Every process in a UNIX-like system, just like every file, has an owner (the user, either real or a system "pseudo-user", such as daemon, bin, man, etc) and a group owner. The group owner for a user's files is typically that user's primary group, and in a similar fashion, any processes you start are typically owned by your user ID and by your primary group ID.
Sometimes, though, it is necessary to have elevated privileges to run certain commands, but it is not desirable to give full administrative rights. For example, the passwd command needs access to the system's shadow password file, so that it can update your password. Obviously, you don't want to give every user root privileges, just so they can reset their password - that would undoubtedly lead to chaos! Instead, there needs to be another way to temporarily grant elevated privileges to users to perform certain tasks. That is what the SETUID and SETGID bits are for. It is a way to tell the kernel to temporarily raise the user's privileges, for the duration of the marked command's execution. A SETUID binary will be executed with the privileges of the owner of the executable file (usually root), and a SETGID binary will be executed with the group privileges of the group owner of the executable file. In the case of the passwd command, which belongs to root and is SETUID, it allows normal users to directly affect the contents of the password file, in a controlled and predictable manner, by executing with root privileges. There are numerous other SETUID commands on UNIX-like systems (chsh, screen, ping, su, etc), all of which require elevated privileges to operate correctly. There are also a few SETGID programs, where the kernel temporarily changes the GID of the process, to allow access to logfiles, etc. sendmail is such a utility.
The sticky bit serves a slightly different purpose. Its most common use is to ensure that only the user account that created a file may delete it. Think about the /tmp directory. It has very liberal permissions, which allow anyone to create files there. This is good, and allows users' processes to create temporary files (screen, ssh, etc, keep state information in /tmp). To protect a user's temp files, /tmp has the sticky bit set, so that only I can delete my files, and only you can delete yours. Of course, root can do anything, but we have to hope that the sysadmin isn't deranged!
For normal files (that is, for non-executable files), there is little point in setting the SETUID/SETGID bits. SETGID on directories on some systems controls the default group owner for new files created in that directory.
When you use chown in a manner that changes only the group, then it acts the same as chgrp. The owner of a file or directory can change the group to any group he is a member of.
It works like that because both the chown and chgrp commands use the same underlying chown syscall, which allows changing both owner and group. The syscall is what applies the permission check. The only difference between the chown and chgrp commands is the syntax you use to specify the change you want to make.
Mark can't change the group back to SK001778 because he isn't a member of group SK001778 (and he isn't root, which isn't restricted by group membership).
Best Answer
It really comes down to what makes up a process in Unix. A process can come into existence in one of 2 ways. Either via the
fork()function or through one of theexec()functions in C.fork()fork()basically just makes a copy of the current process, but assigns it a new process ID (PID). It's a child of the original process. You can see this relationship in the output ofps:Here you can see that
gnome-terminalis the parent process (PID = 5255) and thatbashis it's child (PID = 18422, PPID = 5255).NOTE: PPID = Parent Process ID.
When a process forks from its parent, it "inherits" certain things, such as copies of all the file descriptors that the parent currently has for open files and the parent's user and group IDs.
NOTE: The last 2 are what identify what file and group permissions this process will have when accessing the file system.
So if a process just inherits its user and group ID from its parent, then why isn't everything just owned by root or a single user? This is where
exec()comes in.exec()Part #1The
exec()family of functions, specificallyexecve(), "replace" a current process image with a new process image. The terminology "process image" is really just a file, i.e. an executable on disk. So this is how a bash script can execute a program such as/usr/bin/time.So what about the user ID and group ID? Well to understand that let's first discuss the concept of "Persona".
Persona
At any time, each process has an effective user ID, an effective group ID, and a set of supplementary group IDs. These IDs determine the privileges of the process. They are collectively called the persona of the process, because they determine "who it is" for purposes of access control.
exec()Part #2So in addition to being able to swap out the "process image",
exec()can also change the user & group IDs from the original "real" ones to "effective" ones.An example
For this demonstration I'm going to show you what happens when we start out in a shell as our default UID/GID, and then spawn a child shell using one of my supplementary GIDs, making it the child shell's effective GID.
To perform this I'm going to make use of the unix command
newgrp.newgrpallows you to spawn a new shell passing it the supplementary group that I'd like to make my effective GID.For starters:
We can see that this shell is currently configured with my default UID/GID of
saml&saml. Touching some files shows that this is the case as well:Now we make our supplementary group
jupiterthe effective GID:Now if we touch some files:
We see that the shell's effective GID is
jupiter, so any interactions with the disk result in files being created withjupiterrather than my normal default group ofsaml.References