log information to its standard output. As far as I can tell this data is not being stored anywhere.

I would like to write or append the stdout of $DAEMON to a file somewhere.

The only solution I know is to tell start-stop-daemon to call a shellscript instead of $DAEMON directly; the script then calls $DAEMON and writes to the logfile. But that requires an extra script which, like modifying the daemon itself, seems the wrong way to solve such a common task.

Answer 1

Usually start-stop-daemon closes the standard file descriptors when running in the background. From the man page of start-stop-daemon:

-C, --no-close
Do not close any file descriptor when forcing the daemon into the background. Used for debugging purposes to see the process output, or to redirect file descriptors to log the process output. Only relevant when using --background.

This one worked for me:

    start-stop-daemon -b -C -o -c \ 
         $DAEMON_USER -S -x $DAEMON > $DAEMON_LOG 2>&1

Answer 2

You need to do:

start-stop-daemon --start --quiet --chuid $DAEMONUSER    \
    --make-pidfile --pidfile $PIDFILE --background       \
    --exec /bin/bash -- -c "$DAEMON $DAEMON_ARGS > /var/log/some.log 2>&1"

Also if you use --chuid or --user, make sure the user can write to /var/log or the existing /var/log/some.log. The best way is to have that user own a /var/log/subdir/ though.

Answer 3

I'm not sure if "$DAEMON $DAEMON_ARGS > /var/log/some.log 2>&1" will ever close the file descriptor for the log file... which means if your daemon runs forever, I'm not sure that logrotate or other mechanisms for cleaning up disk space would work. Since it's > instead of >>, the suggested command would also truncate existing logs on restart. If you want to see why the daemon crashed, and it restarts automatically, that might not be very helpful.

Another option might be "$DAEMON | logger". logger is a command that will log to syslog (/var/log/messages). If you need stderr too, I think you could use "$DAEMON 1>&2 | logger"

Answer 4

How about:

sudo -u myuser -i start-stop-daemon ...

Answer 5

It seems you should be able to use now the --no-close parameter when starting start-stop-daemon to capture the daemon output. This new feature is available in the dpkg package since version 1.16.5 on Debian:

Add new --no-close option to disable closing fds on --background.

This enabled the caller to see process messages for debugging purposes, or to be able to redirect file descriptors to log files, syslog or similar.

Answer 6

Quoting an old mailing list:

https://lists.ubuntu.com/archives/ubuntu-uk/2005-June/000037.html

An easy -- and if you want to use start-stop-daemon perhaps the only -- way around it is to create a small script containing:

#!/bin/sh
exec /home/boinc/boinc/boinc > /home/boinc/log/boinc.log

and then use that script as the argument to start-stop-daemon.

Perhaps the real question however is whether it is really necessary to use start-stop-daemon in the first place?