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.

```bash
for var in ${list}
do
    your_commands
done
```

Example: Loop through a list of users.

```bash
touch for_users.sh
```

Open `for_users.sh` and add:

```bash
#!/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:

```bash
chmod +x for_users.sh
./for_users.sh
```

Output:
```
Dev Team Six
softwareshinobi
troy
```

Loop through a range of numbers:

```bash
touch for_numbers.sh
```

Open `for_numbers.sh` and add:

```bash
#!/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.

```bash
while [[ your_condition ]]
do
    your_commands
done
```

Example: Counter from 1 to 10.

```bash
touch while_counter.sh
```

Open `while_counter.sh` and add:

```bash
#!/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.

```bash
touch while_input.sh
```

Open `while_input.sh` and add:

```bash
#!/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.

```bash
until [[ your_condition ]]
do
    your_commands
done
```

Example: Counter from 1 to 10.

```bash
touch until_loop.sh
```

Open `until_loop.sh` and add:

```bash
#!/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`

```bash
touch continue_example.sh
```

Open `continue_example.sh` and add:

```bash
#!/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`

```bash
touch break_example.sh
```

Open `break_example.sh` and add:

```bash
#!/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.

```bash
touch nested_break_example.sh
```

Open `nested_break_example.sh` and add:

```bash
#!/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.