Chapter 7: Advanced Shell Programming

1. Chapter 7:Advanced Shell Programming

2. Ensuring the Correct Shell Runs the Script • Each UNIX/Linux user can choose which shell they use by default • Ensure the correct shell is used to run a script • Not all shells support the same commands and programming statements • The first line of a script should specify which shell to use

3. Setting the Default Shell • System administrator establishes the default shell for a user account • User shell set in /etc/passwd file • File can only be edited (carefully!) by system administrator • Some systems provide management software to assist in setting default shells

4. Using Bash Login and Logout Scripts • When Bash is your default shell, scripts run automatically upon login or re-entry • .bash_profile • .bashrc (also runs in a subshell) • Administrator controls /etc/bashrc and /etc/profile • .bash_logout runs when user logs out • Often used to clear the screen

5. Defining the Shell (Sha-Bang) • A shell script is little more than a list of commands that are run in sequence. • Conventionally, a shellscript should start with a line such as the following: #!/bin/bash • This indicates that the script should be run in the bash shell regardless of which interactive shell the user has chosen. • This is important, since the syntax of different shells can vary greatly.

6. A Simple Example • Here's a very simple example of a shell script with comments. It just runs a few simple commands #!/bin/bash # Tell the user what is happening echo "hello, $USER. Here is a directory listing” # list files echo "listing files in the current directory, $PWD" ls

7. More on Shell Variables #!/bin/bash X=ABC echo "$Xabc" • This gives no output. What went wrong ? • The answer is that the shell thought that we were asking for the variable Xabc, which is un-initialized. • The way to deal with this is to put braces around X to separate it from the other characters. • The following gives the desired result: #!/bin/bash X=ABC echo "${X}abc"

8. Setting Defaults for Using the vi Editor • To use the vi editor for code development, configure .exrc in your home directory • Automatically sets up the vi environment • Set the number of tab spaces to use when nesting lines of code • Display line numbers

9. Some vi tips for .exrc • To view the configurations of the parameters in your current vi session, go into the command mode and issue the command set all . :set all or :setall

10. Some vi tips for .exrc • Setting syntax on/off • syntax off • Precede each line with a number • set number • Set word wrap 3 characters from edge of screen • set wm=3 • Create an abbreviation/shortcut • ab wwr William W. Richards

11. Spell Checking with vi :map V :w^M:!ispell % ^M:e!^M^M

12. Reading in the Date :ab DATE ^M^[:.!date '+\%a \%d \%h \%Y'^MkJA

13. Clearing the Screen • The clear command is useful for clearing the screen, but there is a faster way • Store the output of the clear command in a variable and then echo the contents of the variable on the screen • About ten times faster than the actual command since the system does not have to locate and execute the clear command

14. Clearing the Screen • How do you do this? • Simple… CLEAR=`/usr/bin/clear` export CLEAR echo $CLEAR

15. Chapter 8:Exploring UNIX/Linux Utilities

16. Objectives • Understand many of the UNIX/Linux utilities that are available and how they are classified • Use the dd utility to copy and convert files • Make a bootable removable disk • Monitor hard disk usage

17. Objectives (continued) • Use system status utilities • Monitor and manage processes • Check the spelling of text in a document • Use the cmp command to compare the contents of two files • Format text to create and use a man page

18. Understanding UNIX/Linux Utilities • UNIX/Linux utilities let you • Create and manage files • Run programs • Produce reports • Monitor and maintain the system • Recover from a range of errors • New utilities are continually being added in order to make UNIX/Linux run more efficiently

19. Understanding UNIX/Linux Utilities (continued) • Classified into eight major areas: • File processing • System status • Networking • Communications • Security • Programming • Source code management • Miscellaneous

