Table of contents
- Introduction
- Syntax for While and Until loop
- How to use integer conditionals with while and until loop
- How to use while loop to read lines from a file and iterate through it
- How to use continue and the Break statements with while and until loop
- Special options with the break and the continue statement
Introduction
In this article we will be covering in detail on how to use the while and until loop on your bash scripts with practical examples.
While loop is used to execute set of command for ‘n’ number of times as long as the condition is true and the loop exits when the condition is false
Until loop is used to execute a set of commands for ‘n’ number of times as long as the condition is false and the loop exits when the condition is true. We can say it is a reverse of what while loop does
You might be interested in understanding different conditionals we can use with the While and until loop. Our article on Bash conditionals will explain the different conditionals we can consider to use with while loop and until loop. Please click the below link to navigate to that article.
Module-4 How to use conditionals in bash scripting
Syntax for While and Until loop
Following syntax can be used for while loop in Bash
while [condition should be true ]
do
# (command statements)
done
Following syntax can be used for until loop in Bash
until [condition should be false ]
do
# (command statements)
done
How to use integer conditionals with while and until loop
Integer Conditionals with While loop
In this example , we are creating a variable “number=1” then we are entering a while loop as long as we satisfy the condition “number should be less than 3 “. When this condition is false we exit the while loop. But when the condition is true we are printing it on the screen and also incrementing the number by 1 for the next iteration
#script
[root@discoveringsystems while-until-loops]# cat basic-while-loop
#!/bin/bash
number=1
while [ $number -lt 3 ]
do
echo "Number is $number"
((number++))
done
#script execution
[root@discoveringsystems while-until-loops]# ./basic-while-loop
Number is 1
Number is 2
Integer conditionals with until loop
In this example , we are creating the integer variable “number=1” and looping through the value with increments until we hit the conditional to be false. If the condition is true then the until loop exits the loop
#script
[root@discoveringsystems while-until-loops]# cat basic-until-loop
#!/bin/bash
number=1
until [ $number -gt 3 ]
do
echo "Number is $number"
((number++))
done
#script execution
[root@discoveringsystems while-until-loops]# ./basic-until-loop
Number is 1
Number is 2
Number is 3
How to use while loop to read lines from a file and iterate through it
In this example we will see how to use read command along with the while loop to iterate through the lines in a text file provided as input
Following are the contents of the book.txt file , please note that there are some empty lines in between the lines
[root@discoveringsystems while-until-loops]# cat book.txt
this is the first line
this is the second line
this is the third line
The following script takes book.txt as input or the while loop , use the read command to read the contents line by line. While reading a line , it stores the line as a variable Parsed_text. Also I have added to iteration counter to check how many iterations were made while parsing through the book.txt , while reading through the empty lines it is counted as an iteration
#script
[root@discoveringsystems while-until-loops]# cat basic-while-loop
#!/bin/bash
iteration=0
while read Parsed_text
do
echo "$Parsed_text"
echo "Iteration $iteration"
((iteration++))
done < book.txt
While the script is executed we can see that each line was echoed along with their iteration count
#script execution
[root@discoveringsystems while-until-loops]# ./basic-while-loop
this is the first line
Iteration 0
Iteration 1
this is the second line
Iteration 2
Iteration 3
Iteration 4
Iteration 5
this is the third line
Iteration 6
The same script can be also modified in a way that it takes the piped inputs from other commands
In this example , the same script has been modified and this we are using the Cat command to read the same book.txt and piped it to provide that as input to the while loop and then the read command reads line by line and while loop iterates through it
#script
[root@discoveringsystems while-until-loops]# cat basic-while-loop
#!/bin/bash
iteration=0
cat book.txt | while read Parsed_text
do
echo "$Parsed_text"
echo "Iteration $iteration"
((iteration++))
done
#script execution
[root@discoveringsystems while-until-loops]# ./basic-while-loop
this is the first line
Iteration 0
Iteration 1
this is the second line
Iteration 2
Iteration 3
Iteration 4
Iteration 5
this is the third line
Iteration 6
How to use continue and the Break statements with while and until loop
Break statement is used to terminate a loop when that statement is hit due to matching any conditionals we incorporate in the same loop iteration
Continue statement is used to skip the remaining statements part of that loop iteration and move on to the next iteration. To use the continue statement we can use the conditionals part of the loop iteration and we can hit it if the condition is true to the iteration variable or item
How to use break statement with while loop
In the below example ,we were initializing the count to be 0 and it enters the While loop. As long as the condition is true we do the iteration. Then we used the break statement in the while loop after we matched the condition. This takes us out of the while loop.
