CIS120Linux Fundementals

Loops

In Bash scripting, loops are used to repeat a series of commands until a specific condition is met. There are several types of loops available in Bash: for, while, and until loops. These loops can also contain if-else statements to add conditional logic within the loop.

for Loop

A for loop iterates over a list of items and executes a block of code for each item.

Syntax:

for var in list; do
    # Commands
done

Example:

#!/bin/bash

for num in 1 2 3 4 5; do
    echo "Number: $num"
done

This script iterates over the numbers 1 to 5 and prints each number.

while Loop

A while loop executes a block of code as long as the specified condition is true.

Syntax:

while [ condition ]; do
    # Commands
done

Example:

#!/bin/bash

count=1

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

This script prints the count from 1 to 5.

until Loop

An until loop is similar to a while loop but executes the block of code as long as the specified condition is false.

Syntax:

until [ condition ]; do
    # Commands
done

Example:

#!/bin/bash

count=1

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

This script prints the count from 1 to 5, stopping when the count is greater than 5.

Nesting if-else Statements in Loops

You can nest if-else statements within loops to add conditional logic to your loops.

Example with for Loop:

#!/bin/bash

for num in 1 2 3 4 5; do
    if [ $num -eq 3 ]; then
        echo "Found three!"
    else
        echo "Number: $num"
    fi
done

In this script, the for loop iterates over the numbers 1 to 5. When the number is equal to 3, it prints "Found three!" Otherwise, it prints the number.

Example with while Loop:

#!/bin/bash

count=1

while [ $count -le 5 ]; do
    if [ $count -eq 3 ]; then
        echo "Found three!"
    else
        echo "Count: $count"
    fi
    count=$((count + 1))
done

This script prints the count from 1 to 5. When the count is equal to 3, it prints "Found three!" Otherwise, it prints the count.

Example with until Loop:

#!/bin/bash

count=1

until [ $count -gt 5 ]; do
    if [ $count -eq 3 ]; then
        echo "Found three!"
    else
        echo "Count: $count"
    fi
    count=$((count + 1))
done

This script prints the count from 1 to 5, stopping when the count is greater than 5. When the count is equal to 3, it prints "Found three!" Otherwise, it prints the count.

Nested Loops

You can also nest loops within other loops to perform more complex iterations.

Example:

#!/bin/bash

for i in 1 2 3; do
    for j in a b c; do
        echo "Combination: $i$j"
    done
done

This script prints all combinations of numbers 1 to 3 with letters a to c.

Summary

Bash provides various looping constructs like for, while, and until loops to perform repetitive tasks efficiently. These loops can be enhanced with nested if-else statements to introduce conditional logic within the loops. Understanding how to use these loops and conditional statements allows you to write more complex and dynamic Bash scripts.