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String Constants
================
String constants are necessary in any scripting language. They provide
values that are evaluated at startup and never change during rsyslog's
run.
Uses
----
String constants are necessary in many places: comparisons,
configuration parameter values and function arguments, to name a few
important ones.
In string constants, special characters are escaped by prepending a
backslash in front of them -- just in the same way this is done in the C
programming language or PHP.
If in doubt how to properly escape, use the `RainerScript String Escape
Online
Tool <http://www.rsyslog.com/rainerscript-constant-string-escaper/>`_.
Types
-----
Rsyslog provides different types of string constants, closely inspired
by the shell:
- single quotes
Values are used unaltered, except for escape sequences, which are
escaped.
- double quotes
Right now, equivalent to single quotes, but $ signs need to be escaped.
If not escaped, a syntax error will be generated and rsyslog startup
be aborted in most cases.
The idea is to support environment variables just like the shell does
in later releases.
- backticks
This was added in 8.33.0. The idea is to provide a useful subset of
what the shell does. Right now, only the following is supported:
- `echo $VARNAME` - It will evaluate the environment variable and use
it as string constant. If the variable is not found, an empty string
is generated (this is **not** an error).
Starting with 8.37.0, the `echo` case has been enhanced. It is now
more along the lines of what bash does. It supports multiple
environment variable expansions as well as constant text
between them.
An example:
* env SOMEPATH is set to "/var/log/custompath"
* config is: param=echo $SOMEPATH/myfile
* param than is expanded to "/var/log/custompath/myfile"
Note, however, that some common bash features are not supported.
Most importantly, `${VAR}` does not work. Also, environment variables
are only terminated by whitespace or `/`. Neither are things like
`$(pwd)` supported. The idea of this parameter is not to provide a
full-blown bash-equivalent, but provide some functionality that is
usually considered useful for customizing rsyslog configuration with
outside data. That said, we are still interested in extending the
coverage if clear need and reasoning is provided.
- `cat filename` - It will evaluate to the content of the given file.
Only a single file name is supported. If the file is not readable,
it will evaluate to an empty string.
Any other construct will currently lead to an error message.
Note that there must be exactly one space between "echo" or "cat" and
the other parameter.
Backticks are especially useful for configuration files that are
auto-generated but need to contain a small set of special functionality.
For an example of this in action, have a look at the rsyslog docker
appliance available at
https://github.com/rsyslog/rsyslog-docker/tree/master/appliance/alpine.
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