Defines a logical expression which determines which agents may execute builds of this project. This expression, when tested against the name and labels of each available agent, will be either true or false. If the expression evaluates to true, then that agent will be allowed to execute builds of this project.

If this project should always be built on a specific agent, or on the Jenkins master, then you can just enter the agent's name, or `master`, respectively.

However, you should generally avoid using the Name of an agent here, preferring to target the Labels of an agent. As documented on the configuration page for each agent, and the Configure System page for the master, labels can be used to represent which operating system the agent is running on, its CPU architecture, or any number of other characteristics.
Using labels removes the need to re-configure the label expression entered here each time that you add, remove, or rename agents.

A label expression can be as simple as entering a single label or agent name, for example `android-builder`, or `linux-machine-42`.
You can also make use of various operators to create more complex expressions.

### Supported operators

The following operators are supported, in descending order of precedence:

`(expression)`
  parentheses — used to explicitly define the associativity of an expression
`!expression`
  NOT — negation; the result of `expression` must not be true
`a && b`
  AND — both of the expressions `a` and `b` must be true
`a || b`
  OR — either of the expressions `a` or `b` may be true
`a -> b`
  "implies" operator — equivalent to `!a || b`.
  For example, `windows -> x64` could be thought of as "if a Windows agent is used, then that agent must be 64-bit", while still allowing this project to be executed on any agents that do not have the `windows` label, regardless of whether they have also have an `x64` label
`a <-> b`
  "if and only if" operator — equivalent to `a && b || !a && !b`
  For example, `windows <-> dc2` could be thought of as "if a Windows agent is used, then that agent must be in datacenter 2, but if a non-Windows agent is used, then it must not be in datacenter 2"

### Notes

- All operators are left-associative, i.e. `a -> b -> c` is equivalent to `(a -> b) -> c`.
- Labels or agent names can be surrounded with quotation marks if they contain characters that would conflict with the operator syntax.
  For example, `"osx (10.11)" || "Windows Server"`.
- Expressions can be written without whitespace, but including it is recommended for readability; Jenkins will ignore whitespace when evaluating expressions.
- Matching labels or agent names with wildcards or regular expressions is not supported.
- An empty expression will always evaluate to true, matching all agents.

### Examples

`master`
  Builds of this project may be executed only on the Jenkins master
`linux-machine-42`
  Builds of this project may be executed only on the agent with the name `linux-machine-42` (or on any machine that happens to have a label called `linux-machine-42`)
`windows && jdk9`
  Builds of this project may be executed only on any Windows agent that has version 9 of the Java Development Kit installed (assuming that agents with JDK 9 installed have been given a `jdk9` label)
`postgres && !vm && (linux || freebsd)`
  Builds of this project may be executed only any on Linux or FreeBSD agent, so long as they are not a virtual machine, and they have PostgreSQL installed (assuming that each agent has the appropriate labels — in particular, each agent running in a virtual machine must have the `vm` label in order for this example to work as expected)