---
title: What are User Ids in Linux and Unix?
description: In Linux and Unix, the kernel identifies users by their user id. User ids are integers. User ids are mapped to user names in user mode programs.
---

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# [What are User Ids in Linux and Unix?](https://www.ssh.com/academy/iam/user-id)

 Written by [Admin](https://www.ssh.com/academy/author/admin) | Nov 5, 2019 5:00:00 AM

In Linux and Unix, the kernel identifies users by their user id. User ids are integers. User ids are mapped to user names in user mode programs.

[Root users](https://www.ssh.com/academy/pam/root-user-account) have user id 0.

Many other operating systems also use user ids. In those other systems, user ids may also be strings.

[View full post](https://www.ssh.com/academy/iam/user-id)

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