Bash Scripting Fundamentals

1. Introduction to Bash

1.1 What is Bash?

Bash (Bourne Again Shell) is a command processor that typically runs in a text window where the user types commands that cause actions.

Bash can also read and execute commands from a file, called a script.

1.2 Creating and Running a Script

• Creating a script: Use a text editor to create a file with the .sh extension.

 nano script.sh 

• Making the script executable:

 chmod +x script.sh 

• Running the script:

 ./script.sh 

2. Basic Commands

2.1 Echo

• Description: Prints text to the terminal.

 echo "Hello, World!" 

2.2 Variables

• Description: Storing and using data.

 NAME="John" echo $NAME 

2.3 Comments

• Description: Adding comments to a script for readability.

 # This is a comment echo "This is a script" 

3. Input and Output

3.1 Reading User Input

• Description: Taking input from the user.

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

3.2 Redirecting Output

• Description: Redirecting command output to a file.

 echo "This is a test" > file.txt 

3.3 Appending Output

• Description: Appending command output to a file.

 echo "This is a test" >> file.txt 

3.4 Redirecting Input

• Description: Using a file as input to a command.

 wc -l < file.txt 

3.5 Piping

• Description: Using the output of one command as input to another.

 cat file.txt | grep "test" 

4. Control Structures

4.1 Conditional Statements

• Description: Making decisions in scripts.

 if [ "$NAME" == "John" ]; then echo "Hello, John!" else echo "You are not John." fi 

4.2 Loops

• Description: Repeating a set of commands.

4.2.1 For Loop

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

4.2.2 While Loop

 COUNTER=0 while [ $COUNTER -lt 5 ]; do echo "Counter: $COUNTER" COUNTER=$((COUNTER + 1)) done 

4.2.3 Until Loop

 COUNTER=0 until [ $COUNTER -ge 5 ]; do echo "Counter: $COUNTER" COUNTER=$((COUNTER + 1)) done 

5. Functions

5.1 Defining and Calling Functions

• Description: Grouping commands into a reusable block.

 my_function() { echo "Hello from my_function" } my_function 

6. Data Structures

6.1 Arrays

• Description: Using arrays to store multiple values.

 NAMES=("John" "Jane" "Doe") echo "First Name: ${NAMES[0]}" 

6.2 Tuples (Simulated with Arrays)

• Description: Using arrays to simulate tuples.

 TUPLE=("Alice" 25 "Developer") echo "Name: ${TUPLE[0]}, Age: ${TUPLE[1]}, Job: ${TUPLE[2]}" 

6.3 Sets (Simulated with Arrays and Associative Arrays)

• Description: Using arrays and associative arrays to simulate sets.

 declare -A SET SET["Apple"]=1 SET["Banana"]=1 if [[ ${SET["Apple"]} ]]; then echo "Apple is in the set." fi 

7. Operators

7.1 Mathematical Operators

• Description: Performing arithmetic operations.

 A=5 B=3 SUM=$((A + B)) echo "Sum: $SUM" 

7.2 Logical Operators

• Description: Using logical operators in conditions.

 if [ $A -eq 5 ] && [ $B -eq 3 ]; then echo "Both conditions are true." fi 

7.3 Semantic Operators

• Description: Using comparison operators.

 if [ "$A" -eq 5 ]; then echo "A is 5" fi if [ "$A" -ne 5 ]; then echo "A is not 5" fi 

8. Type Casting

• Description: Type casting in Bash is generally about ensuring variable content is treated correctly.

 VAR="123" NUM=$((VAR + 0)) # Cast string to number for arithmetic echo $NUM 

9. API Calls

• Description: Making HTTP requests using tools like curl.

 RESPONSE=$(curl -s -X GET "https://api.example.com/data") echo "Response: $RESPONSE" 

10. Automating Tasks

10.1 Cron Jobs

• Description: Scheduling scripts to run automatically.

 # Edit the crontab file crontab -e # Add a job to run every day at midnight 0 0 * * * /path/to/script.sh 

10.2 Using at

• Description: Scheduling a one-time task.

 echo "/path/to/script.sh" | at now + 5 minutes 

10.3 Background Jobs

• Description: Running scripts in the background.

 ./script.sh & 

11. Advanced Topics

11.1 Using sed and awk

• Description: Text processing with sed and awk.

