Beginner's Guide to Shell Scripting

This is a full beginner's course on shell scripting that will take you from the very basics to writing simple scripts and connecting them in Unix-based systems.

1. Introduction to the Shell

What is a Shell?

A shell is a command-line interpreter that allows users to interact with the operating system. It processes user commands and requests the operating system to perform the specified tasks.

Types of Shells

Shell	Description
Bash	Bourne Again Shell - Default on many Unix-based systems.
Zsh	Z Shell - Offers improvements over Bash.
Fish	Friendly Interactive Shell - User-friendly features.
Ksh	Korn Shell - Combines features of Bourne and C Shells.

To check which shell you're using, type:

echo $SHELL

Navigating the Terminal

Basic navigation commands:

Command	Description
`pwd`	Show current directory.
`cd`	Change directory.
`ls`	List files in a directory.

Basic Commands

Example Script: Basic Commands

#!/bin/bash

# Display current directory
echo "Current Directory: $(pwd)"

# List files
echo "Files in Directory:"
ls

2. Writing Your First Shell Script

What is a Shell Script?

A shell script is a text file containing a sequence of commands for the shell to execute.

How to Create and Run a Script

Example Script: Hello World

#!/bin/bash
echo "Hello, World!"

Steps:

Create the script file:
```
nano hello_world.sh
```
Add the content above and save the file.
Make the script executable:
```
chmod +x hello_world.sh
```
Run the script:
```
./hello_world.sh
```

Comments and Documentation

Use # for comments.

#!/bin/bash

# This script prints Hello, World!
echo "Hello, World!"

3. Variables in Shell

Defining Variables

Example Script: Defining Variables

#!/bin/bash

NAME="Adamo"
AGE=21

Using Variables

echo "My name is $NAME and I am $AGE years old."

Complete Script: Using Variables

#!/bin/bash

NAME="Adamo"
AGE=21

echo "My name is $NAME and I am $AGE years old."

Special Variables

Variable	Description
`$0`	The name of the script.
`$1` - `$9`	The first to ninth arguments to the script.
`$@`	All the arguments.
`$#`	Number of arguments.
`$$`	Process ID of the script.
`$?`	Exit status of the last command.

Example Script: Special Variables

#!/bin/bash

echo "Script name: $0"
echo "First argument: $1"
echo "All arguments: $@"
echo "Number of arguments: $#"

Quoting

Quote Type	Description
`' '` (Single Quotes)	Prevents variable expansion.
`" "` (Double Quotes)	Allows variable expansion.
` ` (Backticks)	Executes commands inside.

Example Script: Quoting

#!/bin/bash

NAME="Adamo"

echo 'Hello $NAME'    # Outputs: Hello $NAME
echo "Hello $NAME"    # Outputs: Hello Adamo

DATE=`date`
echo "Today is $DATE"

4. Conditionals and Control Flow

`if` Statements

Syntax:

if [ condition ]; then
  # commands
fi

Example Script: Simple `if` Statement

#!/bin/bash

NAME="Adamo"

if [ "$NAME" == "Adamo" ]; then
  echo "Hello Adamo!"
fi

`else` and `elif` Statements

Syntax:

if [ condition ]; then
  # commands
elif [ condition ]; then
  # commands
else
  # commands
fi

Example Script: `if-elif-else`

#!/bin/bash

NAME=$1  # Get the first argument

if [ "$NAME" == "Adamo" ]; then
  echo "Hello Adamo!"
elif [ "$NAME" == "Sofia" ]; then
  echo "Hello Sofia!"
else
  echo "Hello Stranger!"
fi

Using Conditions (`test`, `[ ]`, `[[ ]]`)

test, [ ], and [[ ]] are used to evaluate conditions in shell scripts, but they have some differences:

