Essential Programs in UNIX and Linux

1. Lesson 2-Touring Essential Programs

2. Overview • Development of UNIX and Linux. • Commands to execute utilities. • Communicating instructions to the shell. • Navigating the file system. • Examining and managing files. • Accessing the programmer’s manual. • Accessing Internet resources.

3. Development of UNIX and Linux The UNIX operating system, developed in the 1970s at Bell Laboratories, consisted of the following features: • A main program, called kernel, to control the central processing unit (CPU) and other hardware. • A collection of user and system programs called utilities. • A structure, called file system, for keeping and locating data on the hard drive. • Unix was a multi-user and multi-tasking operating system.

4. Development of UNIX and Linux • Linus Torvalds wrote a UNIX look-alike operating system called Linux. • The operating system is available for free in its basic form for download, or at a low cost from several distributors.

5. Development of UNIX and Linux • The fundamental programs in Linux employ the same code, but different distributors add special features and installation programs. • Some of different flavors of Linux operating system are RedHat, Lindows, SuSe, Mandrake, etc.

6. Commands to Execute Utilities • Introduction to the shell. • Issuing commands to a shell. • Passing information to a utility. • Listing processes.

7. Introduction to the Shell • The shell is a program, which displays the prompt on the screen. • The shell reads the instructions of the user and interprets them to the remainder of the system. • Usually the $ sign is the prompt in many systems. • The shell processes and executes the command keyed in by a user.

8. Issuing Commands to a Shell • Various utilities can be executed from the shell prompt. • The “whoami” utility displays a user’s login name or username on the screen. • The “date” utility displays the current date and time on the screen. • The “hostname” utility is used to display the name of the system the user is working on.

9. Issuing Commands to a Shell • The “clear” utility is used to clear all the text on the screen and display the prompt at the top of the screen or window. • The Delete key or the Ctrl-H key can be used to correct errors that occur while entering commands. • The “who” utility is used to display a list of users who have currently logged on.

10. Issuing Commands to a Shell Output of who Command

11. Issuing Commands to a Shell • A port is a physical location at the back of the computer. • Each port has a designation that usually begins with the letters tty and a number. • A terminal connected through a network uses an electronic or pseudo port named pts.

12. Issuing Commands to a Shell • The command/utility keyed in by a user is followed by the ENTER key. • The shell interprets the ENTER as the end of the command/utility. • The shell starts a child process to execute the command/utility.

13. Issuing Commands to a Shell • The shell locates the path of the utility/command and executes it. • The output is, by default, connected to the screen, unless we redirect it. • After executing the command/utility, it returns the command prompt again.

14. Issuing Commands to a Shell How the shell works

15. Passing Information to a Utility • When any information is passed to a utility, the shell runs the utility, and passes the information that comes after the command to the utility. • Information passed to the utility is called an argument. • Arguments provide instructions to utilities. • Multiple arguments can also be passed to a utility.

16. Passing Information to a Utility • The “cal” program provides the users with the current month’s calendar. • Multiple arguments can be passed to the cal utility. • When two arguments are passed to the cal program, the program interprets the first argument as the month and the second as the year.

17. Passing Information to a Utility • The “look” command is used to locate words in the dictionary file that begin with that word, and output all the matching words. • The look command requires one argument – the word that needs to be located.

18. Listing Processes • A process is an instance of a running program code. • The “ps” command can be used to obtain a list all the current processes, with some information about each process. • The output displays the process id, the port, CPU time, and the code that the process is running.

19. Listing Processes • The “–aux” and “–ef” command options are used with the “ps” command to display the processes running on the entire system, including system processes, and a list of currently logged on users. • Arguments that begin with a minus (-) sign are options. • The ps option is useful for system administrators in trouble-shooting the system.

20. Communicating Instructions to the Shell • Every user is given a place, called the home directory, to work with and save files. • The home directory is the default workspace of a user. • The home directory contains the names of files and subdirectories created by and owned by a user.

21. Communicating Instructions to the Shell • Working with files. • Redirecting output from a utility. • Determining the role of tokens on command-lines. • Starting a child shell. • Reissuing commands. • Using nicknames for commands.

22. Working with Files • The “ls” (list) command lists the contents of the current directory’s standard files. • The “more” command displays the contents of a file one part at a time. • The command requires one argument – the name of the file.

23. Working with Files • The spacebar can be used to display additional text in a file displayed by more. • The “q” key can be used to quit the ‘more’ utility. • The “wc” (word count) utility is used to count the number of lines, words, and characters in a particular file. • The command requires one parameter – the name of the file.

24. Working with Files • The “–l” option can be used with the wc command to only count the number of lines in a file. • The “-c” option is used to count the number of characters in the file. • The “-w” option is used to count the number of words in the file.

25. Redirecting Output from a Utility • By default, the results provided by any utility are displayed on the user’s screen. • Instructions can be given to redirect the output of a utility to a file. • The command to redirect the output of a file is “utility > filename”. • The > is the instruction to redirect the output to a new file.

26. Redirecting Output from a Utility • The double redirect (>>) can be used to append the output of a utility to the end of an existing file. • The | (pipe) command can be used to redirect the output of one utility to another utility. • The argument following the pipeline must be a utility.

