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Linux UID & GID

Links 104 Linux Index

UID & GID

  • UID: User Identifier
  • GID: Group Identifier
  • A UID (user identifier) is a number assigned by Linux to each user on the system.
    • This number is used to identify the user to the system and to determine which system resources the user can access.
    • It is user's representation in the linux kernel.
  • UIDs are stored in the /etc/passwd file:
Root has a UID and GID of 0.
  • These numbers are what give the root account its overwhelming power. If you don't believe me, rename the root account to goonygoogoo, or whatever you choose, and then create a new user account named root, allowing the system to assign the next available UID and GID to it. This account has no more power than any other user account on the system.
  • It's not the name, but the UID and the GID that give the administration account its power. To further test this assertion, assign a test user account with 0 for the UID and GID, and that user is now root, regardless of the account name.
  • Most Linux distributions reserve the first 100 UIDs for system use.
  • New users are assigned UIDs starting from 500 or 1000.
    • For example, new users in Ubuntu start from 1000:
    • The theory behind this arbitrary assignment is that anything below 1000 is reserved for system accounts, services, and other special accounts, and regular user UIDs and GIDs stay above 1000.
    • attachments/Pasted image 20220923232709.jpg
  • Groups in Linux are defined by GIDs (group IDs).
  • Just like with UIDs, the first 100 GIDs are usually reserved for system use.
  • GIDs are stored in the /etc/groups file
  • We can find the id of the user using id
  • Processes have owners just like files.
    • Only the owner (or the root user) of a process can send process signals to it.

References

Last updated: 2022-09-23