A Guide to Unix Using Linux Fourth Edition

A Guide to Unix Using Linux Fourth Edition

Chapter 7Advanced Shell Programming

A Guide to Unix Using Linux, Fourth Edition 2

Objectives

• Perform program design and analysis using flowcharts and pseudocode

• Use techniques to ensure a script is employing the correct shell

• Set the default shell

• Configure Bash login and logout scripts

• Set defaults for the vi editor

• Use the test command for programming functions


Objectives (continued)

• Format record output

• Delete records using a script

• Set up a quick screen-clearing technique

• Create a program algorithm to solve a cursor-repositioning problem

• Develop and test a program to eliminate duplicate records

• Create shell functions and use them in a program


Understanding Program Design and Analysis

• Design process:– Second step in program development cycle

• After creating the specifications for program

– Develop computer program by analyzing best way to achieve desired results

– Two popular and proven analysis tools:• Flowchart

• Pseudocode


Flowcharting

• Flowchart– Logic diagram– Uses set of standard symbols– Visually explains sequence of

events from start of process to its end point

– Documents:• Processes• Procedures• Program sequence


Flowcharting (continued)


Writing Pseudocode

• After creating a flowchart, you write pseudocode– Creates a model that you can later use as a basis for

a real program

Display "What is your favorite vegetable? " on the screenEnter data into veg_nameIf veg_name is equal to "broccoli"Then Display "Broccoli is a healthy choice." on the screenElse Display "Don’t forget to eat your broccoli also." on thescreenEnd If



Ensuring the Correct Shell Runs the Script

• UNIX/Linux users can choose their shell of preference

• To ensure that the correct shell is used to interpret your script:– Include command that sets the particular shell to use

on the first line of the script• #!/bin/bash


Setting the Default Shell

• Shell set up by default is established by system administrator– /etc/passwd filetrbrown:x:500:500:Thomas Brown:/home/trbrown:/bin/bash

• To edit /etc/passwd file:– Use vi or Emacs

• Must be very careful!

• Make a backup copy first

– Use a GUI• e.g., User Manager tool in GNOME desktop, YaST


Setting the Default Shell (continued)


Using Bash Login and Logout Scripts

• In Bash, two scripts run when you log in:– .bash_profile and .bashrc

• Edit files with a text editor

• Unlike .bash_profile, .bashrc is also run each time you start a subshell

• /etc/.bashrc, /etc/bashrc, or /etc/bash.bashrc files set system defaults– Not always available

• .bash_logout file (in your home directory) executes commands when user logs out


Using Bash Login and Logout Scripts (continued)


Setting Defaults for Using the vi Editor

• .exrc used to automatically set up vi environment– Located in your home directory– Example:

set numberset tabstop=3set shell=/bin/bash


Using the test Command


Performing Relational Integer Tests with the test Command

• Exit status: numeric value that command returns to OS when it finishes– Interpreting exit status for test command:

• 0 (zero) test result is true

• 1 test result is false

• Use echo $? to view most recent exit status


Performing Relational Integer Tests with the test Command (continued)


Performing String Tests with the test Command

• These tests are useful in scripts to test contents of variables– Example: ensure that a variable contains a specific

value


Testing Files with the test Command


Performing Boolean Tests with the test Command

• Boolean operator: logical operator that symbolizes AND, OR, or NOT to evaluate a relationship– Result of evaluation is either true or false

• Examples:– test expression1 -a expression2– test expression1 -o expression2– test !expression


Formatting Record Output

• To format record output, use translate utility, tr

• Two examples:1) tr [a-z] [A-Z] < counters– Sends contents of counters file as input to tr – Then, converts lowercase characters to uppercase

2) cat names | tr ":" " "– Sends output of cat to tr – Pipes (|) contents of names file to tr– tr replaces each occurrence of “:” with a space


Deleting Phone Records

• sed takes contents of input file and applies actions to file’s contents– Actions provided as options and arguments– Results are sent to standard output device

• Simple way to delete a phone record:– Use -d option

Enter phone numberUse sed -d to delete the matching phone number and output toa temporary file, fConfirm acceptanceIf the output is accepted, copy the temporary file f back tocorp_phones (overlaying it)


Clearing the Screen


Creating an Algorithm to Place the Cursor

Read information into field2 While field2 equals "-" Move cursor to position of previous field, field1 Clear current information displayed in field1 Read new information into field1 If field1 = "q" Then Exit program End If Move cursor to position of field2 Read information into field2End While

tput cup 5 18; read lname while test "$lname" = "-" do tput cup 4 18; echo " " tput cup 4 18; read phonenum tput cup 5 18; read lname done


Protecting Against Entering Duplicate Data

• A program should always check its input to ensure the user has entered acceptable information– Input validation



Using Shell Functions

• Shell function: group of commands stored in memory and assigned a name– Shell scripts can use function name to execute the

commands– Use shell functions to isolate reusable code sections

• Reduce typing and debugging time– Example:

datenow(){date}


Defining a Function from the Command Line

• You can define functions from the command line:

• However, functions are usually stored in script files – Loaded into memory when you log in

• Arguments are passed to functions in the same manner as any other shell procedure:

[martin@localhost ~]$ datenow() <Enter>> { <Enter>> date <Enter>> } <Enter>[martin@localhost ~]$

[martin@localhost ~]$ datenow "Today’s date and time are:"Today’s date and time are:Mon Feb 9 21:49:45 MST 2009


Creating Functions Inside Shell Scripts

• Example: a shell script called .myfuncs

• You may start .myfuncs from your .bash_profile or .bashrc login script, or the command line

sort_name(){sort -k 2 -t: corp_phones}sort_job(){sort -k 6 -t: corp_phones}sort_dept(){sort -k 5 -t: corp_phones}


Troubleshooting a Shell Script

• Some tips to help you troubleshoot a script:– Ensure script has execute permissions– Be certain first line of script specifies shell to use– Use the sh -n, -v, and -x troubleshooting options– Look for typographic errors– Look for errors in the use of particular characters

• ;, ‘, `, ‘’, “”, #, <, >

– Check for syntax errors in the use of commands– Look for the use of command options that are not

supported in your distribution of UNIX/Linux– Check initial and exit value of looping logic


Summary

• Two most popular and proven analysis tools: flowchart and pseudocode

• Have the first line in a script file specify the shell

• Use test to validate the existence of directories and files as well as to compare numeric and string values

• tr changes characters typed at keyboard

• sed reads a file as its input and outputs the file’s modified contents

• Shell functions can make programmer more efficient


Command Summary


Command Summary (continued)