How to Use While Loop in Bash: Syntax and Examples

In bash, a while loop repeatedly executes a block of code as long as the given condition is true. The syntax is while [ condition ]; do commands; done. This loop is useful for running commands multiple times until a condition changes.

Syntax

The basic syntax of a while loop in bash is:

• while [ condition ]: Checks the condition before each loop iteration.
• do: Starts the block of commands to run repeatedly.
• commands: The commands to execute while the condition is true.
• done: Ends the loop block.

The condition is usually a test command that returns true or false.

bash

while [ condition ]
do
  commands
 done

Example

This example counts from 1 to 5 using a while loop. It shows how the loop runs commands repeatedly while the condition is true.

bash

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

Output

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

Common Pitfalls

Common mistakes when using while loops in bash include:

• Forgetting spaces around square brackets in the condition, e.g., [ $count -le 5 ] must have spaces.
• Not updating the variable inside the loop, causing an infinite loop.
• Using incorrect test operators or syntax.

Here is a wrong and right example:

bash

# Wrong: missing spaces and no increment (infinite loop)
count=1
while [$count -le 5]
do
  echo $count
done

# Right: spaces and increment
count=1
while [ $count -le 5 ]
do
  echo $count
  ((count++))
done

Quick Reference

Element	Description	Example
while [ condition ]	Loop runs while condition is true	while [ $x -lt 10 ]
do	Start commands block	do
commands	Commands to execute each loop	echo $x; ((x++))
done	End loop block	done

Key Takeaways

Use spaces around brackets in the condition: [ condition ].

Update variables inside the loop to avoid infinite loops.

The while loop runs commands repeatedly while the condition is true.

Use (( )) for arithmetic operations inside the loop.

Always close the loop with done.