How to Write Loop in Bash with Examples

How to make automation with bash for loop under Linux/UNIX operating system? How do I use break, continue and the parameter for loop control expression? How to act on files using for loop?

For loop is a very useful tool to solve many problems in the programming world and therefore we will solve some problems in the real world. In Linux we use loops via Bash, Python to make automation like password script, counting script. In this article, we will explain all of the kind of loops for Bash.

1) for loop

We can use For loop to read all elements in a list or part of them and displaying these elements on the screen.

Syntax of For loop

for variable in element1 element2 element3 Or <List>
do
commands
done

We will make a script to send greeting message for each day of the week, so we will use for loop to read each day and print it out. Create file welcome.sh with nano editor and paste code below.

  #!/bin/bash
  for day in Sunday Saturday Monday Tuesday Wednesday Thursday Friday
  do
     echo "Welcome $day day"
  done

Run the welcome.sh file with bash command.

$ bash welcome.sh

output
Welcome Sunday day
Welcome Saturday day
Welcome Monday day
Welcome Tuesday day
Welcome Wednesday day
Welcome Thursday day
Welcome Friday day

The for loop will take each item in the list respectively, and assign this item to the variable $day, after that execute the code between do and done then go back to the top, assign the next item in the list and repeat over.

We can use a range with for loop to put start point and end point.

  #!/bin/bash
  for i in {10..20}
  do
     echo "Welcome $i times"
  done

$ bash range.sh

output
Welcome 10 times
Welcome 11 times
Welcome 12 times
Welcome 13 times
Welcome 14 times
Welcome 15 times
Welcome 16 times
Welcome 17 times
Welcome 18 times
Welcome 19 times
Welcome 20 times

If we want to count even number from 1 to 20, we will add step for range.

  #!/bin/bash
  for i in {10..20..2}
  do
     echo "Welcome $i times"
  done

$ bash even.sh

output
Welcome 10 times
Welcome 12 times
Welcome 14 times
Welcome 16 times
Welcome 18 times
Welcome 20 times

Another type of using step with for loop.

  #!/bin/bash
  for ((i=1;i<=25;i+=5))
  do
      echo $i
  done

$ bash step5.sh

output
1
6
11
16
21

And for negative step we can use the following type.

  #!/bin/bash
  for ((i=25;i>=1;i-=5))
  do
      echo $i
  done

$ bash step-5.sh

output
25
20
15
10
5

Examples on For loop

1) Acting on files using for loop

Bash For loop is the best way when we are working on files.

#!/bin/bash
for file in ~/*.txt
do
    echo $file
done

$ bash find_txt.sh

output
/root/cat.txt
/root/echo.txt
/root/file.txt
/root/f.txt
/root/nano.txt
/root/printf.txt

2) One line For loop

we can execute For loop in one line, we will rename all *.txt files to remove the file extension.

$ for filename in *.txt; do mv "$filename" "${filename%.txt}"; done

Or in script

  !#/bin/bash
  for filename in *.txt
  do
    mv "$filename" "${filename%.txt}"
  done

3) Reading Command-line arguments

When we are executing For loop script, we can enter arguments.

  for myvalue in $*
  do
    echo "Argument: $myvalue"
  done

$ bash linoxide.sh I Love LinOxide

output
Argument: I
Argument: Love
Argument: LinOxide

4) Reading odd and even number

We will write script to read the even and the odd numbers.

for (( n=10; n<=15; n++ ))
do
  if (( $n%2==0 ))
  then
    echo "$n is even"
  else
    echo "$n is odd"
  fi
done

$ bash even_odd.sh

output
10 is even
11 is odd
12 is even
13 is odd
14 is even
15 is odd

2) while loop

While loop depend on the condition is true, if the condition is false the interpreter get out from the loop.

Syntax of while loop

while [ <some condition> ]do
<commands>
done

We will count from 10 to 20 and print out the results. So we will put a condition that the counter less than or equal 20.

  #!/bin/bash
  # Basic while loop
  counter=10
  while [ $counter -le 20 ]
  do
    echo Number : $counter
    ((counter++))
  done

$ bash while.sh

output
Number : 10
Number : 11
Number : 12
Number : 13
Number : 14
Number : 15
Number : 16
Number : 17
Number : 18
Number : 19
Number : 20

3) Until loop

Until loop like while loop but the interpreter excute the commands within it until the condition becomes true.

Syntax of until loop

until [ <some condition> ]do
<commands>
done

We will count from 10 to 20 and print out the results. So we will put a condition that the counter greater than or equal 20.

  #!/bin/bash
  # Basic Until loop
  counter=10
  until [ $counter -gt 20 ]
  do
    echo Number : $counter
    ((counter++))
  done

$ bash until.sh

output
Number : 10
Number : 11
Number : 12
Number : 13
Number : 14
Number : 15
Number : 16
Number : 17
Number : 18
Number : 19
Number : 20

4) Controlling loops

We can use Break or Continue to control loops.

Break statement

for Break statement, we can get out from the loop and no need to complete the loop when we use if statement inside the loop.

#!/bin/bash
# Basic loop use break
counter=10
until [ $counter -gt 20 ]
do
   echo Number : $counter
   if [ $counter -eq 15 ]
   then
       echo Done
       break
   fi
   ((counter++))
done

$ bash break.sh

output
Number : 10
Number : 11
Number : 12
Number : 13
Number : 14
Number : 15
Done

Continue statement

For Continue statement, we can go on the loop and no need to finish the loop when we are using if statement inside the loop.

#!/bin/sh

NUMS="1 2 3 4 5 6 7"

for NUM in $NUMS
do
   Q=`expr $NUM % 2`
   if [ $Q -eq 0 ]
   then
      echo "A number is an even number!!"
      continue
   fi
   echo "Found odd number"
done

$ bash continue.sh

output
Found odd number
A number is an even number!!
Found odd number
A number is an even number!!
Found odd number
A number is an even number!!
Found odd number

5) Select loop

Select loop like while and until loop but allows you to create a simple menu system.

Syntax of select loop

select var in <list>
do
<commands>
done

We will make three options and use select loop to choose from them.

  #!/bin/bash
  # select script

  PS3='Please enter your choice: '
  options=("Option 1" "Option 2" "Option 3" "Quit")
  select opt in "${options[@]}"
  do
      case $opt in
          "Option 1")
              echo "you choose choice 1"
              ;;
          "Option 2")
              echo "you choose choice 2"
              ;;
          "Option 3")
              echo "you choose choice $REPLY which is $opt"
              ;;
          "Quit")
              break
              ;;
          *) echo "invalid option $REPLY";;
      esac
  done

$ bash select.sh

output
1) Option 1
2) Option 2
3) Option 3
4) Quit
Please enter your choice: 1
you chose choice 1
Please enter your choice: 5
invalid option 5
Please enter your choice: 4

Conclusion

Bash For loop used in synchronization, making password, backup and etc...
Do while is same as while but the interpreter executes the first code without any conditions
Break statement is very important for getting out from the loop
Select statement is useful when we have many options

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