Starting with rsyslog 7.1.3, variables can be manipulated inside rsyslog.conf. Note that the native message properties (like $msg) cannot be modified, but the CEE/lumberjack-type properties (those $!name variables) can. As variable modification is primarily an aid for template generation and modification, we do not consider this a restriction, as native message properties can simply be copied to the CEE tree if this is needed.
Note that CEE/lumberjack properties, as implemented in rsyslog, can be hierarchical and levels are delimited by the bang sign (based on lumberjack recommendations). So "!uid" is the uid field in the CEE root, whereas "!usr!uid" is the uid field inside the usr container. Nesting can be as deep as desired. Currently, all parts of the CEE tree can be accessed. In later versions, this may require the setting of a global option. If that will happen depends on the feedback we receive.
So: how does it work?
To set a variable, simply use
set varname = expression;
Where expression can be an arbitrary complex expression, just like in an "if" statement. Note the semicolon at the end: this is unfortunately necessary and a diversion from the other config statements. However, this is the price we need to pay to remain upward compatible with the old style config format.
set $!usr!level2!var1 = "test";
set $!usr!level2!var1 = $msg; # update variable with native MSG field
set $!usr!level2!var2 = 1+1; # set !usr!level2!var2 = 2
set $!usr!level2 = $fromhost; # invalid
The last example is invalid and will not be executed because it tries to replace a complete container (!usr!level2) with a single value. This is rejected and not executed. Note that this problem is not detected during config read time but rather during execution time (in less trivial samples, it can not be reliable detected without execution).
Note that string concatenation is currently NOT supported, but it will be soon in the next releases. Also, full JSON containers cannot yet be copied. If this is tried to, the resulting variable will receive a string representation, which most probably is not what you wanted (and you will get a different result in future versions).
There is also an accompanying "unset" statement to remove a variable that is no longer needed. This is primarily meant to restructure a CEE container. It's syntax simply is:
Again, note the semicolon at the end. A concrete example is
which removes a single element. But full containers can also be removed:
Note that all variables are assigned to the message currently being processed. There currently is no way to set global variables (but this may become available based on feedback I see).