Bash Loops in Bash

A loop runs a block of code multiple times without writing it repeatedly. Instead of typing the same command 100 times, a loop handles it in a few lines. Bash provides three main types of loops: for, while, and until.

for Loop

A for loop iterates over a list of values. Each item in the list takes a turn being assigned to the loop variable, and the code block runs once per item.

Syntax

for variable in list
do
  # commands
done

Example – Loop Over a List of Words

#!/bin/bash
for fruit in Apple Banana Mango Orange
do
  echo "Fruit: $fruit"
done

Output:

Fruit: Apple
Fruit: Banana
Fruit: Mango
Fruit: Orange

Example – Loop Over a Number Range

#!/bin/bash
for num in {1..5}
do
  echo "Number: $num"
done

Output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

for Loop with Step Value

#!/bin/bash
for num in {2..10..2}
do
  echo $num
done

Output:

2
4
6
8
10

C-style for Loop

This style works exactly like a for loop in C or Java.

#!/bin/bash
for (( i=1; i<=5; i++ ))
do
  echo "Count: $i"
done

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

while Loop

A while loop keeps running as long as a condition remains true. It checks the condition before every iteration.

Syntax

while [ condition ]
do
  # commands
done

Example – Countdown Timer

#!/bin/bash
count=5
while [ $count -gt 0 ]
do
  echo "Countdown: $count"
  ((count--))
done
echo "Launch!"

Output:

Countdown: 5
Countdown: 4
Countdown: 3
Countdown: 2
Countdown: 1
Launch!

until Loop

An until loop is the opposite of while. It runs as long as the condition is false. It stops when the condition becomes true.

Syntax

until [ condition ]
do
  # commands
done

Example

#!/bin/bash
num=1
until [ $num -gt 5 ]
do
  echo "Number: $num"
  ((num++))
done

Output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

Loop Type Comparison

┌──────────────────────────────────────────────────────┐
│              Loop Type Summary                       │
│                                                      │
│  for    → Use when the number of iterations is known │
│           e.g., loop 5 times, loop over a list       │
│                                                      │
│  while  → Use when iterations depend on a condition  │
│           e.g., keep running until user types quit   │
│                                                      │
│  until  → Use when stopping on a TRUE condition      │
│           e.g., wait until a file appears            │
└──────────────────────────────────────────────────────┘

Loop Control – break and continue

break – Exit the Loop Early

The break statement immediately exits the loop, even if the condition is still true.

#!/bin/bash
for num in {1..10}
do
  if [ $num -eq 6 ]; then
    echo "Stopping at $num"
    break
  fi
  echo "Number: $num"
done

Output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5
Stopping at 6

continue – Skip the Current Iteration

The continue statement skips the rest of the current iteration and moves to the next one.

#!/bin/bash
for num in {1..6}
do
  if [ $num -eq 3 ]; then
    echo "Skipping $num"
    continue
  fi
  echo "Number: $num"
done

Output:

Number: 1
Number: 2
Skipping 3
Number: 4
Number: 5
Number: 6

Looping Over Files

#!/bin/bash
for file in /home/john/documents/*.txt
do
  echo "Found file: $file"
done

Infinite Loop with while true

An infinite loop runs forever until a break or Ctrl+C stops it. This pattern is common in scripts that wait for user input or monitor a process.

#!/bin/bash
while true
do
  read -p "Type 'quit' to exit: " input
  if [ "$input" = "quit" ]; then
    echo "Goodbye!"
    break
  fi
  echo "You typed: $input"
done

Nested Loops – Loop Inside a Loop

#!/bin/bash
for row in 1 2 3
do
  for col in A B C
  do
    echo "Row $row, Col $col"
  done
done

Output:

Row 1, Col A
Row 1, Col B
Row 1, Col C
Row 2, Col A
Row 2, Col B
Row 2, Col C
Row 3, Col A
Row 3, Col B
Row 3, Col C

Practical Example – Multiplication Table

#!/bin/bash
read -p "Enter a number: " n
for i in {1..10}
do
  printf "%d x %2d = %d\n" $n $i $((n * i))
done

Output (for n=5):

5 x  1 = 5
5 x  2 = 10
5 x  3 = 15
...
5 x 10 = 50

Key Takeaways

• Use for to loop over a list or a known number of times.
• Use while to loop as long as a condition is true.
• Use until to loop until a condition becomes true.
• Use break to exit a loop and continue to skip the current iteration.
• The while true pattern creates an infinite loop that must be stopped with break or Ctrl+C.