Introduction to Shell Script Programming

Introduction to Shell Script Programming

Objectives • Understand the program development cycle • Compare UNIX/Linux shells for creating scripts • Use shell variables, operators, and wildcard characters

Objectives (continued) • Use shell logic structures • Employ shell scripting to create a menu • Use commands to help debug shell scripts

Objectives (continued) • Explain ways to customize your personal environment • Use the trap command

Previewing the Application • Shell scripts can be used to customize your environment • Will develop a simulated employee information system using shell scripts • Will gain experience with shell variables, shell script operators, and logic structures

The Program Development Cycle • The program development cycle is the process of developing an application • First step is to create program specifications • Second step is to create the program design • Third step is developing the code, which is written, tested, and debugged

The Program Development Cycle

Using High-Level Languages • High-level languages are computer languages that use English-like expressions • Examples are; COBOL, C, C++ • High-level language statements are stored in a source file, which programmers create using an editor

Using High-Level Languages (continued) • 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 files • If 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 UNIX/Linux Shell Scripts • Unlike high-level language programs, shell scripts do not have to be converted into machine language • The UNIX/Linux shell acts as an interpreter when reading script files • Interpreters read statements in script files and immediately translate them into executable instructions and run them

Using UNIX/Linux Shell Scripts(continued) • After creating shell script, the OS is instructed that the file is an executable shell script via the chmod 755 command. • Script 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

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 effort • Shell scripts can be used to prototype applications that will later be moved to compiled languages

Using Comments • Comments are important! • Provide useful documentation to both the programmer and to others who need to understand or debug the code • To use, start comment line with a #

The Programming Shell All Linux versions use the Bash shell as the default

Variables • Variables are symbolic names that represent values stored in memory • Three 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

Environment and Configuration Variables • Environment and configuration variables can be used to set up and personalize your sessions • The printenv command shows current environment and configuration variables

Environment and Configuration Variables (continued) Use the printenv variable to see a list of environment variables

Environment and Configuration Variables (continued)

Shell Variables • Variables that you can define and manipulate for use with program commands in a shell • Observe basic guidelines for handling and naming shell variables

Shell Operators • Bash shell operators are in four groups: • Defining • Evaluating • Arithmetic • Redirection

Defining Operators • Used to assign a value to a variable • Most common is = (equal sign) • Use quotation marks with strings • Backquote says execute the command inside the backquotes and store the result in the variable

Evaluating Operators • Used for determining the contents of a variable • echo $variablename will show the value of variablename • Double quotes can be used, but not single quotes

Arithmetic Operators

Arithmetic Operators (continued) • Regular mathematical precedence rules apply to arithmetic operators To store arithmetic values in a variable, use let statement

Redirection Operators • The > redirection operator overwrites an existing file • -o noclobber option of setcommand will prevent overwriting

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 exportcommand

Modifying the PATH Variable • PATH variable controls where your shell will look for shell scripts • You can add directories to your PATH • Special directories for scripts • Your current working directory

More About Wildcard Characters • Shell scripts often use wildcard characters • Wildcard characters are called glob characters and are a part of glob patterns • Glob 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

Shell Logic Structures • Four basic logic structures needed for program development are: • Sequential logic • Decision logic • Looping logic • Case logic

Sequential Logic • Commands are executed in the order in which they appear in the script or program • The only break in this sequence comes when a branch instruction changes the flow of execution by redirecting to another location in the script or program • Used for simple, straightforward command sequences

Decision Logic • Enables your script or program to execute a statement or series of statements only if a certain condition exists • In many cases, the condition depends upon the result of a command or on a comparison • The if statement is the primary decision-making control structure in this type of logic

Looping Logic • A control structure repeats until some condition exists or some action occurs • Two 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

Looping Logic (continued) The for loop repeats for however many values there are in the specified set of values

Looping Logic (continued) • Program control structures can be entered from the command line • Wildcard characters can be used in loops • The while loop is set up to test repeatedly for a matching condition • The while loop is used when code must be repeatedly executed an undetermined number of times

Case Logic • The case logic structure simplifies the selection from a list of choices • It allows the script to perform one of many actions, depending on the value of a variable • Two semicolons (;;) terminate the actions taken after the case matches what is being tested

Using Shell Scripting to Create a Menu • Often useful to create a menu that branches to specific shell scripts • The 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

Debugging a Shell Script • A shell script will not execute if there is an error in one or more commands • Running 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

Customizing YourPersonal Environment • When programming and shell scripting, customizing your environment by modifying the initial settings in the login scripts provides many benefits • Login scripts run just after logging in • Setting up personal bin directories and modify editor defaults are common customizations

Customizing Your Personal Environment (continued) • 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 login • The .bashrc script is executed each time a shell is generated, such as when shell scripts are run

The trap Command • The trap command causes a shell program to automatically remove temporary files created when shell scripts run • Programmers often set up a subdirectory to store temporary files, and when a script file exits, trap removes the files • Having files removed from a temporary directory like this is considered “good housekeeping”

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

Summary • A high-level language uses English-like expressions and must be converted into a low-level language before being executed • The shell interprets shell scripts • Linux shells are derived from the UNIX Bourne, Korn and C shells, and bash is the default

Summary (continued) • UNIX/Linux uses three types of variables: configuration, environment, and shell • Shell operators include defining, evaluating, arithmetic, and redirection • Wildcard characters are used in shell scripts • The logic structures supported are: sequential, decision, looping and case

Summary (continued) • The tput command manages cursor placement on the screen • Programmers and system administrators often customize the .bashrc file • Aliases simplify common commands can be entered into the .bashrc • Use the trap command to remove temporary files after the script exits

Introduction to Shell Script Programming