Expression Language

Stylus uses a custom expression language for evaluating conditions in monitor configurations. This language supports arithmetic operations, logical operations, string manipulation, and comparisons.

Overview

The expression language is currently only used in the SNMP and ping monitors.

Data Types

The expression language supports two main data types:

• Integers: Whole numbers (e.g., 42, -17, 0)
• Strings: Text values enclosed in quotes (e.g., "hello", 'world')

Literals

Numbers

42        // Positive integer
-17       // Negative integer
0         // Zero

Strings

Strings can be enclosed in single or double quotes:

"hello world"    // Double quotes
'hello world'    // Single quotes

Escape Sequences

Strings support escape sequences:

"hello \"world\""    // Escaped double quote
'hello \'world\''    // Escaped single quote
"hello \\world\\"    // Escaped backslash

Boolean Values

true      // Boolean true (evaluates to 1)
false     // Boolean false (evaluates to 0)

Arithmetic Operations

Basic Arithmetic

a + b     // Addition (numbers) or concatenation (strings)
a - b     // Subtraction
a * b     // Multiplication
a / b     // Division
a ^ b     // Exponentiation (a raised to power b)
-a        // Negation

Examples

2 + 3           // 5
"hello" + " " + "world"    // "hello world"
10 - 3          // 7
4 * 5           // 20
15 / 3          // 5
2 ^ 3           // 8
-5              // -5

Comparison Operations

Comparison Operators

a == b    // Equal to
a != b    // Not equal to
a > b     // Greater than
a < b     // Less than
a >= b    // Greater than or equal to
a <= b    // Less than or equal to

Examples

5 == 5           // true
"up" == "down"   // false
10 > 5           // true
"abc" < "def"    // true
7 >= 7           // true
3 <= 10          // true

Logical Operations

Logical Operators

a and b   // Logical AND
a or b    // Logical OR
not a     // Logical NOT

Examples

true and true    // true
true and false   // false
true or false    // true
false or false   // false
not true         // false
not false        // true

String Functions

String Manipulation

startswith(str, prefix)    // Check if string starts with prefix
endswith(str, suffix)      // Check if string ends with suffix
contains(str, substr)      // Check if string contains substring
length(str)                // Get string length

Examples

startswith("hello world", "hello")    // true
endswith("hello world", "world")      // true
contains("hello world", "lo wo")      // true
length("hello")                       // 5

Type Conversion Functions

Type Conversion

str(value)    // Convert value to string
int(value)    // Convert value to integer

Examples

str(42)       // "42"
int("123")    // 123
str(true)     // "1"
int("abc")    // 0 (default for failed conversion)

Precedence and Associativity

The expression language follows Python-like precedence rules (from lowest to highest):

1. or
2. and
3. not
4. Comparisons (==, !=, >, <, >=, <=)
5. Addition/Subtraction (+, -)
6. Multiplication/Division (*, /)
7. Exponentiation (^)
8. Functions, parentheses, literals, variables

Examples

a == 1 and b == 2 or c == 0    // ((a == 1) and (b == 2)) or (c == 0)
not a == 1                      // not (a == 1)
2 + 3 * 4                       // 2 + (3 * 4) = 14
2 ^ 3 + 1                       // (2 ^ 3) + 1 = 9

Truthiness

Values are considered "truthy" or "falsy" in logical operations:

• Falsy values: 0, "" (empty string), false
• Truthy values: Any non-zero number, any non-empty string, true

Examples

0 and "hello"      // false (0 is falsy)
1 and "hello"      // true (both are truthy)
"" or 42           // true (42 is truthy)
not 0              // true
not "hello"        // false

Context Variables

Depending on the context, the expression language has access to different local variables.

SNMP Examples

The SNMP monitor makes the OIDs for each interface available as context variables.

// Check if interface is up and admin enabled
ifOperStatus == "up" and ifAdminStatus == "up"

// Check if interface is Ethernet and not loopback
ifType == "ethernetCsmacd" and not contains(ifDescr, "Loopback")

// Check if interface speed is less than 1Gbps
ifSpeed < 1000000000

// Check if interface description contains specific text
contains(ifDescr, "10G Ethernet Adapter")