While and Until Loops in Shell

Summary: Execute code repeatedly using while and until.

When automating tasks or scripting in the shell, loops allow you to repeat actions efficiently. Among the most commonly used loops are the while and until constructs. Understanding their differences, syntax, and practical uses is essential for any shell scripter. In this article, we’ll explore how while and until loops work in shell scripts with real examples.

The while Loop

A while loop executes its body as long as a specified condition is true. It’s ideal when you want your script to repeat a block of code for an undetermined number of times, stopping once a condition is no longer satisfied.

Syntax

while [ condition ]; do
  # Commands to execute
done

Example: Countdown Timer

Let’s create a simple countdown timer that starts from 5 and stops at 0.

count=5
while [ $count -gt 0 ]; do
  echo "Countdown: $count"
  sleep 1
  count=$((count - 1))
done
echo "Blast off!"

How it works:

  • The loop checks if count is greater than 0.
  • It prints the current count, waits for 1 second, and decrements the count.
  • Once count reaches 0, the condition fails, and the loop exits.

Common Use Cases

  • Monitoring files or system status until a condition changes
  • Reading lines from a file
  • Waiting for user input or a process to complete

The until Loop

The until loop is nearly the opposite of the while loop. Its body executes repeatedly as long as the condition is false. It only stops when the condition becomes true.

Syntax

until [ condition ]; do
  # Commands to execute
done

Example: Waiting for a File to Exist

Suppose you want your script to wait until a specific file appears in a directory:

until [ -f /tmp/target-file.txt ]; do
  echo "Waiting for target-file.txt to be created..."
  sleep 2
done
echo "File found!"

How it works:

  • The -f test checks if the file exists.
  • As long as the file does not exist ([ -f ... ] is false), the script prints a message and waits.
  • When the file is created, the condition becomes true and the loop ends.

Common Use Cases

  • Waiting for a resource to appear or become available
  • Retrying commands until successful
  • Polling for changes in the system or environment

Comparison: When to Use Each Loop

Use This When You Want To...
while Keep looping WHILE a condition is true
until Keep looping UNTIL a condition is true

In essence, the two loops can often achieve the same outcomes with opposite conditions, but choosing the one that directly maps to your intent improves readability.

Advanced: Reading Files with while

A practical and popular usage is reading files line by line:

while IFS= read -r line; do
  echo "Line: $line"
done < myfile.txt
  • The loop reads each line from myfile.txt and processes it.
  • Using IFS= and -r prevents accidental trimming and backslash interpretation.

Good Practices

  • Avoid Infinite Loops: Ensure your condition can eventually become false (while) or true (until), or use a break statement inside the loop.
  • Quoting Variables: Always quote variables in test expressions to avoid issues with whitespace or empty values. 
    while [ "$input" != "q" ]; do
  # do something
done

Conclusion

while and until loops are foundational tools for automation in shell scripts. Remember:

  • Use while for repeating actions while a condition is true.
  • Use until for repeating actions until a condition becomes true.

Choosing the right loop for the job makes your script clearer and more robust. Happy scripting!

Further Reading:

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