Advanced Shell Programming

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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)

• 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 cursor-repositioning problem

• Develop and test a program to eliminate duplicate records

• Create shell functions and use them in a program

5

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

7

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 translateutility (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 EnteringDuplicate 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 EnteringDuplicate Data (continued)

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Protecting Against EnteringDuplicate 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