chapter 6 introduction to shell script programming
TRANSCRIPT
Guide to UNIX Using Linux, Third Edition
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Objectives
Using shell variables, operators, and wildcard charactersUsing shell logic structuresEmploy shell scripting to create a menuUse commands to help debug shell scriptsCustomizing your personal environmentUsing the trap commandDevelop a menu-based application
Guide to UNIX Using Linux, Third Edition
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The Program Development Cycle
The program development cycle is the process of developing an application 1st step is to create program
specifications 2nd step is to create the program
design 3rd step is developing the code, which
is written, tested, and debugged
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Using High-Level Languages
High-level languages are computer languages that use English-like expressionsExamples are; COBOL, C, C++High-level language statements are stored in a source file, which programmers create using an editor
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High-level source files must be converted into a low-level machine language file A compiler is a program that converts source files into executable machine-language filesIf a source file contains syntax errors, it cannot be converted into an executable file A programmer must correct these
errors before the program can be run
Using High-Level Languages
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Using UNIX/Linux Shell Scripts
Unlike high-level language programs, shell scripts do not have to be converted into machine languageThe UNIX/Linux shell acts as an interpreter when reading script filesInterpreters read statements in script files and immediately translate them into executable instructions and run them
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Using UNIX/Linux Shell Scripts
After creating shell script, the OS is instructed that the file is an executable shell script via the chmod commandScript files can be run in several ways: Set the path variable and type the
script name at the command prompt Type ./filename if script is in current
directory Type the script name preceded by the
full path
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Prototyping an Application
A prototype is a running model of your application Less detail, less design time than a
full application Shows potential without full
programming effortShell scripts can be used to prototype applications that will later be moved to compiled languages
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Using Comments
Comments are important!Provide useful documentation to both the programmer and to others who need to understand or debug the codeTo use, start comment line with a #
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The Programming Shell
All Linux versions use the Bash shell as the default
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Variables
Variables are symbolic names that represent values stored in memoryThree types of variables: Configuration variables store information
about the setup of the OS Environment variables hold information
about your login session Shell variables are created at the
command prompt or in shell scripts and are used to temporarily store information
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Variables
Use the printenv variable to see a list of environment variables.
You can also use env
Look at the man pages. What is different between the two commands?
Turn to page 195 (2nd Ed.) or page 269 (3rd Ed.)
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Environment and Configuration Variables (continued)
Use the printenv variable to see a list of environment variables
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Shell Variables
Shell Variables are variables that you can define and manipulate for use with program commands in a shellObserve basic guidelines for handling and naming shell variables I recommend that you use all UPPERCASE
characters when naming your variables
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Shell Variables
Variables are handled differently depending on the syntaxType: echo $USERNAME echo “$USERNAME” echo ’$USERNAME’ echo `$USERNAME`
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Shell Operators
Bash shell operators are in four groups: Defining operators Evaluating operators Arithmetic operators Redirection operators
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Defining Operators
Used to assign a value to a variableMost common is = (equal sign)Use quotation marks with stringsBackquote says execute the command inside the backquotes and store the result in the variable
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Evaluating Operators
Used for determining the contents of a variableecho $variablename will show the value of variablenameDouble quotes can be used, but not single quotes
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Arithmetic Operators (continued)
Regular mathematical precedence rules apply to arithmetic operatorsTo store arithmetic values in a variable, use let statement let x=6+4*2 echo $x
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Redirection Operators
The > redirection operator overwrites an existing file-o noclobber option of set command will prevent overwriting
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Exporting Shell Variables to the Environment
Shell scripts cannot automatically access variables created and assigned On the command line By other scripts
Make variables global in your environment by using the export command
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Modifying the PATH Variable
PATH variable controls where your shell will look for shell scriptsYou can add directories to your PATH Special directories for scripts Your current working directory
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More About Wildcard Characters
Shell scripts often use wildcard charactersWildcard characters are called glob characters and are a part of glob patternsGlob patterns are intended to match filenames and words Question mark (?) matches one character Asterisk (*) matches zero or more
characters [chars] defines a class of characters, the
glob pattern matches any character in the class
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Shell Logic Structures
Four basic logic structures needed for program development are: Sequential logic Decision logic Looping logic Case logic
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Sequential Logic
Commands are executed in the order in which they appear in the script or programThe only break in this sequence comes when a branch instruction changes the flow of execution by redirecting to another location in the script or programUsed for simple, straightforward command sequences
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Decision Logic
Enables your script or program to execute a statement or series of statements only if a certain condition existsIn many cases, the condition depends upon the result of a command or on a comparisonThe if statement is the primary decision-making control structure in this type of logic
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Looping Logic
A control structure repeats until some condition exists or some action occursTwo common looping mechanisms: for loops cycle through a range of
values until the last in a set of values is reached
The while loop cycles as long as a particular condition exists
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Looping Logic (continued)
The for loop repeats for however many values there are in the specified set of values
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Program control structures can be entered from the command lineWildcard characters can be used in loopsThe while loop is set up to test repeatedly for a matching conditionThe while loop is used when code must be repeatedly executed an undetermined number of times
Looping Logic (continued)
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Case Logic
The case logic structure simplifies the selection from a list of choicesIt allows the script to perform one of many actions, depending on the value of a variableTwo semicolons (;;) terminate the actions taken after the case matches what is being tested
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Using Shell Scripting to Create a Menu
Often useful to create a menu that branches to specific shell scriptsThe tput command is useful when creating menus Can initialize the terminal display to place
text and prompts in specific locations and respond to the user
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Debugging a Shell Script
A shell script will not execute if there is an error in one or more commandsRunning a shell script using sh enables quick debugging of problems sh -v option displays lines of code in the
script as they are read by the interpreter sh -x option displays the command and
its arguments line by line as they are run
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Customizing YourPersonal Environment
When programming and shell scripting, customizing your environment by modifying the initial settings in the login scripts provides many benefitsLogin scripts run just after logging inSetting up personal bin directories and modify editor defaults are common customizations
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Customizing Your Personal Environment
An alias is a name that represents another command The .bashrc file in your home directory is used to establish customizations that take effect at each loginThe .bashrc script is executed each time a shell is generated, such as when shell scripts are run
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The trap Command
The trap command causes a shell program to automatically remove temporary files created when shell scripts runProgrammers often set up a subdirectory to store temporary files, and when a script file exits, trap removes the filesHaving files removed from a temporary directory like this is considered “good housekeeping”
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Putting It All Together in an Application
Applications require you to: Assign and manage variables Use shell operators Employ shell logic structures Use additional wildcard characters Use tput for managing screen
initialization Use trap to clean up temporary files
Will use these skills to build a shell script application in Hands-on Project
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Chapter Summary
A high-level language uses English-like expressions and must be converted into a low-level language before being executedThe shell interprets shell scriptsLinux shells are derived from the UNIX Bourne, Korn and C shells, and bash is the default
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UNIX/Linux uses three types of variables: configuration, environment, and shellShell operators include defining, evaluating, arithmetic, and redirectionWildcard characters are used in shell scriptsThe logic structures supported are: sequential, decision, looping and case
Chapter Summary (continued)
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The tput command manages cursor placement on the screenProgrammers and system administrators often customize the .bashrc fileAliases simplify common commands can be entered into the .bashrc Use the trap command to remove temporary files after the script exits
Chapter Summary (continued)