Chapter Seven
Advanced Shell
Programming
Lesson A
Developing a
Fully Featured Program
2
Objectives
Use flowcharting and pseudocode tools
Learn to write scripts that tell the system which
shell to use as an interpreter
Use the test command to compare values and
validate file existence
Use the translate command, tr, to display a
record with duplicate fields
Use the sed command in a script to delete
phone records
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Analyzing the Program
A computer program is developed by
analyzing the best way to achieve the
desired results – standard programming
tools help with this process
The two most popular and proven analysis
tools are:
– The program flowchart
– Pseudocode
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Flowcharting
Each step in the program is represented by a
symbol in the flowchart
The shape of the symbol indicates the type of
operation being performed
– Arrows connect the symbols and indicate the direction
in which the program flows
– Input and output operations are represented by
trapezoids and decision structures as diamonds
Flowcharts are manually created using a
drawing template
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Writing Pseudocode
After flowcharting, the next step in designing a
program is to write pseudocode
Pseudocode instructions are similar to actual
programming statements and are used to create
a model that is later used as the basis for a real
program
Pseudocode is a design tool only, and never
processed by the computer; therefore there are
no strict rules to follow
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Ensuring the Correct Shell
Runs the Script
Since each UNIX user has the freedom to
choose which shell they will use, it is important
to ensure that the correct shell is used to run the
script
This is because all shells do not support the
same commands and programming statements
The first line of the script is where you instruct a
user to run it with a specific shell
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Using the test Command
The test command makes preliminary checks of
the UNIX internal environment and other useful
comparisons
Place the test command inside the shell script or
execute it directly from the command line
The test command can be used to:
–
–
–
–
Perform relational tests with integers
Test strings
Determine if a file exists and what type of file it is
Perform Boolean tests
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Relational Integer Tests
The test command returns a value known as an exit status,
which is a numeric value and indicates the results of the
test performed: true if 0 (zero) and false if 1 (one)
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String Tests
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Testing Files
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Performing Boolean Tests
AND – returns true (0) if both expressions are true,
otherwise returns false (1)
OR – returns true if either expression is true, otherwise if
neither is true, returns false
! – negates the value of the expression
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Using the test Command
The content of this
file was created in
part due to using
the test command
to determine if a
directory existed
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Using the test Command
The output shown
here was created
in part due to
using the test
command to
determine a value
entered by a user
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Formatting Record Output
Record output is formatted using the
translate utility (tr)
Use tr to:
– Change the characters typed at the keyboard,
character by character
– Work as a filter when the input comes from
the output of another UNIX command
– Redirect standard input to come from a file
rather than the keyboard
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Formatting Record Output
tr was used to
change lowercase
characters to
uppercase and
replace colon
characters with
spaces
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Formatting Record Output
tr was used to
help format the
record output of
the program
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Deleting Phone Records
Bash shell operators are in three groups:
– Defining and Evaluating operators are used to set a
variable to a value and to check variable values
The equal sign (=) is an example
– Arithmetic operators are used to perform
mathematical equations
The plus sign (+) is an example
– Redirecting and piping operators are used to specify
input and output data specifications
The greater than sign (>) is an example
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Deleting Phone Records
The menu has
been updated to
allow for deleting
a phone record
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Deleting Phone Records
Examine the
corp_phones file
before deleting
a record
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Deleting Phone Records
The sed
command is
behind the delete
option
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Deleting Phone Records
The record is no
longer in the file
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Lesson B
Completing the Case Project
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Objectives
Set up a quick screen-clearing technique
Create a program algorithm to solve a cursor
repositioning problem
Develop and test a program to reenter fields during
data entry
Develop and test a program to eliminate duplicate
records
Create shell functions and use them in a program
Load shell functions automatically when you log in
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Clearing the Screen
The clear command is a useful housekeeping
utility for clearing the screen before new screens
appear, but there is a faster way
You can clear the screen faster by storing the
output of the clear command in a variable and
then echoing the contents of the variable on the
screen
– This works about ten times faster than the actual
command since the system does not have to locate
and execute the clear command
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Moving the Cursor
You can allow a user to correct data entered into
a previous data entry field. This will involve
moving the cursor from one field to another
One option would make the minus sign (-) the
user’s means to move back a field
– If a user enters a minus and hits enter, the cursor is
repositioned at the start of the previous field
– To accomplish this, the first step is to create a
program algorithm
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Creating Program
Algorithms
An algorithm is a sequence of commands or
instructions that produces a desired result
A good practice for creating an algorithm would
be to develop both the logic shown in a flowchart
and the expressed conditions necessary to carry
out the logic described in the pseudocode
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Creating Program
Algorithms
Incorrect
information has
been entered by
the user
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Creating Program
Algorithms
The algorithm has
encountered a
minus sign and
moved the cursor
to the previous
field
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Protecting Against Entering
Duplicate Phone Numbers
Input validation is necessary because
users don’t always enter valid data
Programs should always check to ensure
that users enter acceptable information
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Protecting Against Entering
Duplicate Phone Numbers
The phoneadd
program now
does input
validation
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Using Shell Functions
A shell function is a group of commands that is
stored in memory and assigned a name
Shell scripts can use function names to execute
the commands
Shell functions are used to isolate reusable code
sections, so there is no need to duplicate the
same algorithm throughout your program
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Reusing Code
To improve programming productivity, learn to
reuse code
This means that functions and programs
developed should be shared with other
programs and functions as much as possible
This practice helps to prevent duplications, save
time, and reduce errors
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Sorting the Phone List
The code that
generated this list
includes a shell
script which
contains sort
functions
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Chapter Summary
The two most popular and proven analysis
tools are the program flowchart and
pseudocode
Pseudocode is a model of a program
Use the first line in a script file to tell the OS
which shell to use when interpreting the script
Use the test command to validate the
existence of directories and files as well as
compare numeric and string values
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Chapter Summary
The translate utility (tr) changes characters
typed at the keyboard and also works as a filter
when input comes from the output of another
UNIX command
The sed command reads a file as its input and
outputs the file’s modified content
To speed clearing the screen, assign the clear
command sequence to the shell variable
CLEAR that can be set inside the login script
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Chapter Summary
An algorithm is a sequence of
instructions or commands that produces
a desired result
Shell functions can simplify program
code by isolating code that can be
reused throughout one or many
programs
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