Loops are one of the fundamental concepts of programming languages. Loops are handy when you want to run a series of commands a number of times until a particular condition is met.
This tutorial covers the basics of
while loops in Bash. We’ll also show you how to use the
continue statements to alter the flow of a loop.
while loop is used to performs a given set of commands an unknown number of times as long as the given condition evaluates to true.
while loop takes the following form:
while [CONDITION] do [COMMANDS] done
while statement starts with the
while keyword, followed by the conditional expression.
The condition is evaluated before executing the commands. If the condition evaluates to true, commands are executed. Otherwise, if the condition evaluates to false, the loop is terminated, and the program control will be passed to the command that follows.
In the example below, on each iteration, the current value of the variable
i is printed and incremented by one.
i=0 while [ $i -le 2 ] do echo Number: $i ((i++)) done
Tue loop iterates as long as
i is less or equal than two. It will produce the following output:
Number: 0 Number: 1 Number: 2
An infinite loop is a loop that repeats indefinitely and never terminates. If the condition always evaluates to true, you get an infinite loop.
In the following example, we are using the built-in command
: to create an infinite loop.
: always returns true. You can also use the
true built-in or any other statement that always returns true.
while : do echo "Press <CTRL+C> to exit." sleep 1 done
while loop above will run indefinitely. You can terminate the loop by pressing
Here is a single-line equivalent:
while :; do echo 'Press <CTRL+C> to exit.'; sleep 1; done
Read a File Line By Line
One of the most common usages of the
while loop is to read a file, data stream, or variable line by line.
Here is an example that reads the
/etc/passwd file line by line and prints each line:
file=/etc/passwd while read -r line; do echo $line done < "$file"
Instead of controlling the
while loop with a condition, we are using input redirection (
< "$file") to pass a file to the
read command, which controls the loop. The
while loop will run until the last line is read.
When reading file line by line, always use
read with the
-r option to prevent backslash from acting as an escape character.
By default, the
read command trims the leading/trailing whitespace characters (spaces and tabs). Use the
IFS= option before
read to prevent this behavior:
file=/etc/passwd while IFS= read -r line; do echo $line done < "$file"
continue statements can be used to control the while loop execution.
break statement terminates the current loop and passes program control to the command that follows the terminated loop. It is usually used to terminate the loop when a certain condition is met.
In the following example, the execution of the loop will be interrupted once the current iterated item is equal to
i=0 while [ $i -lt 5 ] do echo "Number: $i" ((i++)) if [[ "$i" == '2' ]]; then break fi done echo 'All Done!'
Number: 0 Number: 1 All Done!
continue statement exits the current iteration of a loop and passes program control to the next iteration of the loop.
In the following below, once the current iterated item is equal to
continue statement will cause execution to return to the beginning of the loop and to continue with the next iteration.
i=0 while [ $i -lt 5 ] do ((i++)) if [[ "$i" == '2' ]]; then continue fi echo "Number: $i" done echo 'All Done!'
Number: 1 Number: 3 Number: 4 Number: 5 All Done!
while loop repeatedly executes a given set of commands as long as a condition is true.
If you have any questions or feedback, feel free to leave a comment.