bash scripting
Linux

Bash Scripting Part2 – For and While Loops With Examples

In the previous post, we talked about how to write a bash script, and we saw how bash scripting is awesome. In this post, we will look at the for command, while command, and how to make loops to iterate over a series of values.

for Command

The for command enables you to perform a loop on a list of items. This is often the fundamental format of the for command.

for myvar in vars

do

Code Here

done

In every loop, the variable myvar holds one of the values of the list. The loop iterates until the list is finished.

Iterating Over Simple Values

You can iterate over simple values like this:

Check the results:

bash scripting for loop

Iterating Over Complex Values

Your list maybe contains a comma or two words, and you want to deal with them as one item on the list.

Check the following example:

We quote our strings with double quotations.

We play nice till now, we always do. Just keep reading and practicing.

bash scripting compelx for loop

Command Substitution

By using command substitution using this format $(Linux command) you can store the result in a variable for later use.

Here we get the file content using cat command. Notice that our file contains one word per line, not separated by spaces.

bash scripting loop from command

Here we get the content of the file using command substitution then iterate over the result, assuming that each line has one word.

What about having spaces in one of these lines?

In this case, every word will be considered a field. You need to tell the shell to consider new lines as a separator instead of spaces.

The Field Separator

By default, the following characters treated as fields.

  • Space
  • Tab
  • newline

If your text includes any of these characters, the shell will assume it’s a new field.

Well, you can change the internal field separator or IFS environment variable. like this:

IFS=$'\n'

It will consider new lines as a separator instead of spaces.

You got it. Bash scripting is easy.

bash scripting passwd file

The separator is colons in /etc/passwd file which contains the user’s information, you can assign it like this:

IFS=:

Bash scripting is awesome, right?

Iterating Over Directory Files

If you want to list the files in /home directory, you can use the for loop like this:

From the previous post, you should know the if statement and how to check for files and folders, so if you don’t know, I recommend you to review it bash script step by step.

bash scripting directory iteration

Here we use wildcard character which is the asterisk * and this is called in bash scripting file globbing which means All files with all names.

Notice that in the if statements here we quote our variables with quotations because maybe the file or the folder name contains spaces.

As you see the result, all files and directories in that folder are listed.

for Command C-Style

If you know C language, you may find that the for loop here is some weird because you are familiar with this syntax:

for (var= 0; var < 5; var++)

{

printf(“number is %d\n”, var);

}

Well, you can use the same syntax but with a little difference, here’s the syntax.

for (( variable = start ; condition ; iteration step))

So it looks like this:

for (( var = 1; var < 5; var++ ))

And this is an example:

And this is the output:

bash scripting c-style

The while Command

The for loop is not the only way for looping in bash scripting. The while loop does the same job but it checks for a condition before every iteration.

The while loop command takes the following structure:

while condition

do

commands

done

and here is an example:

The script is simple; it starts with the while command to check if number is greater than zero, then the loop will run and the number value will be decreased every time by 1 and on every loop iteration it will print the value of number, Once the number value is zero the loop will exit.

bash scripting while loop

If we don’t decrease the value of var1, it will be the same value and the loop will be infinite.

Nesting Loops

You can type loops inside loops. This is called the nested loop.

Here’s an example of nested loops:

The outer loop hits first, then goes into the internal loop and completes it and go back to the outer loop and so on.

bash scripting nested loops

Iterate Over File Content

This is the most common usage for the for loop in bash scripting.

We can iterate over file content, for example, iterate over /etc/passwd file and see the output:

Here we have two loops, the first loop iterate over the lines of the file and the separator is the newline, the second iteration is over the words on the line itself and the separator is the colon :

bash scirpting file data

You can apply this idea when you have a CSV or any comma separated values file. The idea is the same; you just have to change the separator to fit your needs.

Controlling the Loop

Maybe after the loop starts you want to stop at a specific value, will you wait until the loop is finished? Of course no, there are two commands help us in this:

  • break command
  • continue command

The break Command

The break command is used to exit from any loop, like the while and the until loops.

The loop runs until it reaches 14 then the break command exits the loop.

bash scirpting break command

And the same for the while loop:

The break command exits the while loop and that happens when the execution reaches the if statement.

bash scripting break while

The continue command

You can use the continue command to stop executing the remaining commands inside a loop without exiting the loop.

Check the following example:

When the if condition is true, the continue command runs each iteration, and lines after the continue command never run until the condition becomes false.

bash scripting continue command

Redirecting the Loop Output

You can use the done command to send the loop output to a file like this:

The shell creates the file myfile.txt and the output is redirected to the file, and if we check that file we will find our loop output inside it.

bash sciprintg process output

Let’s employ our bash scripting knowledge in something useful.

Useful Examples

Finding executables

To get all executable files on your system, you can iterate over the directories in the PATH variable. We discussed for loop and if statements and file separator so our toolset is ready. Let’s combine them together and make something useful.

This is just awesome. We were able to get all the executables on the system that we can run.

bash scripting finding executables

Now nothing stops you except your imagination.

I hope you learn a new thing or at least review your knowledge if you forget it. My last word for you, keep reading and practicing.

Thank you.

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  • clfapujc

    In the last example there is:

    for folder in $PATH
    How does the script know that we are looking for actual folders. We never used -d to specify we wanted directories.

    • When we use the asterisk * it means all files and directories in that folder.
      so we get files and directories BOTH of them without filtering .
      the filtration to get the folders is on the second if statement which is.

      for file in $folder/*

      then we search for the executable with -x

      • clfapujc

        Now I get it. Thanks.
        For a nearly total noob it is confusing 😛

    • Amani Hamis

      If I understood your question: I think the $PATH global variable only contains a list of folders in which the executables are stored, separated by “:”. To see these folders on your system just type echo $PATH. On my system, it returns the string “/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin”
      None of those can be a file, so he didn’t need to filter.

      • Great comment really.
        Thanks for contribution.

        • Amani Hamis

          GREAT articles btw! I’m learning SO MUCH!
          I started at Linux file system->Main Linux Commands->Main Linux Commands (Part 2) -> Linux Environment Variables -> Linux Command Line Tricks -> Bash Script Step-by-Step ->Bash Scripting Part 2 -> Part 3 where I am currently 🙂
          Thanks, and don’t stop delivering!

          • Great to know that.
            I do my best to post quality content.
            Hope the community love it.
            Regards.

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