Tutorial 12 - Shell Scripting - Part 2

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ADDITIONAL SHELL SCRIPTING

Main Objectives of this Practice Tutorial

• Use the if-elif-else control flow statement in a shell script.

• Use the for loop control using a list with command substitution.

• Use the while loop in a shell script.

• Use the exit and break statements in a shell script.

• Explain how to configure and use a .bashrc start-up file.

Tutorial Reference Material

Course Notes		Linux Command/Shortcut Reference		YouTube Videos
Course Notes: PDF | PPTX		Control Flow Statements if-elif-else for Loop while Loop Additional Statements exit break	Startup Files Purpose Examples	Brauer Instructional Videos: Bash Shell Scripting - Part 2

KEY CONCEPTS

IF-ELIF-ELSE STATEMENT

The elif statement can be used to perform additional conditional tests of the previous test condition tests FALSE. This statement is used to make your logic control-flow statement to be more adaptable.

How it Works:
If the test condition returns a TRUE value, then the Linux Commands between
then and else statements are executed.

If the test returns a FALSE value, then a new condition is tested again,
and action is taken if the result is TRUE, then the Linux Commands between
then and else statements are executed. Additional elif statements can be used if additional conditional testing is required . Eventually, an action will be taken
when the final test condition is FALSE.

Example:

num1=5
num2=10
if test $num1 –lt $num2
then
   echo “Less Than”
elif test $num1 –gt $num2
then
   echo “Greater Than”
else    echo “Equal to”
fi

FOR LOOP USING COMMAND SUBSTITUTION

Let’s issue the for loop with a list using command substitution.
In the example below, we will use command substitution to issue the ls command and
have that output (filenames) become arguments for the for loop.

Example:

for x in $(ls)
do
   echo “The item is: $x”
done

WHILE LOOP

The while loop is useful to loop based on the result from a test condition or command result.
This type of loop is very useful for error-checking.

How it Works:
The condition/expression is evaluated, and if the condition/expression is TRUE,
the code within … the block is executed. This repeats until the condition/expression becomes FALSE.
Reference: https://en.wikipedia.org/wiki/While_loop

Example 1:

answer=10
read –p “pick a number between 1 and 10: “ guess
while test $guess –eq 10
do    read –p “Try again: “ guess
done
echo “You are correct”

Example 2:

value=1
while [ $value -le 5 ]
do
  echo "$value"
  ((value=value+1)) # could also use ((value++))
done
1
2
3
4
5

EXIT & BREAK STATEMENTS

exit Statement

The exit statement is used to terminate a shell script.
This statement is very useful when combined with logic in a shell script.
The exit command can contain an argument to provide the exit status of your shell script.

Example:

if [ $# -ne 1 ]
then
  echo "USAGE: $0 [arg]"
  exit 1
fi

break Statement

The break statement is used to terminate a loop.
Although the loop terminates, the shell script will continue running.

Example:
read -p "Enter a number: " number
while [ $number -ne 5 ]
do
   read -p "Try again. Enter a number: " number
   if [ $number -eq 5 ]
   then
     break
   fi
done

START-UP FILES

Shell configuration (start-up) files are scripts that are run when you log in, log out, or start a new shell.
The start-up files can be used, for example, to set the prompt and screen display, create local variables,
or create temporary Linux commands (aliases)

The /etc/profile file belongs to the root user and is the first start-up file that executes when you log in, regardless of shell.

User-specific config start-up files are in the user's home directory:

• ~/.bash_profile runs when you log in.
  
• The ~/.bashrc runs when you start an interactive Bash shell.

Logout Files

There is a file that resets or restores the shell environment (including shut-down of programs running in the shell) when the user logs out of their shell. User-specific logout start-up files are in the user's home directory: ~/.bash_logout