My Bash Scripting Notes — All the basics at one place
This is my Bash Scripting Notes, tried to write down every important detail. I hope you derive value from it✌️🤝
1. Basic Script Structure
Shebang: Every script starts with #!/bin/bash, telling the system to use the bash interpreter. (e.g., 1_firstscript.sh)
Comments: Use # for single-line comments to explain your code.
Commands: Scripts execute commands sequentially, like in a terminal. (e.g., echo, uptime, free, df)
2. Output and Display
echo: Prints text to the console. (e.g., echo “Welcome to bash script.”)
Quotes: Single quotes treat content literally, while double quotes allow variable expansion. (e.g., echo “Welcome $USER” vs. echo ‘Welcome $USER’)
3. Variables
Declaration: No data type declaration needed, just assign a value. (e.g., TEMPDIR=”/tmp/webfiles”)
Access: Use $ before the variable name. (e.g., echo $TEMPDIR)
Command Substitution: Enclose a command in $(…) to capture its output and assign it to a variable. (e.g., FREERAM=$(free -m | grep Mem | awk ‘{print $4}’)
4. User Input
read: Reads input from the user and stores it in a variable. (e.g., read SKILL)
Prompt: Use -p flag with read to display a prompt message. (e.g., read -p ‘Username: ‘ USR)
Hidden Input: Use -s flag with read for passwords. (e.g., read -sp ‘Password: ‘ pass)
5. Conditional Statements
if: Executes a block of code if the condition is true. (e.g., if [ $NUM -gt 100 ])
else: Provides an alternative code block if the if condition is false.
elif: Chains multiple conditions. (e.g., elif [ $value -gt 1 ])
Comparison Operators:
- -eq: Equal to
- -ne: Not equal to
- -gt: Greater than
- -lt: Less than
- -ge: Greater than or equal to
- -le: Less than or equal to
File Test Operators:
- -f: Check if file exists and is a regular file
6. Loops
for: Iterates over a list of values.
while: Executes a block of code as long as the condition is true.
Condition-based: (e.g., while [ $counter -lt 5 ])
Infinite loop: Use while true to create a loop that runs indefinitely until explicitly stopped.
7. Exit Status
$?: Holds the exit status of the last executed command (0 for success, non-zero for failure). (e.g., if [ $? -eq 0 ])
8. Script Arguments
$0, $1, $2, …: Access script arguments by their position. $0 is the script name itself.
Passing Arguments: Provide arguments after the script name when executing. (e.g., ./my_script.sh arg1 arg2)
9. Redirection and Pipes
>: Redirects output to a file, overwriting the file if it exists. (e.g., sudo yum install wget unzip httpd -y > /dev/null)
>>: Appends output to a file.
|: Pipes the output of one command as input to another. (e.g., free -m | grep Mem | awk ‘{print $4}’)
10. Example Use Cases
System Administration: Automate tasks like installing software, managing services (starting, stopping), monitoring system resources, and deploying applications.
Web Development: Deploy websites, manage web servers, and automate web-related tasks.
Data Processing: Process text files, extract information, and manipulate data using commands like grep, awk, sed.
11. Best Practices
- Use meaningful variable names.
- Comment your code thoroughly.
- Validate user input to prevent errors.
- Test your scripts thoroughly.
- Consider using functions to organize code into reusable blocks.
- Use appropriate error handling techniques.
Sample script:
#!/bin/bash
# 1. Basic Script Structure
# Shebang
# Comments: This script demonstrates various bash scripting concepts.
# 2. Output and Display
echo "Welcome to the mega bash script demo."
# Single vs Double Quotes
echo "Current user: $USER"
echo 'Current user: $USER'
# 3. Variables
TEMPDIR="/tmp/webfiles"
echo "Temporary directory is $TEMPDIR"
# Command Substitution
FREERAM=$(free -m | grep Mem | awk '{print $4}')
echo "Free RAM: ${FREERAM}MB"
# 4. User Input
read -p "Enter your name: " USERNAME
read -sp "Enter your password: " PASSWORD
echo
echo "Hello, $USERNAME"
# 5. Conditional Statements
read -p "Enter a number: " NUM
if [ "$NUM" -gt 100 ]; then
echo "The number is greater than 100"
elif [ "$NUM" -gt 50 ]; then
echo "The number is greater than 50 but less than or equal to 100"
else
echo "The number is 50 or less"
fi
# File Test Operators
if [ -f "$TEMPDIR/sample.txt" ]; then
echo "sample.txt exists."
else
echo "sample.txt does not exist."
fi
# 6. Loops
for VAR in java .net python ruby php; do
echo "Learning $VAR"
done
counter=0
while [ "$counter" -lt 5 ]; do
echo "Counter: $counter"
((counter++))
done
# Infinite Loop
# Uncomment the below lines to test the infinite loop
# while true; do
# echo "Press [CTRL+C] to stop.."
# sleep 1
# done
# 7. Exit Status
echo "Checking if /tmp directory exists."
ls /tmp
if [ $? -eq 0 ]; then
echo "/tmp directory exists."
else
echo "/tmp directory does not exist."
fi
# 8. Script Arguments
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
# Usage: ./mega_script.sh arg1 arg2
# 9. Redirection and Pipes
echo "Creating a directory structure in $TEMPDIR"
mkdir -p $TEMPDIR
# Redirect output to a file
echo "Installation logs" > $TEMPDIR/install.log
# Append output to a file
echo "Appending a new line" >> $TEMPDIR/install.log
# Pipes
df -h | grep "/dev/sda1" > $TEMPDIR/disk_usage.txt
# 10. Example Use Cases
# System Administration Example
echo "Installing software (simulated)…"
# sudo yum install wget unzip httpd -y > /dev/null
# Web Development Example
echo "Setting up web server (simulated)…"
# sudo systemctl start httpd
# Data Processing Example
echo "Processing data (simulated)…"
# grep "pattern" file.txt | awk '{print $1}'
# 11. Best Practices
# Meaningful variable names, thorough commenting, user input validation, testing, functions, and error handling.
# Example function
function greet_user {
echo "Hello, $1!"
}
greet_user $USERNAME
# Error handling example
function check_file {
if [ -f "$1" ]; then
echo "File $1 exists."
else
echo "File $1 does not exist."
fi
}
check_file "$TEMPDIR/sample.txt"Any suggestions are welcome. Let’s make this as complete as we can make keeping brevity in check.
