Transcript Shell Scripting

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
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Objectives (continued)
• Use shell logic structures
• Employ shell scripting to create a menu
• Use commands to help debug shell scripts
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Objectives (continued)
• Explain ways to customize your personal
environment
• Use the trap command
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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
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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
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The Program Development Cycle
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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
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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
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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
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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
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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 effort
• Shell 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 code
• To 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 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
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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
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Environment and Configuration
Variables (continued)
Use the printenv
variable to see a
list of environment
variables
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Environment and Configuration
Variables (continued)
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Environment and Configuration
Variables (continued)
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Environment and Configuration
Variables (continued)
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Environment and Configuration
Variables (continued)
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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
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Shell Operators
• Bash shell operators are in four groups:
– Defining
– Evaluating
– Arithmetic
– Redirection
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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
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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
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Arithmetic Operators
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Arithmetic Operators (continued)
• Regular mathematical precedence rules apply to
arithmetic operators
To store arithmetic
values in a variable,
use let statement
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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
scripts
• You 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 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
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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 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
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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
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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
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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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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
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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
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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
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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
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Customizing Your
Personal 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
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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
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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”
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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
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Summary
• A high-level language uses English-like
expressions and must be converted into a lowlevel 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
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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
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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
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