I wonder what happens what happens when I am changing my password on a Linux sys

Question

I wonder what happens what happens when I am changing my password on a Linux system. Basically all passwords are stored in a file called /etc/shadow or /etc/master.passwd in BSD-like systems as I do remember. Both of them are owned by root/wheel with 600 permissions.

So when I change my password using passwd command in terminal it should somehow access this file with root permissions. Then read and write to it.

I am interested in how does it get the root permissions and which process (or processes) are responsible for password change.

Explanation / Answer

Basically it's using the suid bit. If you check the passwd command in your machine:

-rwsr-xr-x 1 root root 43K Feb 15 2011 passwd

SUID (Set owner User ID up on execution) is a special type of file permissions given to a file. Normally in Linux/Unix when a program runs, it inherits access permissions from the logged in user. SUID is defined as giving temporary permissions to a user to run a program/file with the permissions of the file owner rather that the user who runs it. In simple words users will get file owner

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