How To Create Files Of A Certain Size In Linux

A while ago, I had 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 a file to be exactly 100 MB in size. After a bit of web search, I found out how to create files of a certain size in Linux and 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, using truncate command, 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 truncate man pages.

$ man truncate

2. Create files of a particular 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 fallocate 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 head command.

$ man head

4. Create 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 particular 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 dd command man pages.

$ man dd

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