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Linux and many other operating systems use various identifiers. An identifier is a token or number used to designate some object or entity. Such an object or entity may be a user, group, process, etc. The Linux kernel prefers to use identifiers when referring to some object. For instance, the "kill" command is a system call that stops a process, but does so after being given the process's identifier. This syscall (as well as many other kernel calls) will only accept an identifier and not a name. The Linux kernel has a variety of types of identifiers as you will see.
Group ID (GID) - Every group is given a GID. Many Unix systems have different GID maximums (GIDs are not unlimited). Many distros have limit of 0-65535 or 0-4294967295. This limit is due to how GIDs are stored. As many of you know, Linux is written in C/C++ and Assembly. A GID is (depending on the system) an unsigned 16-bit integer or unsigned 32-bit integer. Every user must belong to at least one group. The group a user primarily belongs to is defined in /etc/passwd. Additional groups are listed in /etc/group.
Process ID (PID) - Every process whether it be a daemon, GUI application, kernel, etc. has a process ID. The kernel always has a PID of 0 and init (or an alternative) has a PID of 1. When a process is no longer running, its PID can be reused by a new process. There is a limit to the number of available PIDs, but the limit is so high that users would run out of memory and/or CPU resources before the PID limit can be reached.
Parent Process ID (PPID) - A process has a PPID which is the same number as the PID belonging to the processes parent. For example, if I open Xterm which has a PID of 123 and I start Firefox in Xterm by typing "firefox", then Firefox's PPID will be 123 which is Xterm's PID. The PPID is a way of indicating which process (the parent) started the child process.
User ID (UID) - Every user has a UID. Like GIDs, the number of UID's is limited by the type of integer that defines a UID. Again like GIDs, many distros have limit of 0-65535 or 0-4294967295 as an unsigned 16-bit integer or unsigned 32-bit integer, respectively. The Root user always has a UID of 0. The UIDs are stored in /etc/passwd.
Effective User ID (EUID) - When a process or user is trying to access or create some object, the kernel checks the EUID to see if the user or process has permissions to perform the desired action. In most cases, the EUID is the exact same thing as the UID. However, the EUID may differ from the UID. Assume a regular user uses a command that requires Root privileges. The user's permissions are temporarily upgraded (if allowed) to perform th special function (like "sudo"). Users cannot change their UID while being logged in. This is why the EUID is needed, so users can change their EUID when needed. Remember, the UID is used to identify the user/process and EUID is used to identify the permissions. In other words, the UID 0 is Root and an EUID of 0 s Root's file permissions.
FUN FACT: The "whoami" command shows who you are based on your Effective User ID, not your User ID.
Real User ID (RUID) - The Real User ID is like EUID, but is related to sending signals to processes. For instance, a process can only send signals to other processes with the same RUID. When a user starts a program, the program has the same RUID as the user. The RUID is the same as the UID, hence the name "Real User ID". The RUID is not changed. Sometimes the EUID may need to change to the user's ID, and the EUID gets this info from the RUID. The RUID has the same limits as the UID (being an unsigned 32-bit integer).
Saved/Set User ID (SUID) - Remember how the Effective User ID (EUID) changes? Well, sometimes it may need to change to have the same value as the RUID temporarily. It would be inefficient to go through the process of re-elevating privileges again, so the EUID will go back to the ID saved in the SUID. For illustration, a process or user may perform some action that needs elevated privileges. The EUID is changed, but then something needs to be done under the RUID and then change back. Due to security reasons, a user/progress cannot assign itself privileges back. This is where SUID comes into play. When, the EUID needs to switch back to the elevated privileges, the ID is retrieved from the Saved User ID (or Set User ID). However, such elevated privileges can be revoked even from the SUID. By default, the SUID will have the same value as the EUID.
NOTE: Saved IDs and Set IDs are the same.
Real Group ID (RGID) - The Real Group ID is like the Real User ID, but this applies to groups.
Effective Group ID (EGID) - This is just like an Effective User ID, but this is for groups.
Saved Group ID (SGID) - The SGID uses the same concepts as Saved/Set User IDs.
Session ID (SID) - A Session ID is used for communication purposes so only the authenticated processes can communicate together on that particular session only. This helps to prevent hijackers and malware from intercepting a data transaction. The only processes that can communicate on that "conversation" are those with the session ID for that particular communication session. Session IDs are common between HTTP connections.
Process Group ID (PGID) - A Process Group ID is the ID given to a set of processes that group together. The PGID is the same as the PID of the process that formed/declared the group via the setsid() kernel call. This is kind of like groups (as in GID), but PGID are not defined/preset in a file like Group IDs (/etc/passwd and /etc/group). Process groups are needed for inter-process communication purposes. Process groups prevent race conditions as well as other obstacles that may harm communication.
Filesystem User ID (FSUID) - When a filesystem operation is to be performed on a file or the filesystem itself, the FSUID is used rather than the EUID. Effective User IDs (EUID) are used for the data and FSUIDs are needed to modify the file itself (like change permissions and such).
Universally Unique ID (UUID) - This 16-octet number has a variety of uses. Many Linux users may have seen a storage unit's UUID. UUIDs are needed to provide some kind of general unique ID that is permanent (unless changed by the user or programmer). For example, Gparted can be used to change a memory card's or hard-drive's UUID. Mac addresses are not the same as UUIDs, although they seem the same. A Mac address is made up of six sets of hexadecimal pairs, like this - ##:##:##:##:##:##. An online UUID generator can be found here (http://www.uuidgenerator.net/). Or, you can do it locally as a fan of mine suggested on Google Plus - cat /proc/sys/kernel/random/uuid
Partition ID - The different types of filesystems have a unique ID. This ID is usually used by partition formators to format a filesystem. For example, swap partitions have a partition ID of 82. However, not every filesystem uses a unique ID; xiafs, ext2, ext3, Reiserfs, and others share ID 83.
USB ID - Different USB devices also have an ID. The different IDs are listed here (http://www.linux-usb.org/usb.ids). Other types of hardware (like PCI devices) have ID of their own, but I will not bore you with all of the different kinds of hardware IDs since I am sure you get the point - PCI IDs, SCSI IDs, etc.
- General Unixoid Topics - http://dcjtech.info/topic/general-unixoid-topics/
- getsid - Get Session ID Utility - http://dcjtech.info/topic/getsid-get-session-id-utility/