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<h2>Output Channels</h2>
Output Channels are a new concept first introduced in rsyslog
0.9.0. As of this writing, it is most likely that they will
be replaced by something different in the future. So if you
use them, be prepared to change you configuration file syntax when you
upgrade to a later release. 
 
The idea behind output channel definitions is that it shall provide an
umbrella for any type of output that the user might want. In essence, 
this is the "file" part of selector lines (and this is why we are not
sure output channel syntax will stay after the next review). There is a 
difference, though: selector channels both have filter conditions
(currently facility and severity) as well as the output destination.
they can only be used to write to files - not pipes, ttys or whatever
Output channels define the output definition, only. As of this build,
else. If we stick with output channels, this will change over time.
In concept, an output channel includes everything needed to
know about an output actions. In practice, the current implementation
only carries 
a filename, a maximum file size and a command to be issued when this
file size is reached. More things might be present in future version,
which might also change the syntax of the directive.
Output channels are defined via an $outchannel directive. It's
syntax is as follows: 
 
$outchannel name,file-name,max-size,action-on-max-size 
 
name is the name of the output channel (not the file), file-name is the
file name to be written to, max-size the maximum allowed size and
action-on-max-size a command to be issued when the max size is reached.
This command always has exactly one parameter. The binary is that part
of action-on-max-size before the first space, its parameter is
everything behind that space. 
 
Please note that max-size is queried BEFORE writing the log message to
the file. So be sure to set this limit reasonably low so that any
message might fit. For the current release, setting it 1k lower than
you expected is helpful. The max-size must always be specified in bytes
- there are no special symbols (like 1k, 1m,...) at this point of
development. 
 
Keep in mind that $outchannel just defines a channel with "name". It
does not activate it. To do so, you must use a selector line (see
below). That selector line includes the channel name plus an $ sign in
front of it. A sample might be: 
 
*.* :omfile:$mychannel 
 
In its current form, output channels primarily provide the ability to
size-limit an output file. To do so, specify a maximum size. When this
size is reached, rsyslogd will execute the action-on-max-size command
and then reopen the file and retry. The command should be something
like a <a href="log_rotation_fix_size.html">log rotation
script</a> or a similar thing.
If there is no action-on-max-size command or the command did
not resolve the situation, the file is closed and never reopened by
rsyslogd (except, of course, by huping it). This logic was integrated
when we first experienced severe issues with files larger 2gb, which
could lead to rsyslogd dumping core. In such cases, it is more
appropriate to stop writing to a single file. Meanwhile, rsyslogd has
been fixed to support files larger 2gb, but obviously only on file
systems and operating system versions that do so. So it can still make
sense to enforce a 2gb file size limit.

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This documentation is part of the
<a href="http://www.rsyslog.com/">rsyslog</a> project. 
Copyright &copy; 2008 by <a href="http://www.gerhards.net/rainer">Rainer Gerhards</a> and
<a href="http://www.adiscon.com/">Adiscon</a>. Released under the GNU GPL
version 2 or higher.
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