What’s the difference between a normal user and a system user?
Some documentation I’m going through has a boolean switch on whether or not a user is a ‘system’ user or a ‘normal’ user (defaulting to ‘normal’). What is the difference between these two modes of user-ship? I don’t need to learn what a user is or why you need them (even ‘fake’ ones), but this particular distinction isn’t intuitive to me.
3 Answers 3
That is not a technical difference but an organizational decision. E.g. it makes sense to show normal users in a login dialog (so that you can click them instead of having to type the user name) but it wouldn’t to show system accounts (the UIDs under which daemons and other automatic processes run) there.
Thus a border is defined or rather two ranges for the UIDs for the two groups. In openSUSE the file /etc/login.defs contains these lines:
# Min/max values for automatic uid selection in useradd # # SYS_UID_MIN to SYS_UID_MAX inclusive is the range for # UIDs for dynamically allocated administrative and system accounts. # UID_MIN to UID_MAX inclusive is the range of UIDs of dynamically # allocated user accounts. # UID_MIN 1000 UID_MAX 60000 # System accounts SYS_UID_MIN 100 SYS_UID_MAX 499
# Min/max values for automatic gid selection in groupadd # # SYS_GID_MIN to SYS_GID_MAX inclusive is the range for # GIDs for dynamically allocated administrative and system groups. # GID_MIN to GID_MAX inclusive is the range of GIDs of dynamically # allocated groups. # GID_MIN 1000 GID_MAX 60000 # System accounts SYS_GID_MIN 100 SYS_GID_MAX 499
Operating System — Linux
Linux is one of popular version of UNIX operating System. It is open source as its source code is freely available. It is free to use. Linux was designed considering UNIX compatibility. Its functionality list is quite similar to that of UNIX.
Components of Linux System
Linux Operating System has primarily three components
- Kernel − Kernel is the core part of Linux. It is responsible for all major activities of this operating system. It consists of various modules and it interacts directly with the underlying hardware. Kernel provides the required abstraction to hide low level hardware details to system or application programs.
- System Library − System libraries are special functions or programs using which application programs or system utilities accesses Kernel’s features. These libraries implement most of the functionalities of the operating system and do not requires kernel module’s code access rights.
- System Utility − System Utility programs are responsible to do specialized, individual level tasks.
Kernel Mode vs User Mode
Kernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer. This code represents a single process, executes in single address space and do not require any context switch and hence is very efficient and fast. Kernel runs each processes and provides system services to processes, provides protected access to hardware to processes.
Support code which is not required to run in kernel mode is in System Library. User programs and other system programs works in User Mode which has no access to system hardware and kernel code. User programs/ utilities use System libraries to access Kernel functions to get system’s low level tasks.
Basic Features
Following are some of the important features of Linux Operating System.
- Portable − Portability means software can works on different types of hardware in same way. Linux kernel and application programs supports their installation on any kind of hardware platform.
- Open Source − Linux source code is freely available and it is community based development project. Multiple teams work in collaboration to enhance the capability of Linux operating system and it is continuously evolving.
- Multi-User − Linux is a multiuser system means multiple users can access system resources like memory/ ram/ application programs at same time.
- Multiprogramming − Linux is a multiprogramming system means multiple applications can run at same time.
- Hierarchical File System − Linux provides a standard file structure in which system files/ user files are arranged.
- Shell − Linux provides a special interpreter program which can be used to execute commands of the operating system. It can be used to do various types of operations, call application programs. etc.
- Security − Linux provides user security using authentication features like password protection/ controlled access to specific files/ encryption of data.
Architecture
The following illustration shows the architecture of a Linux system −
The architecture of a Linux System consists of the following layers −
- Hardware layer − Hardware consists of all peripheral devices (RAM/ HDD/ CPU etc).
- Kernel − It is the core component of Operating System, interacts directly with hardware, provides low level services to upper layer components.
- Shell − An interface to kernel, hiding complexity of kernel’s functions from users. The shell takes commands from the user and executes kernel’s functions.
- Utilities − Utility programs that provide the user most of the functionalities of an operating systems.