Difference between EUID and UID?

Question

EUID is not the same as UID. At what context are these both are used in the script?

I tried to get the values by echo "UID is $UID and EUID is $EUID", but only space came as output. My machine runs Ubuntu 12.04 LTS. Seen at some sites that this is usually used to check whether it is root user and all but not able to get proper difference.

A [link](https://muffinresearch.co.uk/linux-changing-uids-and-gids-for-user/) to know how to change uid and gid. `usermod -u groupmod -g find / -user -exec chown -h {} \; find / -group -exec chgrp -h {} \; usermod -g ` — Arun Chettoor, Dec 28 '14 at 15:33

score 47 · Answer 1 · answered Dec 27 '14 at 18:01

47

They're different when a program is running set-uid. Effective UID is the user you changed to, UID is the original user.

answered Dec 27 '14 at 18:01

Barmar

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So why my current shell not displaying any integer. Could you please explain a little bit more! – Arun Chettoor Dec 27 '14 at 18:06
I don't know. It works for me on OS X running bash 3.2.48 and Debian running bash 4.1.5. – Barmar Dec 27 '14 at 18:08
When I am running inside a script it is not coming, but when i write it directly on the shell it is working! – Arun Chettoor Dec 27 '14 at 18:10
Works for me both ways. – Barmar Dec 27 '14 at 18:16
OK. May be I should get a guy who runs Ubuntu 12.04 lts to check it! – Arun Chettoor Dec 27 '14 at 18:19

Fernando · Accepted Answer · 2022-08-17T15:59:36.103

18

UID is the ID of the user that executed the program.

EUID (Effective UID) is the user ID the process is executing. Usually both are equal, unless using a program with SetUID to for example increase your privileges. A common case where UID and EUID are different would be executing sudo.

EUID and UID variables only work on bash, not in dash (in Debian based distros as Ubuntu sh is usually a symlink to dash).

If you are running the script interactively you might not have bash configured as your default shell, run bash before trying.

If you are running it from console:

bash script.sh

If you are running it using its path (for example ./script.sh) ensure the first line of the script is:

#!/bin/bash

And not:

#!/bin/sh

For a more generic way to do it -that works on any shell- check: https://askubuntu.com/questions/15853/how-can-a-script-check-if-its-being-run-as-root

In that post the command id is mentioned, where:

id -u    # is the EUID
id -u -r # is the UID

edited Aug 17 '22 at 15:59

answered Dec 27 '14 at 18:49

Fernando

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which one only works on bash ?? EUID or UID ? you're not clear – Matt Jun 06 '18 at 10:12
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The subject is "Difference between EUID and UID?" but you just tell OP how to get uid or euid or when it works (in bash). Don't know why this is the preferred answer.. – ikwyl6 Feb 19 '20 at 02:14
@Matt I wanted to edit the answer of Fernando but the "Suggested edit queue is full". Both `EUID` and `UID` variables are only available in `bash` and `zsh` – SebMa Nov 06 '20 at 13:12

Difference between EUID and UID?

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