mjhjmtu

Recently, I've noticed a strange thing, and I'm asking you to explain such behavior.

# Ok, I logged in as root

# Am I root? Sure.
╭─root@s1 ~
╰─# whoami && id
root
uid=0(root) gid=0(root) groups=0(root)


# Now i want to log in as user1
╭─root@s1 ~
╰─# su - user1


# What? nginx instead of user1?
╭─nginx@s1 ~
╰─$ whoami && id
nginx
uid=498(nginx) gid=500(user1) groups=500(user1)


# So, what is my home dir?
╭─nginx@s1 ~
╰─$ echo $HOME
/home/user1

So the question is: why whoami and id commands shows me different user?

I'm not an administrator of this server and I don't know what he did with the server but this question interesting to me.

Is it safe ?

This won't break the OS, you can spy on nginx (and nginx on you). It is more a hindrance to human (hence your question) than dangerous to OS.

For unix/linux kernel, process and files are onwed by user-id (I'll drop group for simplicity).

When using ps or ls -l thoses commands (ps and ls) will read /etc/passwd to match id against username. The rule, if an id is mentioned more than once is to stop at first occurence.

How can two user have same id ?

default option for useradd will prevent those conflicting issues, however

1. you can force useradd to use an existing id (-o, --non-unique options)


2. you can edit /etc/passwd manualy, or by have a script include a bunch of user

In your case, it is likely admin didn't do this purposfully, they simply have shell, lots of host to manage, and didn't bother to check for conflicting issue.

nginx might be installed upon host creation (install process will create user nginx), thus adding

nginx:x:498:498::/var/www:/bin/false

to /etc/passwd

then latter on, users are added by a combination of editing /etc/passwd and mkdir+chown, adding in turn

user1:x:498:500::/home/user1:/bin/zsh
user2:x:499:500::/home/user2:/bin/zsh

to /etc/passwd

Why would one do that on purpose ?

You might whish to have more than one login shell (e.g. bash, csh, fish, kde, ...), in this case you can use multiple entry like :

archemar:x:101:101:Archemar:/home/archemar/bash:/bin/bash
arcsh:x:101:101:Archemar:/home/archemar/csh:/bin/csh
arfish:x:101:101:Archemar:/home/archemar/fish:/bin/fish
arkde1:x:101:101:Archemar:/home/archemar/kde1:/bin/bash
arkde2:x:101:101:Archemar:/home/archemar/kde2:/bin/bash

This would allow me to use different login and keep specific configuration file and edit each other without sudo. On the downside there are five different password, this is not the proper way to switch KDE's env, and there is a chsh command should I need to switch shell.

Above configuration would be (should be) frown upon by sysadmin, different $HOME value, will yield different PATH=$HOME/bin:.. (which is what you are looking), yet you still have to have a common part in $PATH, and in the end it is not likely you configuration would be easiest.

You can also be running automatic software tests and want to keep separate directories (home dir in this case), with a unique id for test user.

but, I mean ...

pwck (password check) will issue a warning