27. Determining the Role of Tokens on Command-Lines • Each word or object on a command-line is called a token. • The shell is programmed to read the initial token as a utility program to run or some action to be taken. • The location on the command-line determines how the shell interprets each token.

28. Starting a Child Shell List of Various Shells in UNIX/Linux

29. Reissuing Commands • The “!!” command or the “r” command can be used to re-execute a previously entered command. • The “!!” command works only on a csh, a tch, or a bash shell. • The “r” command works only on a Korn shell. • The sh shell does not allow a user to re-execute previously entered commands.

30. Reissuing Commands • The Up arrow key can be used to display previously entered commands one at a time. • The shell keeps track of the commands that we issue at the prompt. • The “history” command can be used to provide a list of all the commands entered previously, and every command has a number associated with it.

31. Reissuing Commands Various Options to Execute Previous Commands

32. Using Nicknames for Commands • An “alias” or alternate name can be used for commands that are hard to remember. • The alias command can be used to provide a list of all the current aliases. • The command “unalias alias name” can be used to remove an alias.

33. Command Shell alias alias name utility name Csh and tch alias alias name=utility name Bash and ksh Using Nicknames for Commands Nicknames for commands in UNIX/Linux

34. Navigating the File System • The arrangement of files and folders in a system is called the file system. • The “pwd” (present working directory) command can be used to get the location of the user’s home directory. • The result displays the path from the root to the present working directory.

35. Navigating the File System • The topmost directory is called the “root,” and is identified as /. • The “ls /” command can be used to obtain a list of all the files and directories in the root directory.

36. Navigating the File System • The “mkdir” (make directory) command is used to create a new directory. • The command requires one argument – the name to be given to the directory. • The “–F” option can be used with the ‘ls’ command to include a / (slash) after all the directory names, and also to identify other kinds of files.

37. Navigating the File System • The “cd” command can be used to make a directory the current directory. • The command requires one parameter – the name of the directory that needs to be made the current directory. • The cd command is used to return to the home directory. It does not require any parameters.

38. Examining and Managing Files Finding matching text: • A particular word in a file can be located by giving the command “/word to be located”. • The “n” key can be used to locate the next instance of the same word in the file. • The “b” key can be used to move one screen backwards.

39. Examining and Managing Files • The “head” utility can be used to read the first ten lines of a file. • The “tail” utility can be used to view the last ten lines of a file. • Both the utilities require one argument – the name of the file to be read. • The number of lines to be read can also be passed as an argument to the utilities.

40. Examining and Managing Files • The cat utility can be used to view the entire contents of a file. • The utility requires one argument – the name of the file to be read. • This utility is preferable when a file is small.

41. Examining and Managing Files Copying files: • The “cp” command can be used to make a copy of an existing file, as well as copy files to another subdirectory. • Two arguments are required to make a duplicate of a file – the name of the existing file and the name to be given to the duplicated file. • The arguments required to copy a file to another subdirectory are – the name of the file to be copied and the target directory.

42. Examining and Managing Files Removing files: • The “rm” command can be used to delete a file. • The command requires one argument – the name of the file to be deleted.

43. Examining and Managing Files Removing files (continued): • The rm command does its work and displays the prompt. It displays no message. • The “-i” option can be used with the command to ask for confirmation before deleting any filenames listed as an argument. • The command also accepts multiple file names as arguments.

44. Examining and Managing Files Renaming files: • The “mv” command can be used to rename files, as well as to move files from one location to another. • Two arguments are required while renaming the files – the current name of the file and the new name to be given to the file. • The arguments required for moving the files are – the name of the files to be moved and the destination directory.

45. Examining and Managing Files Deciphering utility error messages: • Error messages that occur due to the failure of a utility are shown in the screen by default. • Every process has an "output door", where it writes output, and an error door, where it writes error messages. • By default, the "error door" is connected to the screen, unless we redirect it.

46. Examining and Managing Files Printing a file: • The “lp” or the “lpr” command can be used to print a file. • The command requires one argument – the name of the file to be printed. • The “–P” option with the “lpr” command, and the “-d” option with the lp command, can be used to specify the name of the printer to be used.

47. Accessing the Programmer’s Manual • The UNIX and Linux systems include an extensive collection of powerful utility programs, system features, application languages, and support libraries. • The UNIX programmer’s manual provides the information needed to employ the exact syntax of a particular option or command format for a utility.

48. Accessing the Programmer’s Manual • The manual contains a detailed documentation on the uses and functions of utility programs, application programs, and libraries. • The manual also contains information on UNIX system files and system programming libraries. • It also includes supplementary information on related special files and commands for each entry.

49. Accessing the Programmer’s Manual • The man command can be used to provide an online manual entry for a utility or a command. • The command requires one argument – the name of the utility or the command. • The “man –k” command can be used to search the manual pages’ descriptions for keywords.

50. Accessing Internet Resources The Internet includes many useful sites that provide information on: • Recent news in the Linux world (www.linux.org/www.linux.com). • Interactive Web tutorials. • Recent software updates and Linux code documentation.