Advanced Shell
Programming
Objectives
• Use techniques to ensure a script is employing the
correct shell
• Set the default shell
• Configure Bash login and logout scripts
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Objectives (continued)
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•
•
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Set defaults for the vi editor
Use the test command for programming functions
Format record output
Delete records using a script
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Objectives (continued)
• Set up a quick screen-clearing technique
• Create a program algorithm to solve a cursorrepositioning problem
• Develop and test a program to eliminate duplicate
records
• Create shell functions and use them in a program
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Ensuring the Correct Shell Runs
the Script
• Each UNIX/Linux user can choose which shell they
use by default
– Ensure the correct shell is used to run a script
– Not all shells support the same commands and
programming statements
• The first line of a script should specify which shell to
use
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Setting the Default Shell
• System administrator establishes the default shell
for a user account
– In /etc/passwd file
• File can only be edited (carefully!) by system
administrator
• Some systems provide management software to
assist in setting default shells
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Using Bash Login and Logout Scripts
• When Bash is your default shell, scripts run
automatically upon login or re-entry
– .bash_profile
– .bashrc (also runs in a subshell)
• Administrator controls /etc/bashrc and /etc/profile
• .bash_logout runs when user logs out
– Often used to clear the screen
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Using Bash Login and Logout Scripts
(continued)
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Setting Defaults for Using the vi Editor
• To use the vi editor for code development, configure
.exrc in your home directory
• Automatically sets up the vi environment
– Set the number of tab spaces to use when
nesting lines of code
– Display line numbers
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Using the test Command
• Place the test command inside the shell script or
execute it directly from the command line
• Some uses of the test command are:
– Perform relational tests with integers
– Test strings
– Determine if a file or directory exists and what
type of file it is
– Perform Boolean tests
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Performing Relational Integer Tests
with the test Command
The test command returns an exit status that indicates the
result of the test: 0 (zero) if true and 1 (one) if false
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Performing Relational Integer Tests
with the test Command (continued)
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Performing String Tests with the test
Command
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Testing Files with the test Command
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Performing Boolean Tests with the test
Command
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
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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/Linux 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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Deleting Phone Records
• The sed command
– Takes the contents of an input file and applies
specific actions to the file’s contents
– Sends results to standard output
• The -d option of sed can be used to delete matching
records from the output
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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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Clearing the Screen
• The clear command is useful for clearing the screen,
but there is a faster way
– Store the output of the clear command in a
variable and then echo the contents of the
variable on the screen
– 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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Clearing the Screen (continued)
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Creating an Algorithm to Place the
Cursor
• To correct data entered into a previous data entry
field, move the cursor back to the erroneous 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 an Algorithm to Place the
Cursor (continued)
• An algorithm is a sequence of commands or
instructions that produces a desired result
• A good practice for creating an algorithm is to
develop both the logic shown in a flowchart and the
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
(continued)
The algorithm has
encountered a
minus sign and
moved the cursor to
the previous field
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Protecting Against Entering
Duplicate Data
• 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 Data (continued)
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Protecting Against Entering
Duplicate Data (continued)
The phoneadd
program now does
input validation
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Using Shell Functions
• A shell function is a group of commands stored in
memory and assigned a name
• Shell scripts can use function names to execute the
commands
• Shell functions isolate reusable code sections; no
need to duplicate the same algorithm throughout
your program
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Defining a Function from the Command
Line
• To enter a function from the command line
– Enter the name of the function
– Enter internal code in brackets
• Shell will prompt with a > for next line until closing
bracket } is entered
• Arguments are positional ($1 - $9)
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Creating Functions Inside Shell Scripts
• Creating a function inside a shell script supports code
reuse
• Functions can be automatically loaded or defined upon
login using .bashrc or .bash_profile scripts
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Summary
• Use the first line in a script to tell the
OS which shell to use with the script
• test command validates the existence
of directories and files and compares
numeric and string values
• sed reads a file as its input and
outputs the file’s modified content
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Summary (continued)
• translate (tr) changes characters typed at the
keyboard and filters input from the output of
another UNIX/Linux command
• To speed clearing the screen, assign the output of
the clear command to the shell variable CLEAR
• Algorithm: a sequence of instructions or
commands that produces a desired result
• Shell functions can isolate program code to be
reused in multiple programs
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