1. `test` and `[ ]`

test is a command that evaluates conditions. It can also be written using the [ ] shorthand.
Both test and [ ] support basic comparisons and are POSIX-compliant, meaning they work on all Unix-like systems.
Example:

if test "$NUM1" -eq "$

NUM2"; then
  echo "Numbers are equal"
fi

# Equivalent to
if [ "$NUM1" -eq "$NUM2" ]; then
  echo "Numbers are equal"
fi

2. `[[ ]]`

[[ ]] is an enhanced version of [ ] with additional functionality, such as pattern matching and better support for complex expressions.
It is bash-specific (not POSIX), meaning it won't work on all shells, but it is more powerful.
Example:

if [[ "$STRING" =~ ^[a-z]+$ ]]; then
  echo "String is lowercase"
fi

3. Key Differences:

[[ ]] allows for more complex conditionals and doesn’t require escaping certain characters, while [ ] is more limited and requires careful quoting and escaping.
[[ ]] is considered safer when using string comparisons because it prevents many unexpected issues caused by shell expansions.

Example Script: Comparing Numbers and Strings

#!/bin/bash

NUM1=10
NUM2=20

if [ "$NUM1" -lt "$NUM2" ]; then
  echo "$NUM1 is less than $NUM2"
fi

STR="hello"

if [[ "$STR" =~ ^[a-z]+$ ]]; then
  echo "$STR contains only lowercase letters"
fi

Comparison Operators:

Operator	Description	For Numbers	For Strings
`-eq`	Equal	✔️
`-ne`	Not equal	✔️
`-gt`	Greater than	✔️
`-lt`	Less than	✔️
`-ge`	Greater or equal	✔️
`-le`	Less or equal	✔️
`=`	Equal		✔️
`!=`	Not equal		✔️
`-z`	String is empty		✔️
`-n`	String is not empty		✔️

Example Script: Using Conditions

#!/bin/bash

NUM1=10
NUM2=20

if [ $NUM1 -lt $NUM2 ]; then
  echo "$NUM1 is less than $NUM2"
fi

STR="Hello"

if [ -n "$STR" ]; then
  echo "String is not empty"
fi

`case` Statements

Syntax:

case variable in
  pattern1)
    # commands
    ;;
  pattern2)
    # commands
    ;;
  *)
    # default commands
    ;;
esac

Example Script: `case` Statement

#!/bin/bash

COLOR=$1  # Get the color from the first argument

case $COLOR in
  "red")
    echo "The color is red."
    ;;
  "blue")
    echo "The color is blue."
    ;;
  "green")
    echo "The color is green."
    ;;
  *)
    echo "Unknown color."
    ;;
esac

5. Loops

`for` Loops

Syntax:

for variable in list; do
  # commands
done

Example Script: `for` Loop

#!/bin/bash

for i in {1..5}; do
  echo "Number: $i"
done

`while` Loops

Syntax:

while [ condition ]; do
  # commands
done

Example Script: `while` Loop

#!/bin/bash

i=1
while [ $i -le 5 ]; do
  echo "Number: $i"
  i=$((i + 1))
done

`until` Loops

Syntax:

until [ condition ]; do
  # commands
done

Example Script: `until` Loop

#!/bin/bash

i=1
until [ $i -gt 5 ]; do
  echo "Number: $i"
  i=$((i + 1))
done

Breaking Out of Loops

Use break and continue to control loop execution.

Example Script: Break and Continue

#!/bin/bash

for i in {1..10}; do
  if [ $i -eq 5 ]; then
    echo "Skipping number 5"
    continue
  fi
  if [ $i -gt 8 ]; then
    echo "Breaking at number $i"
    break
  fi
  echo "Number: $i"
done

6. Functions in Shell Scripts

Defining Functions

Syntax:

function_name() {
  # commands
}

Example Script: Defining a Function

#!/bin/bash

greet() {
  echo "Hello, $1!"
}

Calling Functions

greet "Adamo"

Passing Arguments to Functions

Complete Script: Functions with Arguments

#!/bin/bash

greet() {
  echo "Hello, $1!"
}

greet "Adamo"
greet "Sofia"

7. Input and Output

Echoing Text

#!/bin/bash

echo "This is a message."

Reading User Input

Example Script: Reading Input

#!/bin/bash

read -p "Enter your name: " NAME
echo "Hello, $NAME!"

