Introduction to UNIX System PDF

Chapter 2 Introduction to the UNIX system 1  UNIX users  UNIX is a multi-user system that enables several users to work simultaneously on the same OS.  User management is very important to avoid an anarchic structure in the use of the system resources.  Each user has: a na...

Chapter 2 Introduction to the UNIX system 1  UNIX users  UNIX is a multi-user system that enables several users to work simultaneously on the same OS.  User management is very important to avoid an anarchic structure in the use of the system resources.  Each user has: a name (login), a password and an identifier (UID).  UNIX users are organized into groups and assigned permissions.  Each group is identified by a unique number (GID).  The GID and UID can be defined when the group and user are created, respectively. If not, an ID greater than 999 is chosen. IDs below 999 are reserved for the system. 2  UNIX users  A user cannot be attached to a group that does not exist.  If the group to which a user belongs is not specified, the operating system usually creates a group specifically for this user and assigns it the same name as the user.  On a UNIX system, there are two types of user accounts: Super user (root): is the system administrator (programmer, database user, etc.). He/she has all the permissions and can run all the commands without any restriction. Normal user: general users are typically assigned to these accounts and usually have limited access to critical system files and directories. 3 UNIX users Group 0 Root(super-user) Group 1 ID 1001 ID 1006 PWD 1 PWD 6 user1 P1 user6 P 6 Group 3 OS ID 1002 ID 1005 PWD 2 PWD 5 user2 user5 P 5 P2 ID 1003 ID 1004 PWD 3 PWD 4 Group 2 user3 P 3 user4 P4 4 Structure of Unix operating system  The UNIX architecture consists mainly of three components: Karnel: it is a set of programs that manage the SE's key functions. It is the one that interacts directly with the hardware. Processes pass through the kernel to access hardware resources using a set of commands called “system calls”. These system calls control two main components of the kernel: the process manager and the file management system. Shell: The program between the user and the kernel is known as the shell. It translates the many commands that are typed into the terminal session. Applications: The applications and utility layer in Unix includes the word processors, graphics programs, database management programs, commands etc. 5 Structure of Unix operating system Utilitaires Coquille Noyau Hardware Karnel Shell Utilities 6 The SHELL  The Shell is a program that receives commands from a user's keyboard and sends them to the operating system for execution.  There are several types of shell, each with its own syntax and number of commands. The most common is the Bourne shell (sh), We have also C-Shell (csh), Korn Shell (ksh), Posix Shell and, under Linux, bash-shell (bash).  The Shell is in charge of: Interpreting commands; Transmitting commands to the system; Return the result. 7