Bash while Loop
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Loops are one of the fundamental concepts of programming languages. Loops are handy when you want to run a series of commands number of times until a particular condition is met.
This tutorial explains the basics of while loops in Bash as well as the break and continue statements to alter the flow of a loop.
Bash while Loop
The 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.
The Bash while loop takes the following form:
while [CONDITION] do [COMMANDS] done
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 will be terminated and the program control will be passed to the command that follows.
In the example below, on each iteration the loop prints the current value of the variable
i and increments the variable 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
Infinite while Loop
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
: which always returns true to create an infinite loop. You can also use the
true built-in command or any other statement that always returns true.
while : do echo "Press <CTRL+C> to exit." sleep 1 done
The 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.
In the following example the while loop will read the
/etc/passwd file line by line and print 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 which is the command that 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 to act as an escape character.
By default the
read command trims the leading/trailing whitespace characters (spaces and tabs). Use the
IFS= option before the
read command to prevent this behavior
file=/etc/passwd while IFS= read -r line; do echo $line done < "$file"
Break and Continue Statements
The break and continue statements can be used to control the while loop execution.
The 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!
The 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
2 the 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!
By now you should have a good understanding of how to use the bash while loop.
If you have any questions or feedback, feel free to leave a comment.