11.1.1 sed

 sed 's/old/new/g' file.txt 

11.1.2 awk

 awk '{print $1}' file.txt 

11.2 Traps

• Description: Capturing signals.

 trap "echo 'Script interrupted'; exit" SIGINT while true; do echo "Running..." sleep 1 done 

12. Debugging

12.1 Enabling Debugging

• Description: Running a script in debug mode.

 bash -x script.sh 

12.2 Using set

• Description: Setting options for debugging.

 set -x # Enable debugging set +x # Disable debugging 

12.3 Error Handling

• Description: Handling errors in scripts.

 command || { echo "Command failed"; exit 1; } 

12.4 Using trap for Cleanup

• Description: Cleaning up resources on script exit.

 cleanup() { echo "Cleaning up..." # perform cleanup here } trap cleanup EXIT 

12.5 Logging

• Description: Creating log files for your scripts.

 LOG_FILE="/var/log/script.log" echo "Script started" >> $LOG_FILE 

13. More on Arrays

13.1 Array Operations

• Description: Array length and iteration.

 ARR=("one" "two" "three") echo "Array length: ${#ARR[@]}" for ITEM in "${ARR[@]}"; do echo $ITEM done 

13.2 Associative Arrays

• Description: Using associative arrays.

 declare -A ASSOC_ARRAY ASSOC_ARRAY["name"]="John" ASSOC_ARRAY["age"]="30" echo "Name: ${ASSOC_ARRAY["name"]}" echo "Age: ${ASSOC_ARRAY["age"]}" 

13.3 Multidimensional Arrays

• Description: Simulating multidimensional arrays.

 ARRAY_2D[0]="1 2 3" ARRAY_2D[1]="4 5 6" ARRAY_2D[2]=" 7 8 9" for ROW in "${ARRAY_2D[@]}"; do for ITEM in $ROW; do echo -n "$ITEM " done echo done 

14. String Operations

14.1 Substring Extraction

• Description: Extracting a substring from a string.

 STRING="Hello World" echo ${STRING:6:5} # Outputs "World" 

14.2 String Length

• Description: Getting the length of a string.

 echo ${#STRING} # Outputs "11" 

14.3 String Replacement

• Description: Replacing part of a string.

 echo ${STRING/World/Bash} # Outputs "Hello Bash" 

15. File Operations

15.1 File Tests

• Description: Testing properties of files.

 if [ -f "file.txt" ]; then echo "file.txt is a regular file." fi if [ -d "directory" ]; then echo "directory is a directory." fi 

15.2 Reading Files

• Description: Reading a file line by line.

 while IFS= read -r line; do echo "$line" done < file.txt 

15.3 Writing to Files

• Description: Writing to a file.

 echo "Hello, World!" > file.txt 

15.4 Appending to Files

• Description: Appending to a file.

 echo "This is a new line" >> file.txt 

16. Network Operations

16.1 Downloading Files

• Description: Using wget or curl to download files.

 wget http://example.com/file.zip curl -O http://example.com/file.zip 

16.2 Uploading Files

• Description: Using curl to upload files.

 curl -F "file=@/path/to/file" http://example.com/upload 

16.3 Checking Internet Connection

• Description: Pinging a server to check for internet connectivity.

 ping -c 4 google.com 

17. Handling Large Data

17.1 Sorting and Uniqueness

• Description: Sorting data and removing duplicates.

 sort file.txt | uniq > sorted.txt 

17.2 Finding Patterns

• Description: Using grep to find patterns.

 grep "pattern" file.txt 

17.3 Splitting Files

• Description: Splitting large files into smaller parts.

 split -l 1000 largefile.txt smallfile_ 

18. Process Management

18.1 Checking Running Processes

• Description: Using ps to list running processes.

 ps aux 

18.2 Killing Processes

• Description: Using kill to terminate processes.

 kill -9 PID 

18.3 Monitoring System Resources

• Description: Using top or htop to monitor system resources.

 top htop # Requires installation 

19. Packaging and Distribution

19.1 Creating Tar Archives

• Description: Archiving files using tar.

 tar -cvf archive.tar file1 file2 directory/ 

19.2 Extracting Tar Archives

• Description: Extracting files from a tar archive.

 tar -xvf archive.tar 

19.3 Compressing Files

• Description: Compressing files using gzip.

 gzip file.txt 

19.4 Decompressing Files

• Description: Decompressing files using gunzip.

 gunzip file.txt.gz 

This extensive guide covers fundamental and advanced topics in Bash scripting