Shell Scripts

Introduction to Shell Scripts

• Shell is the interface between a user and the operating system.
• It converts human-readable commands into system-understandable instructions.
• The Kernel is the innermost part of the OS, managing communication between hardware and software.
• The Shell is the outer layer that interacts with users.
• The terminal runs a shell that provides a command prompt (usually $), where user commands are executed.

What is a Shell Script?

• A shell script is a text file containing UNIX commands.
• It usually starts with the shebang (#!) followed by the shell path (e.g., #!/bin/sh).
• If not specified, the current shell is used.
• Commands execute sequentially or based on control structures.
• Commonly written for the Bourne shell (sh).

Example

#!/bin/sh

echo "Hello World"

Why Use Shell Scripts?

• Automate repetitive tasks — replace a sequence of commands with one script.
• Simplify complex operations — avoid remembering long command sequences.

Unix Shell Types

Bourne Shell Family

• Default prompt: $ character
• Subcategories:
  • Bourne shell (sh)
  • Bourne Again shell (bash)
  • Korn shell (ksh)
  • POSIX shell (sh)

C Shell Family

• Default prompt: % character
• Variants:
  • C shell (csh)
  • TENEX/TOPS C shell (tcsh)

Steps to Write and Execute a Shell Script

1. Open the terminal and navigate to a directory.
2. Create a file with .sh extension.
3. Write the script in the file.
4. Make it executable:

chmod +x filename.sh

5. Execute the script:

./filename.sh

Comments in Shell Script

• Lines starting with # are comments.
• Comments are ignored during execution.

Example

# This is a comment
echo "Script running..."

Determining Your Shell

To find the current shell:

echo $SHELL

The Shebang (#!)

• Defines which shell interpreter will execute the script.
• Example:

#!/usr/bin/env bash

• The file must be made executable before running.

Shell Variables

• A variable stores data like numbers, text, or filenames.
• Variable names can include letters, digits, and underscores.
• Access variable values using $.

Example

name="Othman"
echo "Hello $name"

Read-Only Variables

Once declared as read-only, a variable cannot be changed:

readonly var1="constant"
# Or
declare -r var2="constant"

Reading Variables from Input

echo "Enter your name:"
read name
echo "Hello $name"

Unsetting Variables

Deletes a variable from memory:

unset var

Special Variables

Variable	Description
$0	Script name
$n	nth argument ($1, $2, etc...)
$#	Number of arguments
$*	All arguments as a single string
$@	All arguments individually quoted
$?	Exit status of last command
$$	Process ID of current shell
$!	PID of last background command

Example Script with Output

#!/usr/bin/env bash

echo "Script name: $0"
echo "All arguments: $*"
echo "Number of arguments: $#"
echo "First argument: $1"
echo "Second argument: $2"
echo "Exit status: $?"

Output:

Script name: ./script.sh
All arguments: apple banana
Number of arguments: 2
First argument: apple
Second argument: banana
Exit status: 0

Exit Status

Any number other than 0 indicates a failure.

• Stored in $?.
• 0 = Success.
• 1 = Failure.

Shell Arrays

• Arrays hold multiple values in one variable (bash only).

arr=(apple banana cherry)
echo ${arr[1]}   # Outputs banana
echo ${arr[@]}   # Outputs all elements

Storing Command Output

Use backticks or $():

today=$(date) # Prefered syntax
echo "Today is $today"

year=`date +%Y` # Old syntax
echo "Year is $year"

Arithmetic Operations Using expr Command

• The complete expression should be enclosed between backticks (`)
• There must be spaces between operators and expressions
• Example:

# Correct:
val=`expr 2 + 2`

# Incorrect (no spaces):
val=`expr 2+2`

Modern Arithmetic Features

NOTE

Arithmetic expansion $(()) and arithmetic evaluation (( )) were introduced in Bash 2.0 (1996) as modern alternatives to expr.

Shell Operators

Integer Comparison

Operator	Meaning
-eq	Equal
-ne	Not equal
-gt	Greater than
-ge	Greater than or equal
-lt	Less than
-le	Less than or equal

Boolean Operators

Operator	Meaning	Example
!	NOT	[ ! false ]
-o	OR	[ $a -lt 10 -o $b -gt 100 ]
-a	AND	[ $a -lt 10 -a $b -gt 5 ]

Conditional Statements

If Statement

if [ condition ]; then
  commands
fi

If-Else Statement

if [ condition ]; then
   commands
else
   other_commands
fi

Case Statement

Used for multi-condition checks:

case $var in
  1) echo "One" ;;
  2) echo "Two" ;;
  ?) echo "Missing" ;;
  *) echo "Other" ;;
esac

Loops

While Loop

Executes while a condition is true:

while [ condition ]; do
  commands
done

For Loop

Iterates over a list:

for item in 1 2 3; do
  echo $item
done

Until Loop

Executes until a condition becomes true:

until [ condition ]; do
  commands
done

Select Loop

Displays a numbered menu for user input:

select option in Apple Banana Exit; do
  echo "You chose $option"
  break
done

Even-Odd number using select loop:

PS3='Enter your choice: ' # Custom prompt
select number in {1..10} none; do
  case "${number}" in
  2 | 4 | 6 | 8 | 10) echo "even" ;;
  1 | 3 | 5 | 7 | 9) echo "odd" ;;
  none)
    echo "Bye!"
    break
    ;;
  *) echo "Error invalid selection" ;;
  esac
done

Loop Control Statements

Break Statement

Terminates the loop early:

for i in 1 2 3; do
  if [ $i -eq 2 ]; then
    break
  fi
done

Continue Statement

Skips to next iteration:

for i in 1 2 3; do
  if [ $i -eq 2 ]; then
    continue
  fi
  echo $i
done

Shell Functions

• Functions divide scripts into reusable blocks.
• Parameters are accessed as $1, $2, etc.

Example

myfunc() {
  echo "Hello $1"
}
myfunc "Othman"

Example Script: Factorial

#!/usr/bin/env bash

echo -n "Enter a number: "
read -r a
fact=1
while [ "$a" -ne 0 ]; do
  fact=$((fact * a))
  a=$((a - 1))
done
echo "Factorial: $fact"