linux.softwareshinobi.com/landing/docs/Bash-Scripts/011-bash-loops.md

Loops

Loops are fundamental for automation. Bash provides for, while, and until loops to repeat commands efficiently.

---

For Loops

The for loop iterates through a list of items.

for var in ${list}
do
    your_commands
done

Example: Loop through a list of users.

touch for_users.sh

Open for_users.sh and add:

#!/usr/bin/env bash

users="Dev Team Six softwareshinobi troy"

for user in ${users}
do
    echo "${user}"
done

Save and exit. Make it executable and run:

chmod +x for_users.sh
./for_users.sh

Output:

Dev Team Six
softwareshinobi
troy

Loop through a range of numbers:

touch for_numbers.sh

Open for_numbers.sh and add:

#!/usr/bin/env bash

for num in {1..10}
do
    echo ${num}
done

Save, make executable, and run.

While Loops

A while loop continues as long as its condition remains true.

while [[ your_condition ]]
do
    your_commands
done

Example: Counter from 1 to 10.

touch while_counter.sh

Open while_counter.sh and add:

#!/usr/bin/env bash

counter=1
while [[ $counter -le 10 ]]
do
    echo $counter
    ((counter++)) # Increment counter
done

Save, make executable, run.

Example: Require user input. Loop until a non-empty name is provided.

touch while_input.sh

Open while_input.sh and add:

#!/usr/bin/env bash

read -p "What is your name? " name

while [[ -z "${name}" ]]
do
    echo "Name cannot be blank. Please enter a valid name!"
    read -p "Enter your name again: " name
done

echo "Hi there ${name}!"

Save, make executable, run (test with empty and valid input).

Until Loops

An until loop runs until its condition becomes true.

until [[ your_condition ]]
do
    your_commands
done

Example: Counter from 1 to 10.

touch until_loop.sh

Open until_loop.sh and add:

#!/usr/bin/env bash

count=1
until [[ $count -gt 10 ]]
do
    echo $count
    ((count++))
done

Save, make executable, run.

Continue and Break

Control loop flow with continue (skip current iteration) and break (exit loop entirely).

continue

touch continue_example.sh

Open continue_example.sh and add:

#!/usr/bin/env bash

for i in 1 2 3 4 5
do
    if [[ $i -eq 2 ]]; then
        echo "Skipping number 2"
        continue # Skip to next iteration
    fi
    echo "i is equal to $i"
done

Save, make executable, run.

break

touch break_example.sh

Open break_example.sh and add:

#!/usr/bin/env bash

num=1
while [[ $num -lt 10 ]]; do
    if [[ $num -eq 5 ]]; then
        break # Exit loop
    fi
    ((num++))
done
echo "Loop completed. Num stopped at: $num"

Save, make executable, run.

For nested loops, break N exits N levels of loops. break 2 exits the current and the parent loop.

touch nested_break_example.sh

Open nested_break_example.sh and add:

#!/usr/bin/env bash

for (( a = 1; a < 3; a++ )); do # Outer loop
    echo "Outer loop: $a"
    for (( b = 1; b < 5; b++ )); do # Inner loop
        if [[ $b -gt 2 ]]; then
            echo "  Breaking inner and outer loop at b=$b"
            break 2 # Exits both loops
        fi
        echo "  Inner loop: $b"
    done
done
echo "All loops finished."

Save, make executable, run.