Redirecting Input and Output

Symbol	Description
`>`	Redirect output to a file (overwrite).
`>>`	Redirect output to a file (append).
`<`	Redirect input from a file.
`2>`	Redirect stderr to a file.
`&>`	Redirect both stdout and stderr to a file.

Example Script: Redirecting Output

#!/bin/bash

ls -l > files_list.txt
echo "Directory listing saved to files_list.txt"

8. Working with Files

Creating, Moving, and Deleting Files

Example Script: File Operations

#!/bin/bash

# Create a file
touch myfile.txt

# Move the file
mv myfile.txt mynewfile.txt

# Copy the file
cp mynewfile.txt copy_of_mynewfile.txt

# Delete the files
rm mynewfile.txt copy_of_mynewfile.txt

File Permissions and Ownership

Change file permissions using chmod.

Example Script: Changing Permissions

#!/bin/bash

# Give read, write, and execute permissions to the user
chmod u+rwx script.sh

# Remove write permission from group and others
chmod go-w script.sh

**Using Wildcards (`*`, `?`)**

* matches any number of characters.
? matches a single character.

Example Script: Using Wildcards

#!/bin/bash

# List all .txt files
ls *.txt

# List files starting with 'file' and any single character
ls file?.txt

File Tests

Test Expression	Description
`-e FILE`	File exists.
`-f FILE`	File exists and is a regular file.
`-d DIRECTORY`	Directory exists.
`-s FILE`	File is not empty.
`-r FILE`	File is readable.
`-w FILE`	File is writable.
`-x FILE`	File is executable.

Example Script: File Tests

#!/bin/bash

FILE="testfile.txt"

if [ -e "$FILE" ]; then
  echo "$FILE exists."
else
  echo "$FILE does not exist."
fi

if [ -f "$FILE" ]; then
  echo "$FILE is a regular file."
fi

if [ -s "$FILE" ]; then
  echo "$FILE is not empty."
fi

9. Working with Strings and Numbers

String Manipulation

Extract Substring

#!/bin/bash

STR="Hello World"
SUBSTR=${STR:0:5}
echo "Substring: $SUBSTR"  # Outputs: Hello

String Length

#!/bin/bash

STR="Hello"
LENGTH=${#STR}
echo "Length: $LENGTH"  # Outputs: 5

Arithmetic in Shell

Use $(( expression )) for arithmetic operations.

Example Script: Arithmetic Operations

#!/bin/bash

NUM1=10
NUM2=5

SUM=$((NUM1 + NUM2))
DIFF=$((NUM1 - NUM2))
PROD=$((NUM1 * NUM2))
QUOT=$((NUM1 / NUM2))
MOD=$((NUM1 % NUM2))

echo "Sum: $SUM"
echo "Difference: $DIFF"
echo "Product: $PROD"
echo "Quotient: $QUOT"
echo "Remainder: $MOD"

10. Advanced Scripting Techniques

`sed` and `awk` for Text Manipulation

Using `sed` for Search and Replace

#!/bin/bash

# Replace 'old' with 'new' in file.txt
sed 's/old/new/g' file.txt > newfile.txt

Using `awk` for Field Extraction

#!/bin/bash

# Print the first column of a file
awk '{print $1}' file.txt

Using Regular Expressions in Shell Scripts

Example Script: Validating Email with Regex

#!/bin/bash

read -p "Enter your email: " EMAIL

if [[ $EMAIL =~ ^[a-zA-Z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}$ ]]; then
  echo "Valid email."
else
  echo "Invalid email."
fi

Scheduling Tasks with `cron`

To schedule tasks, edit the crontab file.

Example: Running a Script Every Day at 2 AM

Open the crontab editor:
```
crontab -e
```
Add the following line:
```
0 2 * * * /path/to/script.sh
```

Debugging Shell Scripts

Use -x option to trace script execution.

Example: Debugging a Script

bash -x script.sh

11. Additional Topics

Error Handling and Exit Status

Every command returns an exit status (0 for success, non-zero for failure).

Example Script: Checking Exit Status

#!/bin/bash

mkdir new_directory

if [ $? -eq 0 ]; then
  echo "Directory created successfully."
else
  echo "Failed to create directory."
fi

Alternatively, use set -e to exit the script if any command fails.

