Syntax of while loop:
do
commands
done
The starting and ending block of the while loop is defined by do and done keywords in the bash script. The termination condition is defined at the starting of the loop. Open a text editor to write a bash script and test the following while loop examples.
Example-1: Iterate the loop for a fixed number of times
Create a bash file named while1.sh with the following content. Here, the loop will iterate 5 times and print the counter value in each iteration.
# Initialize the counter
n=1
# Iterate the loop for 5 times
while [ $n -le 5 ]
do
# Print the value of n in each iteration
echo "Running $n time"
# Increment the value of n by 1
(( n++ ))
done
Output:
The following output will appear after executing the above script.
Example-2: Using break statement for conditional exit
the break statement is used to exit from the loop early based on a particular condition. Create a bash file named while2.sh with the following code. Here, the loop is defined to iterate 10 times, but the iteration will be stopped when the counter value is 6.
# Initialize the counter
n=1
# Iterate the loop for 10 times
while [ $n -le 10 ]
do
# Check the value of n
if [ $n == 6 ]
then
echo "terminated"
break
fi
# Print the current value of n
echo "Position: $n"
# Increment the value of n by 1
(( n++ ))
done
Output:
The following output will appear after executing the above script.
Example-3: Using continue statement to omit particular step
Create a bash file named while3.sh with the following code. In this example, the loop will iterate for 5 times, but it will not print all 5 positions. When the loop iterates for the 3rd time, the continue statement will be executed, and the loop will go for the next iteration without printing the text of the 3rd position.
# Initialize the counter
n=0
# Iterate the loop for 5 times
while [ $n -le 5 ]
do
# Increment the value of n by 1
(( n++ ))
# Check the value of n
if [ $n == 3 ]
then
continue
fi
# Print the current value of n
echo "Position: $n"
done
Output:
The following output will appear after executing the above script.
Example-4: Read the command-line argument with options
Create a bash file named while4.sh with the following code. Here, the loop is used to read the command-line arguments with options. The script will print the formatted argument values after the execution if the three-argument values pass with the valid option.
# Read the command-line arguments values with option using loop
while getopts n:a:e: OPT
do
case "${OPT}"
in
n) name=${OPTARG};;
a) address=${OPTARG};;
e) email=${OPTARG};;
*) echo "Invalid option"
exit 1;;
esac
done
# Print the argument values
printf "Name:$name\nAddress:$address\nEmail:$email\n"
Output:
The following output will appear after executing the above script.
Example-5: Read file line by line
Create a bash file named while5.sh with the following code. Here, a filename will be given in the first command-line argument at the execution time. If the file exists, then the content of the file will be printed line by line using the loop; otherwise, an error message will be printed.
# Check the command-line argument value is given or not
if [ $# -gt 0 ]; then
# Assign the filename from comand-line argument value
filename=$1
# Read file line by line
while read line; do
# Print each line
echo $line
done < $filename
else
# Print message if no argument is provided
echo "Argument value is missing."
fi
Output:
The following output will appear after executing the above script.
Example-6: Write content into a file
Create a bash file named while6.sh with the following code. Here, the filename will be taken from the user in which the text content will be written. The user has to type Ctrl+D after typing the content of the file.
echo -n "Enter the filename to create: "
# Take the filename that will be created
read filename
# Read the content of the file from the terminal
while read line
do
echo $line >> $filename
done
Output:
The following output will appear after executing the above script.
Example-7: Creating an infinite loop
Sometimes, it is required to declare an infinite loop for various programming purposes. Create a bash file named while7.sh and test the code of the infinite loop. No termination condition is set for the loop in this example. This type of loop is called an infinite loop. Here, an exit statement is used to quit from the infinite loop. So, this loop will be iterated 10 times, and when the iteration value becomes equal to 10, the exit statement will execute for exiting from the infinite loop.
# Initialize the counter
n=1
# Declare an infinite loop
while :
do
printf "The current value of n=$n\n"
if [ $n == 3 ]
then
echo "good"
elif [ $n == 5 ]
then
echo "bad"
elif [ $n == 7 ]
then
echo "ugly"
elif [ $n == 10 ]
then
exit 0
fi
# Increment the value of n by 1
((n++))
done
# Take the filename that will be created
read filename
# Read the content of the file from the terminal
while read line
do
echo $line >> $filename
done
Output:
The following output will appear after executing the above script.
Example-8: Using C-style while loop
Create a bash file named while8.sh with the following code. Here, the while loop has been declared in a c-style format that will iterate 5 times by incrementing the counter value by 10.
# Initialize the counter
n=5
# Define the while in C-style
while((n <= 50))
do
echo $n
# Increment counter by 10
((n=n+10))
done
Output:
The following output will appear after executing the above script.
Conclusion:
Different uses of the while loop have been explained in this tutorial by using multiple examples. I hope the bash user will be able to use this loop properly in their script after practicing these examples.
