Bash Scripting Cheat Sheet

Defining a variable

name="Lewis"

Displaying a variable

echo $name

Command output to a variable

current_date=$(date)
echo $current_date

Conditional Statements and Flow Control

Bash introduces conditional statements (if-else), allowing scripts to make decisions based on specific conditions or comparisons. Combined with logical and comparison operators, conditional statements facilitate decision-making processes, error handling, and more complex script flows.

Using if-else condition

if [ "$name" == "Lewis" ]; then
    echo "Hello, Lewis!"
else
    echo "Hello, stranger!"
fi

Loop Constructs: Automating Repetition

Loop constructs, such as for and while, empower Bash scripts with repetition capabilities. Through loops, scripts can automate iterative tasks, from file processing to data generation and analysis, boosting productivity and efficiency.

Using a for loop

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

Using a while loop

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

Functions: Modular Scripting

Functions are a key component of modular design principles. They act as containers for reusable code segments, improving organisation, readability, and maintainability. By incorporating functions, you can adopt a modular approach to scripting challenges.

Defining a function

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

Calling a function

greet "Lewis"

Pipelines and Redirection: Streamlining Data Flow

Bash scripting utilises pipelines (|) and redirection (>, >>) to combine commands. These allow scripts to channel command outputs, streamline data flow, and manage command outputs.

Using a pipeline

ls -l | grep "txt"

Redirecting output to a file

echo "This is a test" > output.txt

Exploring More Advanced Constructs

Beyond the fundamentals, Bash scripting can be used with advanced constructs, such as case statements, arrays, and parameter expansions.

Using a case statement

case $1 in
    start)
        echo "Starting..."
        ;;
    stop)
        echo "Stopping..."
        ;;
    *)
        echo "Invalid option"
        ;;
esac

Using arrays

colors=("red" "green" "blue")
echo ${colors[0]}  # Output: red

The example script above performs two main tasks:

• Using a case statement: It checks the value of the first command-line argument ($1) and executes different actions based on its value. If the argument is “start,” it prints “Starting…”; if “stop,” it prints “Stopping…”; for any other value, it prints “Invalid option.”

• Working with arrays: The script defines an array named colors with elements “red,” “green,” and “blue.” It then prints the first element of the array, which is “red,” using ${colors[0]}.