Shell Script Arguments Parsing

Use getopts for parsing options.

Example Script: Using `getopts`

#!/bin/bash

while getopts ":u:p:" opt; do
  case $opt in
    u) USERNAME="$OPTARG"
    ;;
    p) PASSWORD="$OPTARG"
    ;;
    \?) echo "Invalid option -$OPTARG" >&2
    ;;
  esac
done

echo "Username: $USERNAME"
echo "Password: $PASSWORD"

Run the script:

./script.sh -u "adamo" -p "password123"

Environment Variables

Environment variables are available to all child processes.

Example Script: Accessing Environment Variables

#!/bin/bash

echo "Home Directory: $HOME"
echo "Current User: $USER"
echo "Path Variable: $PATH"

Trap and Signal Handling

Handle signals using trap.

Example Script: Handling SIGINT

#!/bin/bash

trap "echo 'Script interrupted! Exiting...'; exit 1" SIGINT

echo "Press Ctrl+C to interrupt."

while true; do
  sleep 1
done

Conclusion

This guide covers the fundamentals of shell scripting, providing examples for each topic to help you practice and understand how shell scripts work. By working through these examples, you'll gain a solid foundation in shell scripting and be prepared to tackle more complex scripting tasks.

Happy Scripting!

Additional Resources

Bash Reference Manual: https://www.gnu.org/software/bash/manual/bash.html
Advanced Bash-Scripting Guide: http://tldp.org/LDP/abs/html/

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Beginner's Guide to Shell Scripting

Table of Contents

1. Introduction to the Shell

What is a Shell?

Types of Shells

Navigating the Terminal

Basic Commands

Example Script: Basic Commands

2. Writing Your First Shell Script

What is a Shell Script?

How to Create and Run a Script

Example Script: Hello World

Comments and Documentation

3. Variables in Shell

Defining Variables

Example Script: Defining Variables

Using Variables

Complete Script: Using Variables

Special Variables

Example Script: Special Variables

Quoting

Example Script: Quoting

4. Conditionals and Control Flow

if Statements

Syntax:

Example Script: Simple if Statement

else and elif Statements

Syntax:

Example Script: if-elif-else

Using Conditions (test, [ ], [[ ]])

1. test and [ ]

2. [[ ]]

3. Key Differences:

Example Script: Comparing Numbers and Strings

Comparison Operators:

Example Script: Using Conditions

case Statements

Syntax:

Example Script: case Statement

5. Loops

for Loops

Syntax:

Example Script: for Loop

while Loops

Syntax:

Example Script: while Loop

until Loops

Syntax:

Example Script: until Loop

Breaking Out of Loops

Example Script: Break and Continue

6. Functions in Shell Scripts

Defining Functions

Syntax:

Example Script: Defining a Function

Calling Functions

Passing Arguments to Functions

Complete Script: Functions with Arguments

7. Input and Output

Echoing Text

Reading User Input

Example Script: Reading Input

Redirecting Input and Output

Example Script: Redirecting Output

8. Working with Files

Creating, Moving, and Deleting Files

Example Script: File Operations

File Permissions and Ownership

Example Script: Changing Permissions

Using Wildcards (*, ?)

Example Script: Using Wildcards

File Tests

Example Script: File Tests

9. Working with Strings and Numbers

String Manipulation

`if` Statements

Example Script: Simple `if` Statement

`else` and `elif` Statements

Example Script: `if-elif-else`

Using Conditions (`test`, `[ ]`, `[[ ]]`)

1. `test` and `[ ]`

2. `[[ ]]`

`case` Statements

Example Script: `case` Statement

`for` Loops

Example Script: `for` Loop

`while` Loops

Example Script: `while` Loop

`until` Loops

Example Script: `until` Loop

**Using Wildcards (`*`, `?`)**

`sed` and `awk` for Text Manipulation

Using `sed` for Search and Replace

Using `awk` for Field Extraction

Scheduling Tasks with `cron`

Example Script: Using `getopts`

Packages