Mastering Loops in Bash: `for`, `while`, and `until`

Loops in Bash are essential for automating repetitive tasks, iterating through lists, or executing commands multiple times. Bash provides three primary types of loops: for, while, and until. Each has its own use cases and syntax.

1. for Loop

The for loop in Bash is used to iterate over a list of items (such as numbers, files, or strings) and execute a block of code for each item.

Syntax:

for variable in list
do
  # Commands to execute
done

Example 1: Iterating Over a List of Items

for fruit in apple banana cherry
do
  echo "I love $fruit"
done

Output:

I love apple
I love banana
I love cherry

Example 2: Iterating Over a Range of Numbers (using {})

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

Output:

Number 1
Number 2
Number 3
Number 4
Number 5

Example 3: Iterating with Step Size

You can specify a step size when iterating over a range using the seq command or a specific step in the {} range.

for i in {1..10..2}
do
  echo "Odd number: $i"
done

Output:

Odd number: 1
Odd number: 3
Odd number: 5
Odd number: 7
Odd number: 9

Alternatively, using seq:

for i in $(seq 1 2 10)
do
  echo "Odd number: $i"
done

2. while Loop

The while loop runs as long as a given condition is true. It is useful when you don't know how many times you need to iterate, but you have a condition to check before continuing the loop.

Syntax:

while condition
do
  # Commands to execute
done

Example 1: Basic while Loop

count=1
while [ $count -le 5 ]
do
  echo "Count is $count"
  ((count++))  # Increment count by 1
done

Output:

Count is 1
Count is 2
Count is 3
Count is 4
Count is 5

Example 2: Looping Until a Condition is Met

You can use a while loop to keep iterating as long as a condition is true (or until it's false).

count=5
while [ $count -gt 0 ]
do
  echo "Count is $count"
  ((count--))  # Decrement count by 1
done

Output:

Count is 5
Count is 4
Count is 3
Count is 2
Count is 1

3. until Loop

The until loop works similarly to the while loop, but it continues as long as the condition is false. It’s used when you want to execute commands until a certain condition becomes true.

Syntax:

until condition
do
  # Commands to execute
done

Example 1: Basic until Loop

count=1
until [ $count -gt 5 ]
do
  echo "Count is $count"
  ((count++))  # Increment count by 1
done

Output:

Count is 1
Count is 2
Count is 3
Count is 4
Count is 5

Example 2: Infinite until Loop (with a break)

You can also create an infinite until loop. This is often used with a break statement to stop the loop when a certain condition is met.

count=1
until [ $count -gt 5 ]
do
  echo "Count is $count"
  ((count++))
  if [ $count -eq 3 ]; then
    echo "Stopping at count 3"
    break
  fi
done

Output:

Count is 1
Count is 2
Count is 3
Stopping at count 3

4. Loop Control Statements

• break: Exits the loop prematurely.
• continue: Skips the rest of the current iteration and moves to the next one.

Example with break:

for i in {1..5}
do
  if [ $i -eq 3 ]; then
    echo "Breaking at $i"
    break
  fi
  echo "Number $i"
done

Output:

Number 1
Number 2
Breaking at 3

Example with continue:

for i in {1..5}
do
  if [ $i -eq 3 ]; then
    continue  # Skip the rest of the loop for i=3
  fi
  echo "Number $i"
done

Output:

Number 1
Number 2
Number 4
Number 5

5. Nested Loops

You can nest loops within each other to perform more complex tasks.

Example: Nested for Loops

for i in {1..3}
do
  for j in {1..2}
  do
    echo "i=$i, j=$j"
  done
done

Output:

i=1, j=1
i=1, j=2
i=2, j=1
i=2, j=2
i=3, j=1
i=3, j=2

Example: Nested while Loop

i=1
while [ $i -le 3 ]
do
  j=1
  while [ $j -le 2 ]
  do
    echo "i=$i, j=$j"
    ((j++))
  done
  ((i++))
done

Output:

i=1, j=1
i=1, j=2
i=2, j=1
i=2, j=2
i=3, j=1
i=3, j=2

Summary of Loops in Bash:

1. for loop: Iterates over a list of items (or range) and executes commands for each item.

   • Best for known iterations or ranges.

2. while loop: Executes commands as long as the condition is true.

   • Useful when you want to repeat something until a condition changes.

3. until loop: Executes commands until the condition becomes true.

   • Opposite of the while loop; it stops when the condition is true.

4. Loop control: Use break to exit early or continue to skip the current iteration.

By mastering these loops and their variations, you'll be able to automate a wide range of tasks in Bash effectively!