UNIX System Overview

  • an operating system can be defined as the software that controls the hardware resources of the computer and provides an environment under which programs can run
    • Kernel is relatively small and resides at the core of the environment
    • The interface to the kernel is a layer of software called the system calls
    • Libraries of common functions are built on top of the system call interface, but applications are free to use both
    • The shell is a special application that provides an interface for running other applications
  • In a broad sense, an operating system consists of the kernel and all the other software that makes a computer useful and gives the computer its personality. This other software includes system utilities, applications, shells, libraries of common functions, and so on
  • shell
    • Bourne shell
    • C shell
    • Korn shell
    • Bourne-again shell
    • TENEX C shell
  • File System
    • The UNIX file system is a hierarchical arrangement of directories and files
    • Everything starts in the directory called root, whose name is the single character /
    • A directory is a file that contains directory entries
    • The stat and fstat functions return a structure of information containing all the attributes of a file
  • Filename
    • The names in a directory are called filenames
    • The only two characters that cannot appear in a filename are the slash character (/) and the null character.
    • POSIX.1 recommends restricting filenames to consist of the following characters: letters (a-z, A-Z), numbers (0-9), period (.), dash (-), and underscore (_)
    • Two filenames are automatically created whenever a new directory is created: . (called dot) and .. (called dot-dot). Dot refers to the current directory, and dot-dot refers to the parent directory. In the root directory, dot-dot is the same as dot.
  • Pathname
    • A sequence of one or more filenames, separated by slashes and optionally starting with a slash, forms a pathname
    • absolute pathname
    • relative pathname
  • Working Directory
    • the directory from which all relative pathnames are interpreted
  • Home Directory
    • when we log in, the working directory is set to our home directory
  • File Descriptors
    • File descriptors are normally small non-negative integers that the kernel uses to identify the files accessed by a process.
  • Standard Input, Standard Output, and Standard Error
    • all shells open three descriptors whenever a new program is run
    • then all three are connected to the terminal
  • Unbuffered I/O
    • Unbuffered I/O is provided by the functions open, read, write, lseek, and close
    • These functions all work with file descriptors
  • Programs and Processes
    • Programs
      • A program is an executable file residing on disk in a directory
      • A program is read into memory and is executed by the kernel as a result of one of the seven exec functions
    • Processes and Process ID
      • An executing instance of a program is called a process
      • The UNIX System guarantees that every process has a unique numeric identifier called the process ID
  • Process Control
    • fork
    • exec
    • waitpid
  • Threads and Thread IDs
    • All threads within a process share the same address space, file descriptors, stacks, and process-related attributes
    • Each thread executes on its own stack
    • threads are identified by IDs - Thread IDs
  • Error Handling
    • When an error occurs in one of the UNIX System functions, a negative value is often returned, and the integer errno is usually set to a value that tells why
    • The errors defined in <errno.h> can be divided into two categories: fatal and nonfatal
    • A fatal error has no recovery action. The best we can do is print an error message on the user’s screen or to a log file, and then exit.
    • Nonfatal errors, on the other hand, can sometimes be dealt with more robustly. Most nonfatal errors are temporary, such as a resource shortage, and might not occur when there is less activity on the system.
  • User Identification
    • User ID
      • This user ID is assigned by the system administrator when our login name is assigned, and we cannot change it
      • The user ID is normally assigned to be unique for every user
      • the kernel uses the user ID to check whether we have the appropriate permissions to perform certain operations
      • We call the user whose user ID is 0 either root or the superuser
    • Group ID
      • Groups are normally used to collect users together into projects or departments. This allows the sharing of resources, such as files, among members of the same group.
  • Signals
    • Signals are a technique used to notify a process that some condition has occurred
    • The process has three choices for dealing with the signal
      • Ignore the signal
      • Let the default action occur
      • Provide a function that is called when the signal occurs
  • Time Values
    • Calendar time
      • This value counts the number of seconds since the Epoch: 00:00:00 January 1, 1970
    • Process time
      • This is also called CPU time and measures the central processor resources used by a process. Process time is measured in clock ticks, which have historically been 50, 60, or 100 ticks per second
    • UNIX System maintains three values for a process:
      • Clock time
        • amount of time the process takes to run
      • User CPU time
        • CPU time attributed to user instructions
      • system CPU
        • CPU time attributed to the kernel when it executes on behalf of the process
  • System Calls and Library Functions
    • All implementations of the UNIX System provide a well-defined, limited number of entry points directly into the kernel called system calls
    • Its definition is in the C language
    • The technique used on UNIX systems is for each system call to have a function of the same name in the standard C library