Bash Basics Cheat Sheet

Bash, or the Bourne Again SHell, is a Unix shell and command language that is widely used for scripting and automation on Linux and macOS systems. Below is a quick reference to help you get started with basic Bash commands and scripting.

1. Basics of Bash Commands§

Syntax§

• Command format: <command> [option] [arguments]
• Example: ls -l /home/user

Basic Commands§

File Permissions§

• View permissions: ls -l filename
• Change permissions: chmod [permissions] filename
  • Examples:
    • chmod +x script.sh - make executable
    • chmod 755 filename - set specific permissions

2. Variables and Environment§

Defining Variables§

• Syntax: variable_name=value

  name="John Doe"
  echo $name

Environment Variables§

• Setting an environment variable: export VAR=value
• Accessing variables: $VAR
• Common environment variables:
  • $HOME - User home directory
  • $PATH - Directories Bash looks in to find executables

Special Variables§

3. Basic Control Structures§

Conditional Statements§

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

• Example:

  if [ $age -ge 18 ]; then
    echo "You are an adult."
  else
    echo "You are not an adult."
  fi

Cases§

• Example:

read -p "Enter a choice (start, stop, restart): " action

case "$action" in
  start)
    echo "Starting service..."
    # Commands to start the service
    ;;
  stop)
    echo "Stopping service..."
    # Commands to stop the service
    ;;
  restart)
    echo "Restarting service..."
    # Commands to restart the service
    ;;
  *)
    echo "Invalid option"
    ;;
esac

Loops§

For Loop§

for var in list; do
  # commands
done

• Example:

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

While Loop§

while [ condition ]; do
  # commands
done

• Example:

  count=1
  while [ $count -le 5 ]; do
    echo "Count is $count"
    count=$((count + 1))
  done

4. Functions§

Functions in Bash allow for reusable pieces of code.

Defining a Function§

function_name() {
  # commands
}

Calling a Function§

function_name

• Example:

  greet() {
    echo "Hello, $1!"
  }
  
  greet "Alice"  # Output: Hello, Alice!

5. Basic I/O Redirection§

Redirect Output§

• Redirect standard output: command > file
• Append to file: command >> file
• Redirect standard error: command 2> file
• Redirect both standard output and error: command &> file

Piping§

• Use | to pass output of one command to another.

  ls -l | grep "file.txt"

6. Basic String Operations§

String Comparison§

if [ "$str1" = "$str2" ]; then
  echo "Strings are equal."
fi

String Length§

length=${#str}

Substring Extraction§

substr=${str:position:length}

• Example:

  str="Hello, World!"
  echo ${str:7:5}  # Output: World

7. Common Expressions§

Arithmetic Operations§

Use (( )) for arithmetic operations.

result=$((5 + 3))
echo $result  # Output: 8

Logical Operators§

File Operations§

8. Writing Simple Scripts§

To create and run a Bash script:

1. Create a new file:

   nano script.sh

2. Add the shebang to indicate the script uses Bash:

   #!/bin/bash

3. Write your script and save.

4. Make it executable:

   chmod +x script.sh

5. Run the script:

   ./script.sh

9. Sample Script§

Here’s a simple script that greets the user with the current date:

#!/bin/bash

# Greet the user
echo "Hello, $(whoami)! Today is $(date +"%A, %B %d, %Y")."