Control Structures in Shell Scripts: Mastering Bash Flow Control

Introduction to Control Structures

Control structures are essential components of shell scripting that allow you to control the flow of your script's execution. They enable you to make decisions, repeat actions, and create more complex and powerful scripts. In this article, we'll explore the primary control structures in Bash scripting.

If-Else Statements

If-else statements allow your script to make decisions based on conditions. Here's the basic syntax:

if [ condition ]; then
    # Commands to execute if condition is true
elif [ another_condition ]; then
    # Commands to execute if another_condition is true
else
    # Commands to execute if all conditions are false
fi

Example:

#!/bin/bash

age=25

if [ $age -lt 18 ]; then
    echo "You are a minor."
elif [ $age -ge 18 ] && [ $age -lt 65 ]; then
    echo "You are an adult."
else
    echo "You are a senior citizen."
fi

Case Statements

Case statements are useful when you need to match a variable against several values. They're often cleaner than multiple if-elif statements.

case $variable in
    pattern1)
        # Commands for pattern1
        ;;
    pattern2)
        # Commands for pattern2
        ;;
    *)
        # Default case
        ;;
esac

Example:

#!/bin/bash

fruit="apple"

case $fruit in
    "apple")
        echo "It's an apple."
        ;;
    "banana"|"plantain")
        echo "It's a banana or plantain."
        ;;
    *)
        echo "Unknown fruit."
        ;;
esac

Loops in Bash

For Loops

For loops are used to iterate over a list of items or a range of numbers.

for item in list
do
    # Commands to execute for each item
done

Example:

#!/bin/bash

for color in red green blue
do
    echo "Color: $color"
done

While Loops

While loops execute a set of commands as long as a condition is true.

while [ condition ]
do
    # Commands to execute
done

Example:

#!/bin/bash

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

Until Loops

Until loops are similar to while loops, but they execute until a condition becomes true.

until [ condition ]
do
    # Commands to execute
done

Example:

#!/bin/bash

count=1
until [ $count -gt 5 ]
do
    echo "Count: $count"
    ((count++))
done

Break and Continue

'break' is used to exit a loop prematurely, while 'continue' skips the rest of the current iteration and moves to the next one.

#!/bin/bash

for num in 1 2 3 4 5
do
    if [ $num -eq 3 ]; then
        continue
    fi
    if [ $num -eq 5 ]; then
        break
    fi
    echo "Number: $num"
done

Practical Example: File Type Checker

Let's combine these concepts into a practical script:

#!/bin/bash

echo "Enter a filename:"
read filename

if [ -f "$filename" ]; then
    echo "$filename is a regular file."
    case $(file -b "$filename") in
        *text*)
            echo "It's a text file."
            ;;
        *image*)
            echo "It's an image file."
            ;;
        *)
            echo "It's another type of file."
            ;;
    esac
elif [ -d "$filename" ]; then
    echo "$filename is a directory."
else
    echo "$filename does not exist."
fi

Conclusion

Control structures are powerful tools in shell scripting that allow you to create dynamic and responsive scripts. Practice using these structures to enhance your scripting skills and create more sophisticated Bash programs.

In our next article, we'll explore shell script functions and how they can help you organize and reuse your code. Stay tuned!