---
title: MD variables and expressions
summary: Strings, lists, tables, existence checks and explicit variable namespaces.
section: scripting
kind: guide
status: example
updated: 2026-10-27
tags: ["X4","modding","md","variables"]
sources: ["https://wiki.egosoft.com/X%20Rebirth%20Wiki/Modding%20support/Mission%20Director%20Guide/"]
---
## Values and expressions


Many MD attributes contain expressions. XML quotes delimit the attribute, while inner single quotes make an MD string literal. Lists and tables have different access patterns: `$Items.{1}` reads a list position; `$ByID.{$Key}` reads a named table key.




```xml
<set_value name="$Label" exact="'Hello, X4'" />
<set_value name="$Count" exact="3" />
<set_value name="$Ready" exact="false" />
<set_value name="$Items" exact="[]" />
<set_value name="$ByID" exact="table[]" />
```

## Validate the request shape


The example checks the incoming UI payload inside its handler. A correct list length does not prove element types. Validate the text and switch value before applying a setting.




```xml
<do_if value="typeof $Payload == datatype.list and $Payload.count == 2">
  <set_value name="$Label" exact="$Payload.{1}" />
  <set_value name="$Enabled" exact="$Payload.{2}" />
</do_if>
```

## Existence and scope


`not $Records?` checks variable existence. Objects additionally need `.exists`. A cue reference such as `Init.$Records` explicitly selects context. False, an empty list and a missing variable are distinct states; do not substitute a content check for existence.



## Explicitly choose variable ownership


Do not assume `$Value` and `this.$Value` always refer to the same storage: `$Value` resolves through the namespace. Choose an appropriate `namespace="this"` for instance-local handler data; store shared state in an explicitly chosen context.



Compare two consecutive requests: does the intended field change, and does the first request leak into the second? See [MD libraries](wiki:md-libraries) for a parameterized reusable cue.