# Logstash: how to concatenate static field with dynamic field?

**URL:** <https://discuss.elastic.co/t/logstash-how-to-concatenate-static-field-with-dynamic-field/283183>\
**Category:** Logstash\
**Created:** [September 2, 2021, 2:43pm UTC](https://discuss.elastic.co/t/logstash-how-to-concatenate-static-field-with-dynamic-field/283183 "2021-09-02T14:43:24Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![kelk](https://avatars.discourse-cdn.com/v4/letter/k/13edae/32.png) [@kelk](https://discuss.elastic.co/u/kelk)\
**Post date:** [September 2, 2021, 2:43pm UTC](https://discuss.elastic.co/t/logstash-how-to-concatenate-static-field-with-dynamic-field/283183/1 "2021-09-02T14:43:24Z")

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I'm trying to prepend a static variable to identify certain hosts

```auto
    mutate {
        add_field => { "[host][hostname]" => "UK__[app][host]" }
    }

```

I need to preprend "UK\_\_" to all fields of `app.host` field. But the above gives me outcome as  
`host.hostname: UK_[app][host] ` literally and is not replacing it with the value. how to concatenate static field with dynamic field to create another field?

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<div class="post-metadata">

**Author:** ![kelk](https://avatars.discourse-cdn.com/v4/letter/k/13edae/32.png) [@kelk](https://discuss.elastic.co/u/kelk)\
**Post date:** [September 2, 2021, 2:47pm UTC](https://discuss.elastic.co/t/logstash-how-to-concatenate-static-field-with-dynamic-field/283183/2 "2021-09-02T14:47:37Z")

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Ignore me.. found out a way ! putting curly brackets around it

```auto
mutate {
        add_field => { "[host][hostname]" => "UK__%{[app][host]}" }
    }

```

---

<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [September 30, 2021, 2:48pm UTC](https://discuss.elastic.co/t/logstash-how-to-concatenate-static-field-with-dynamic-field/283183/3 "2021-09-30T14:48:08Z")

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