UIDs (User Identifiers) are assigned by the operating system when a new user account is created. Here’s how the assignment typically works:
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Default Starting Point: Most systems start assigning UIDs from a default value, often 1000 for regular users on many Linux distributions. System users (like
root) usually have lower UIDs (e.g., 0 forroot). -
Incremental Assignment: When a new user is created, the system checks for the next available UID that is not already in use and assigns it to the new user. This ensures that each user has a unique identifier.
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Manual Assignment: Administrators can also manually specify a UID when creating a user account using commands like
useradd. This is useful for maintaining consistency, especially in environments where specific UIDs are required for applications or services. -
Reserved Ranges: Certain ranges of UIDs are reserved for system use. For example, UIDs below 1000 are typically reserved for system accounts and services, while UIDs above 1000 are generally used for regular user accounts.
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Configuration Files: The assignment and management of UIDs are often handled through configuration files like
/etc/passwd, where user account information, including UIDs, is stored.
By following these practices, the system ensures that each user has a unique identifier, which is crucial for managing permissions and access control.
