Expressions — rsyslog 8.2102.0 documentation

# Expressions

The language supports arbitrary complex expressions. All usual operators are supported. The precedence of operations is as follows (with operations being higher in the list being carried out before those lower in the list, e.g. multiplications are done before additions.

- expressions in parenthesis
- not, unary minus
- *, /, % (modulus, as in C)
- +, -, & (string concatenation)
- ==, !=, <>, <, >, <=, >=, contains (strings!), startswith (strings!)
- and
- or

For example, “not a == b” probably returns not what you intended. The script processor will first evaluate “not a” and then compare the resulting boolean to the value of b. What you probably intended to do is “not (a == b)”. And if you just want to test for inequality, we highly suggest to use “!=” or “<>”. Both are exactly the same and are provided so that you can pick whichever you like best. So inequality of a and b should be tested as “a <> b”. The “not” operator should be reserved to cases where it actually is needed to form a complex boolean expression. In those cases, parentheses are highly recommended.

See also

Help with configuring/using `Rsyslog`:

- Mailing list - best route for general questions
- GitHub: rsyslog source project - detailed questions, reporting issues that are believed to be bugs with `Rsyslog`
- Stack Exchange (View, Ask) - experimental support from rsyslog community

See also

Contributing to `Rsyslog`:

- Source project: rsyslog project README.
- Documentation: rsyslog-doc project README

Copyright 2008-2020 Rainer Gerhards (Großrinderfeld), and Others.