How to Use For loops in Bash

Introduction

for loops in Bash execute commands by iterating through a list of items repeatedly before generating a final result. You can use for loops to process files and directories or even to perform batch operations such as automatic backups.

This article explains how to use for loops in Bash.

for Loop Bash Syntax

A for loop statement executes a specific set of commands for each item in a list. Use for loops to iterate over a finite collection of items, such as files or strings using the following syntax.

for variable in list
do
    # Commands to execute for each item in the list
done

In the above for loop structure, the variable takes each value from the list and executes the commands between the do and done body section.

• for: Starts the loop.
• variable: Stores the current item from the list during each iteration.
• in: Specifies the list of items to iterate over.
• list: Sets the list of items.
• do: Marks the start of the loop's body and executes the specified commands.
• done: Marks the end of the loop.

Create for Loops in Bash

for loops work with variables or lists. The loop executes once for each item in the list. A for loop statement stops when it iterates over all items in the list. For example, if a list contains three files, the for loop runs three times before stopping. Follow the steps below to create a for loop statement.

1. Create a for loop that reads data from a variable and iterates the list to output a basic message using the echo command.

   bash Copy

   #!/bin/bash
   
   SERVERS="Server-1 Server-2 Server-3"
   for S in $SERVERS; do
     echo "Updating the packages on: $S"
   done
   

   Explain Code

   Output:

   Updating the packages on: Server-1
   Updating the packages on: Server-2
   Updating the packages on: Server-3

   The above script uses a for loop to iterate over three server names and outputs the Updating the packages on: message using the current list item's value. For example, on the first iteration, the loop outputs the Updating the packages on: Server-1 message.

2. Create a for loop that iterates over a range of numbers.

   bash Copy

   #!/bin/bash
   
   for value in {1..20}
   do
       echo "Number: $value"
   done
   

   Explain Code

   Output:

   Number: 1
   Number: 2
   Number: 3
   Number: 4
   Number: 5
   Number: 6
   Number: 7
   Number: 8
   Number: 9
   Number: 10
   Number: 11
   Number: 12
   Number: 13
   Number: 14
   Number: 15
   Number: 16
   Number: 17
   Number: 18
   Number: 19
   Number: 20

   The above script outputs a list of numbers from 1 to 20 before stopping. The loop updates the value on each iteration with a new number in the brace expansion list. This syntax is useful when processing a list of numbers, files, or repeating tasks in a loop.

3. Create a for loop in bash script that iterates over multiple files in a directory such as /var/www/html and updates the file permissions to 755.

   bash Copy

   #!/bin/bash
   
   for file in /var/www/html/*
   do
       sudo chmod 755 "$file"
       echo "Updated permissions for: $file"
   done
   

   Explain Code

   Output:

   Updated permissions for: /var/www/html/index.html
   Updated permissions for: /var/www/html/index.php

   The above script sets a new value for the file variable by retrieving the list of files from the /var/www/html directory. The script then updates the permissions of each file to 755 and outputs the Updated permissions for: message. For example, if an index.html file exists in the directory, the script sets its permission mode to 755 and outputs the Updated permissions for: index.html message and proceeds to execute the next file in the iteration.

Create Infinite for Loops

Infinite for loops in Bash are control structures that run continously unless an external condition interrupts the execution flow. Infinite loops are suitable for indefinite tasks such as real-time server monitoring, automatic background processes, and applications that require continuous updates.

#!/bin/bash

for (( ; ; ))
do
   echo "This is an Infinite loop [ use CTRL+C to stop it]"
done

Output:

This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This is an Infinite loop [ use CTRL+C to stop it]
This i^C

The above for loop in bash script creates an infinite loop that executes the echo command continuously and outputs the This is an Infinite loop [ use CTRL+C to stop it] message. The for (( ; ; )) declaration defines the start of an infinite loop.

Create Nested for Loops

Nested for loops define advanced functionalities by allowing you to place a for loop statement inside another loop. Nested loops are useful when iterating multiple data dimensions to process a combination of items.

#!/bin/bash

for server_id in A B C; do
    for app in apache mysql php; do
        echo "Server: $server_id can run the $app LAMP package"
    done
done

Output:

Server: A can run the apache LAMP package
Server: A can run the mysql LAMP package
Server: A can run the php LAMP package
Server: B can run the apache LAMP package
Server: B can run the mysql LAMP package
Server: B can run the php LAMP package
Server: C can run the apache LAMP package
Server: C can run the mysql LAMP package
Server: C can run the php LAMP package

The above script uses a nested for loop to iterate over two lists and output a result. The variable $server takes a new value from the outer loop list A B C on each iteration while the variable $app takes a new result from the inner loop list apache mysql php. The output combines the current inner loop and outer loop values to output a message. For example, the first iteration outputs the Server: A can run the apache LAMP package message.

Use Break in for Loops

A break exits a for loop statement prematurely after meeting a specific condition to skip the remaining iterations. For example, the following for loop statement iterates through files in the /etc directory and uses a conditional if statement to test if a list item's value matches the timezone to stop the loop.

#!/bin/bash

for file in /etc/*; do
    if [[ "$file" == "/etc/timezone" ]];
     then
        echo "$file is available, stopping the loop"
        break
    fi
done

Output:

/etc/timezone is available, stopping the loop

Use Continue in for loops.

A continue statement in a for loop skips any remaining commands in the current iteration and runs the next iteration in the loop. For example, the following for loop iterates through numbers 1 to 20 and uses a conditional if statement to test if the current value is equal to 5. If the condition is true, it moves on to the next iteration without executing any other commands.

#!/bin/bash

for value in {1..20}
do
    if [[ $value -eq 5 ]] 
    then
        echo "skipping the number 5"
        continue
    fi
    echo "The current value is equal to $value"
done

Output:

The current value is equal to 1
The current value is equal to 2
The current value is equal to 3
The current value is equal to 4
skipping the number 5
The current value is equal to 6
The current value is equal to 7
The current value is equal to 8
The current value is equal to 9

Use Command Substitution in for Loops

In a for loop statement, you can use command substitution to store and redirect the output of a specific command to a new variable for further processing. For example, when working with files and directories. The following for loop iterates through whitespace-separated entries in the /var/log/syslog file and outputs a Log entry: message with each entry.

#!/bin/bash

for log in $(cat /var/log/syslog); do
    echo "Log entry: $log"
done

Output:

Log entry: Main
Log entry: User
Log entry: Target.
Log entry: Sep
Log entry: 15
Log entry: 23:37:20
Log entry: Lets-Connect
Log entry: systemd[803616]:
Log entry: Startup
Log entry: finished
Log entry: in
Log entry: 137ms.

Use Arrays in for Loops

You can iterate through array elements using for loops. In Bash, an array is an indexed collection of values you can expand. For example, the following for loop expands the app_names array and prints each element.

#!/bin/bash

app_names=("apache" "mysql" "php" "nginx")

for app in "${app_names[@]}"
do
    echo "The application name is: $app"
done

Output:

The application name is: apache
The application name is: mysql
The application name is: php
The application name is: nginx

Conclusion

In this article you used for loops in Bash to iterate over lists. You have also run infinite loops, nested loops, break statements, continue statements, command substitution, and expanded arrays using for loops. Loops automate repetitive tasks such as monitoring files or filtering results in a large list.