The document discusses UNIX shells and their functions as an interface between users and the operating system hardware, including how shells allow for command execution, scripting, and redirection of standard input/output/error. It also covers common shell features like tab completion, history, aliases, environment variables, job control, and process management using commands like ps, kill, and pgrep.

1. Lecture 4<br />ShellUserUNIX shells:<br />HWWhy it is called shell??<br />Layer between the hardware and the user

2. Command Line interface

3. Function:

4. Interpreter ls * ls f1 f2 f3 common in all shells

5. Shell scripts (programs) differ in syntax from one shell and another

6. Features <br />History

7. Auto CompletionBash tab<br />Kshesc \ you must be in vi mode <br />esc = all occurance<br />Redirection

8. Default action of the commandCommand takes the standard input from the keyboard and sends the standard output to the terminal window.<br />You can modify the default action of the standard input, standard output, and standard error within the shell by redirecting stdin, stdout, and stderr.<br />19558090170

9. Input redirection:

11. Forces a command to read the input from a file instead of from the keyboard.

12. Using the less than (<) metacharacter command < filename<br />Output redirection:

13. Sends the output from a command into a file instead of sending the output to the screen. As the command generates error messages, these messages are sent to the standard error. Usually error messages are sent to the terminal screen.

14. Using the greater than (>) metacharacter to direct the standard output to a file instead of printing the output to the screen. If file does not exist, the system creates it. If the file exists, the redirection overwrites the contents of the file.command > filename<br /> >> Append instead of overwrite <br />Redirecting Standard Error

15. No error messages appear on the screen.command 2> /dev/null<br />Redirecting the standard output to a file and the standard error to the same file. command 1> filename 2>&1<br />Redirecting standard error to the file. command 2>filename<br /> Pipe Character BufferIf there is output will be put in the buffer only if required <br /> input ow won’t look at it<br />The piping has no meaning if there is no output before or the after need no input

16. bash-3.00$ cal | date

17. Tue Nov 10 09:46:23 EET 2009

18. -bash-3.00$ date | cat

19. Tue Nov 10 10:25:45 EET 2009

20. -bash-3.00$ cat | date

21. Tue Nov 10 10:26:34 EET 2009

22. here suspend waiting for input when enter

23. -bash-3.00$ touch f1 | touch f2

24. Both will be touched

25. -bash-3.00$ touch f1 | date

26. Tue Nov 10 10:25:45 EET 2009

27. -bash-3.00$ date | touch f2File will be touched but no date output will be appeared<br />-bash-3.00$ ls | rm<br />Usage error<br />-bash-3.00$ ls /etc/passwd | cat<br />/etc/passwd<br />Aliasing

28. alias is within the shell

29. To make an alias:

30. alias c=clear

31. alias ls=’ls –l’

32. To display all alias:

33. alias [no option] view all aliases

34. alias alias name view certain alias

35. To delete all alias:

36. unalias -a delete all aliases

37. unalais certain alias delete certain alais

38. To ignore alias:

39. \ls or /usr/bin/ls to ignore alias

40. Startup and initialization fileUser initialization file<br />The primary purpose of the user initialization files is to define the characteristics of a user’s work environment, such as the command-line prompt, the environment variables, and the windowing environment.<br />The commands in the initialization file is executed :

41. At Login

42. su – usernameUser startup file<br />Startup fileInitialization fileIt is executed whenever you start new shell<br />No startup file

43. Environmental variablesThe shells support two types of variables:<br />Environment variables

44. Variables that provide information about the user’s environment to every shell program that is started.

45. Local variables

46. Variables that affect only the current shell. Any subshell started would not have knowledge of these variables unless it is exported

47. Environment variables LOGNAME Define the user’s login name<br />HOME Sets the path to the user’s home directory. It is the default <br /> argument for the cd command.<br />SHELL Sets the path to the default shell.<br />$ Hold the id no. of current shell<br />0 Hold the name of the current shell<br />PATH Sets the default path that the shell searches to find commands.<br />MAIL Sets the path to the user’s mailbox.<br />TERM Defines the terminal.<br />LPDEST Sets the user’s default printer.<br />PWD Defines the current working directory.<br />PS1 Defines the shell prompt for the Bourne or Korn shell.<br />To set any variables:VARIABLE = value<br />Ex: X=123<br />To export the variable:export VARIABLE = value<br />Or

48. export VARIABLE

49. You can export the variable while assigning it or after you assigned it

50. Once you export a variable it can be seen by another sub shells not another terminals once you close the terminal it will no more being exists

51. To view specific variable:echo $VARIABLE<br />printenv VARIABLE<br />bash-3.00# echo $$

52. 1572

53. bash-3.00# echo $SHELL

54. /sbin/sh

55. bash-3.00# ps -fp $$

56. UID PID PPID C STIME TTY TIME CMD

57. root 1572 1557 0 11:21:44 pts/3 0:00 bash

58. echo $0

59. echo $? Assignmentbash-3.00# x=123 <br />bash-3.00# y=x <br />bash-3.00# echo $x <br />123

60. To view all variables:List all environmental variable (having value) + exported local variableenv<br />printenv<br />List all environmental variable (having value) + exported and un exported local variableexport<br />set <br /> <br />File name globing * : Zero or more matching <br />? : any character matching<br />[]: Range matching [a-c] as example<br />Example

61. ls /usr/bin/*sh

62. ls /usr/bin/w*

63. Job control <br />Any command you run on terminal you cannot write another command unless the previous one is executed

64. So if a command take a lot of time in execution would you wait for it or open new terminal or Run this command in back ground while your terminal control this command in back ground

65. Ex:

66. sleep 100

67. The prevent command will prevent you from writing any command for 100 second

68. Run this command in back ground as you know that it will take long time

69. sleep 100 &

70. If you don’t add & from the beginning and then 2 numbers will be displayed the job id [as when more than one job is running in back ground they are put in stack ]and the process id [ we deal with this number in process management]

71. sleep 100

72. ctl + z : Stop the process in the back ground

73. bg jobid : Run the job in the background

74. fg jobid : Run this job back to the foreground

75. jobs : List all the jobs running with their job id

76. Process managementView process:<br />In order to view the process on your system use psOptions:<br />ps : List the info [pid- process name- consume how much from cpu ] about process running on terminal

77. ps -f : full information about process

78. ps -e : Prints information about every process on the system

79. ps -u username : List the process of certain user

80. ps -p processid : Prints information about this process [ps –p $$]Know process id of process:<br />pgrep processname<br />Send signals process:<br />Kill -l : view all the signals available

81. Kill -signal no pid : Send the signal to process

82. 2Interrupt[ctrl+c] exit without saving can be ignored23Stop running [ctrl+z]25Continue running15“default if no signal is specified with kill” exit after saving can be ignored9Terminate without saving and cannot be ignored

83. pkill -signal nopname : Send the signal to process by the name

84. pkill -u username : To terminate the processes owned by this user