#script
[root@discoveringsystems while-until-loops]# cat basic-while-loop
#!/bin/bash
count=0
while [ $count -lt 5 ]
do
echo "$count"
if (($count==1)); then
echo "we broke the loop using break statement"
break
fi
((count++))
done
#script execution
[root@discoveringsystems while-until-loops]# ./basic-while-loop
0
1
we broke the loop using break statement
How to use break statement with until loop
In the below example ,we were initializing the count to be 0 and it enters the until loop. As long as the condition is False we do the iteration. Then we used the break statement in the until loop after we matched the condition. This takes us out of the until loop
#script
[root@discoveringsystems while-until-loops]# cat basic-until-loop
#!/bin/bash
count=0
until [ $count -gt 5 ]
do
echo "$count"
if (($count==2)); then
echo "we broke the loop using break statement"
break
fi
((count++))
done
#script execution
[root@discoveringsystems while-until-loops]# ./basic-until-loop
0
1
2
we broke the loop using break statement
How to use continue statement with while loop
In the below example ,we were initializing the count to be 0 and it enters the While loop. As long as the condition is true we do the iteration. Then we used the continue statement in the while loop after we matched the condition. This skips the further steps part of that iteration and takes us to the next iteration
#script
[root@discoveringsystems while-until-loops]# cat basic-while-loop
#!/bin/bash
count=0
while [ $count -lt 5 ]
do
((count++))
if (($count==3)); then
echo "we skipped the iteration using continue statement"
continue
fi
echo "$count"
done
#script execution
[root@discoveringsystems while-until-loops]# ./basic-while-loop
1
2
we skipped the iteration using continue statement
4
5
How to use continue statement with until loop
In the below example ,we were initializing the count to be 0 and it enters the until loop. As long as the condition is false we do the iteration. Then we used the continue statement in the until loop after we matched the condition. This skips the further steps part of that iteration and takes us to the next iteration
#script
[root@discoveringsystems while-until-loops]# cat basic-until-loop
#!/bin/bash
count=0
until [ $count -gt 5 ]
do
((count++))
if (($count==3)); then
echo "we skipped the iteration using continue statement"
continue
fi
echo "$count"
done
#script execution
[root@discoveringsystems while-until-loops]# ./basic-until-loop
1
2
we skipped the iteration using continue statement
4
5
6
Special options with the break and the continue statement
Both the break statement and the continue statement has features to help exit multiple nested while , for or until loops ,the syntax is as follows
continue [number of nested loops to skip ]
#Eg : continue 2
break [number of nested loops to break ]
#Eg: break 2
Lets see one practical example of using break 2 in a script. When we specify with 2 or more it will break out of the nested loop
If the bash script has nested while loop like the following , we can see that i made sure the condition matches for the inner while loop and made it to execute the break 2 , so it broke out of both the while loop and the script ended there
#script
[root@discoveringsystems while-until-loops]# cat basic-while-loop
#!/bin/bash
count=0
while [ $count -lt 5 ]
do
echo "$count"
((count++))
count_2=10
while [ $count_2 -lt 15 ]
do
echo "$count_2"
((count_2++))
if (($count_2==13)); then
echo "we broke the iteration using break statement"
break 2
fi
done
done
#script execution
[root@discoveringsystems while-until-loops]# ./basic-while-loop
0
10
11
12
we broke the iteration using break statement