Home Command line utilities How To Create Files Of A Certain Size In Linux
Create Files Of A Certain Size In Linux

How To Create Files Of A Certain Size In Linux

By sk
2298 Views

A while ago, I setup a local file server using PSiTransfer. While using my file server, I wanted to test the upload limit, maximum upload size, and download speed of the files/folders. For that purpose, I needed different size files. I have various size files in my hard drive. But, I want the file to be exactly 100 MB in size. After a bit of Google search and go through over some Linux forums, I found what I wanted. For those wondering, this brief guide explains how to create files of a certain size in Unix-like systems.

Create Files Of A Certain Size In Linux

There are few ways to create files with given size. I will show them all with practical examples.

All commands mentioned in this guide are part of GNU coreutils, so you don't have to install them. These commands comes pre-installed by default.

1. Create files of a certain size using "truncate" command

To create a specific size file, for example 5 MB, run:

$ truncate -s 5M ostechnix.txt

The above command will create a file called ostechnix.txt with size exactly 5MB.

For more details about this command, refer man pages.

$ man truncate

2. Create files of a certain size using "fallocate" command

The another command to create a particular size file is fallocate. Please note that you can only specify file sizes in bytes using fallocate command. To calculate the size for a specific file, for example 5MB, you need to do - 510241024=5242880. Clear? Good!

Now let us create a file of size 5MB using command:

$ fallocate -l 5242880 ostechnix.txt

As Luc Van Rompaey suggested in the comment section, with the bash shell, you can do inline arithmetic, so you won’t have to calculate how many bytes go into 5 MiB beforehand.

So, we can use this as shown below:

$ fallocate -l $((5*1024*1024)) ostechnix.txt

For more details about this command, I suggest you to go through the man pages.

$ man fallocate

3. Create files of a certain size using "head" command

We use head command to output the first part of files, right? Well, we can use this command to create a file of certain size too.

To create a file with 5 MB in size using head command, run:

$ head -c 5MB /dev/urandom > ostechnix.txt

The above command will create 5MB size file filled with random data. You can also create the file with 0s as shown below.

$ head -c 5MB /dev/zero > ostechnix.txt

Refer man pages for further details about this command.

$ man head

4. Ceate files of a certain size using "dd" command

We already knew we can convert and copy a file using dd command. We also use dd command to create a bootable disk. However, we can use this command to create files of certain size as well.

To create a file with size 5MB, run:

$ dd if=/dev/urandom of=ostechnix.txt bs=5MB count=1

Sample output:

1+0 records in
1+0 records out
5000000 bytes (5.0 MB, 4.8 MiB) copied, 0.0402477 s, 124 MB/s

The command will create ostechnix.txt file of size 5MB filed with some random data.

To create a file filled with 0s, you can use:

$ dd if=/dev/zero of=ostechnix.txt bs=5MB count=1

As usual, for details about this command, refer the man pages.

$ man dd

Recommended read:


And, that's all. You know now how to create a file with certain size. As you can see in the above examples, creating files of certain size is no big deal. Hope this helps.

Thanks for stopping by!

Help us to help you:

Have a Good day!!

You May Also Like

6 comments

Luc Van Rompaey July 14, 2017 - 7:36 pm

Just a sidenote: With the bash shell, you can do inline arithmetic, so you won’t have to calculate how many bytes go into 5 MiB beforehand.
Just do:
fallocate -l $(( 5 * 1024 * 1024 )) ostechnix.txt

Reply
SK July 15, 2017 - 11:40 am

Cool. Thank you. I didn’t know this.

Reply
AshkanV May 29, 2019 - 2:34 pm

I think its good to mention that the first 2 command only allocate file with the given size in the file system, which is means no IO overhead.
but the other two create an empty file and extending it by writing data in it, which means LOTS of IO overhead

By the way thanks for good and useful article.

Reply
sk May 29, 2019 - 5:21 pm

Thank you for your clarification.

Reply
Claudio Dobniewski February 6, 2020 - 9:25 pm

Thanks, is very useful info

Reply
Gergely Polonkai May 14, 2020 - 2:47 pm

It also worth noting that truncate creates a file that looks like that size, but doesn’t allocate space. If you want to create a fake block device or a swap file you can’t use it, as the kernel will tell you “it has holes”.

Reply

Leave a Comment

* By using this form you agree with the storage and handling of your data by this website.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

This website uses cookies to improve your experience. We'll assume you're ok with this, but you can opt-out if you wish. Accept Read More