20. Classifying UNIX/Linux Utilities

21. Classifying UNIX/Linux Utilities (continued)

22. Classifying UNIX/Linux Utilities (continued)

23. Classifying UNIX/Linux Utilities (continued)

24. Classifying UNIX/Linux Utilities (continued)

25. Classifying UNIX/Linux Utilities (continued)

26. Classifying UNIX/Linux Utilities (continued)

27. Classifying UNIX/Linux Utilities (continued)

28. Using the dd Command • Allows you to copy a file and change the format of the destination file • Has a rich set of options to handle copies when other methods are inappropriate • An advantage to using the dd command over cp is that all users, not just the administrator, can copy files to and from the floppy drive

29. Checking Hard Disk Usage • To maintain adequate hard disk free space, use these strategies: • Be vigilant against running dangerously low on free space by using the df command • Watch for conspicuous consumption using the du command • Follow a routine schedule for “garbage” collection and removal by using the find and rm commands

30. Using the df Utility The df utility reports on the status of1024-byte blocks that are allocated, used, and available

31. Using the du Utility The du utility summarizes disk usage, expressed in 512-byte blocks (default) or by the number of bytes(-b option)

32. Removing Garbage Files • Garbage files are temporary files that lose their usefulness after several days • Two examples of garbage files are core files (named core) and a.out files • Use the find command to assist you in locating these files and the rm command to remove them

33. Using System Status Utilities • System status commands reflect the system’s performance • System engineers primarily use the data related to system status • Good to know how to obtain and store relevant information to send to system administrator and tune-up specialists

34. Using the top Command • One of the most effective utilities for auditing system performance is the top command • The top command displays a listing of the most CPU-intensive tasks in real time • Updates every five seconds by default

35. Using the top Command (continued) The top utility run without any options specified

36. Using the uptime Command • uptime tells you how long a system has been running since the last time it was booted • Displays current time • How long the system has been up • Number of users on the system • Load average for 1, 5, and 15 minutes

37. Using the free Command The free utility displays the amount of free and used memory in the system

38. Managing Processes • A process is identified through a unique number called a process id (pid) • Unix/Linux offer utilities to run, monitor, and kill processes using pids

39. Monitoring Processes The ps command with the -A option shows a list of all system processes currently running

40. Killing Processes • Administrator with root privileges can kill any user’s processes • User can kill owned processes • Use kill command with the pid of the process • Use kill –9 to stop a process that doesn’t respond to an initial kill command • See man page for descriptions of the signals (remember the man trick)

41. Running Processes in the Background • Can run a process in the background while working with another program in the foreground • To run a program in the background, append the & character to end of the startup command, e.g., top&

42. Checking the Spellingof a Document ispell scans a document, displays errors on the screen and suggests alternative spellings

43. Comparing Files • Use the cmp utility to compare the contents of two files, and report the first difference between them • The cmp command displays the position and line number of this difference • If there are no differences, the cmp command displays nothing

44. Formatting Text in UNIX/Linux • Text formatting in UNIX/Linux involves preparing a text file with embedded typesetting commands and then processing the file • UNIX’s nroff and troff commands were the early standard in formatting programs • An embedded code is a special sequence of characters that is included with the regular text of the file

45. Formatting Text in UNIX/Linux (continued) Linux introduced groff, which implements the features of both nroff and troff

46. Formatting Text in UNIX/Linux (continued) Groff can be used to produce a man page that contains the standard man page sections

47. Formatting Text in UNIX/Linux (continued) Man pages are made available to others by having a privileged user copy it to one of the man page directories

48. Chapter Summary • UNIX/Linux utilities are classified into eight major functional areas • Utility programs are called commands: executed by entering names on the command line • dd command options allow it to handle copies when other copying methods fail • To make a bootable removable disk, use provided utilities such as mkbootdisk

49. Chapter Summary (continued) • df checks and reports on free disk space • du checks for disk usage • Use find to retrieve temporary files and use rm to remove them • top and free provide detailed views of the “internals” of the system that can be redirected to a file for system tune-up

50. Chapter Summary (continued) • Run a program in the background by appending & to the end of a command • ps displays all running processes • kill terminates a specific process • ispell scans for spelling errors • Text formatting involves • Embedding typesetting commands in a file • Processing the file with a program that generates commands for the output device