Linux create file with file size

Linux / UNIX: Create Large 1GB Binary Image File With dd Command

1. fallocate command – Preallocate space to a file.
2. truncate command – Shrink or extend the size of a file to the specified size.
3. dd command – Convert and copy a file i.e. clone/create/overwrite images.
4. df command – Show free disk space.
5. du command – Show disk usage statistics.
6. ls command – List file size.

Tutorial details
Difficulty level	Easy
Root privileges	No
Requirements	fallocate or dd command
Est. reading time	3 minutes

fallocate command syntax

The basic syntax is:
fallocate -l Image_Size_Here /path/to/image.img
Let us see how to create large 1GB binary file with fallocate.

Creating a large file on a Linux using fallocate command

The following command will create 1G file:

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Verify new disk image with the ls command:
$ ls -lh test.img
Sample outputs:

You can use stat or du command to verify block allocation:

dd command syntax

Note : The following information only exists for older version of Linux and/or when fallocate command is not available. If possible use fallocate command only for creating binary images. dd command based method is considered as old and not recommended, but presented here for historical reasons only.

The basic syntax is:
dd if=/path/to/input of=/path/to/output [options] ## OR ##
dd if=/dev/zero of=/path/to/output.img [options] ## OR ##
dd if=/dev/zero of=YOUR-IMAGE-FILE-NAME-HERE bs=1 count=0 seek=Size-HERE

Creating an image file with dd command

First, make sure you’ve sufficient disk space to create a image file using dd:
$ df -H

To create 1MB file (1024kb), enter:
$ dd if=/dev/zero of=test.img bs=1024 count=0 seek=1024

You will get an empty files (also known as “sparse file”) of arbitrary size using above syntax. To create 10MB file , enter:
$ dd if=/dev/zero of=test.img bs=1024 count=0 seek=$[1024*10]

To create 100MB file , enter:
$ dd if=/dev/zero of=test.img bs=1024 count=0 seek=$[1024*100] $ ls -lh test.img

To create 1GB, file:
$ dd if=/dev/zero of=1g.img bs=1 count=0 seek=1G
Sample output:

Verify file size (note bs factor in original dd command):
$ ls -lh 1g.img
$ stat 1g.img
$ du -h 1g.im

dd tip: Create a file with a given size in Unix or Linux

If truncate command not available, try the following syntax:
dd if=/dev/zero of=/path/to/fiie.img bs=YOUR-FILE-SIZE-HERE count=1
In this example, create a file with 1G (on *BSD/OS X use 1g) size:
$ dd if=/dev/zero of=1g.bin bs=1G count=1
Here is what I see:

Verify file size/blocks:
$ stat 1g.bin
Verification outputs:

Summing up

You learned how to create large 1GB (or any size of your choice) binary image file with dd and other command on Linux or Unix-like systems. See man pages for more info:
man dd
man fallocate

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Comments on this entry are closed.

I don’t think this works correctly unless the “count” argument is changed to something greater than 0.

Try something like this:

dd if=/dev/zero of=10Gtest bs=1M count=10000

I don’t think you have read and tested this yourself.

If using “count=0”, as the article suggests, you will still have the file of the specified size.

However, the output will show something like “0+0 records in” and “0+0 records out”.

Then if you check the file size:

it should show the proper size.

It’s even easier, using the format that the commenter Indie suggest:

JW is correct. Without a count setting – its generating a 0 byte file.

ls -alh will show a file size; but compare that to df -h – and you’ll see it doesn’t actually consume any space on disk

Bullshit the 10g one above doesnt create the proper file size at all. Check with the du command it replys the file size is 0.

I think the commenter “Indie” has already clarified this in a previous comment (in agreement with the author).

@JW, @sonassi, @stfu_and_die
If you think you’re right, why don’t tell the author of the article to either remove or edit the article? or write your own article?

Indie August 26, 2011 at 3:02 pm

The ls command will tell you the size of the file while du will tell you the Disk Usage. When you create the empty disk image with the above command it doesn’t write any data so it doesn’t actually allocate any blocks to the file.

If you type “stat disk.img” you’ll see that it’s using 0 blocks and thus no disk space is being consumed by the file, it’s only when you then write data to it that the blocks will then be allocated.

The FAQ has been updated with new and accurate information. However, I kept dd command based method for historical reasons only. I recommend using fallocate command on Linux.

Hope this helps!

can I supress the output of dd command?

Yes, send it to /dev/null.

dd args >/dev/null
OR
dd args >/dev/null 2>&1

Slightly easier version is to use a suffix for the size instead of trying to do the maths.

dd if=/dev/zero of=disk.img bs=1 count=0 seek=1G

to create a 1G file.

Yes this is work but there is concern .

When do ls -lh disk.img file its showing as 1G size but when u run the command

du -h disk.img it shows 0 bytes of size…..

Which one we can believe…i dont know…please clarify if you know about this…

The ls command will tell you the size of the file while du will tell you the Disk Usage. When you create the empty disk image with the above command it doesn’t write any data so it doesn’t actually allocate any blocks to the file.

If you type “stat disk.img” you’ll see that it’s using 0 blocks and thus no disk space is being consumed by the file, it’s only when you then write data to it that the blocks will then be allocated.

I have 2 tb harddrive and i want to clone only 1 tb to my other drive?

what command should i do

Hi, the question is old – but I’m happy to provide a nice answer – maybe this basic advice will help other people as well.
This isn’t exactly possible with dd. If you’d only dd 1tb of the 2tb drive and you’re lucky to be able to mount it, you’ll end up seeing correct filenames – but files will be corrupted entirely.
I’d rather suggest you:
? format your second hard-drive
? look which files/directories are using how much space, recommended tool is ncdu
? create a list of files you want to copy and use rsync to copy the files

How to check if the created file is empty, is there a way to check that?

Using /dev/zero will create a file for you, but the free space seems to remain the same on the disk. If you’re looking to make a filler file as I am in /var/lib/mysql so when the database fills up, I still have some space to work with.. you’ll want to substitute /dev/zero to /dev/random or something like that.

Here’s my attempt at creating a 1G filler file.
df output before:
/dev/mapper/VG01-LV_mysql
30959612 27519608 3439984 89% /var/lib/mysql
command:
dd if=/dev/zero of=1gig-filler.img bs=1000 count=0 seek=$[1000*1000*1]
0+0 records in
0+0 records out
0 bytes (0 B) copied, 3e-05 seconds, 0.0 kB/s

file:
-rw——- 1 root root 1000000000 Feb 13 12:26 1gig-filler.img

df output after (space is slightly different because the db is being used):
/dev/mapper/VG01-LV_mysql
30959612 27521072 3438520 89% /var/lib/mysql

I don’t see any use for using /dev/zero in this scenario. I’ve used it to wipe a disk clean… dd if=/dev/zero of=/dev/sda will clear a disk, but this file doesn’t take up real space.

Actually it might be the count=0 part that is making it not use any real space.. I can’t seem to get df to show me the space actively being used even with /dev/random.. I know I’ve done this successfully before.

How to delete that test file that is being created?

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How To Create Files Of A Certain Size In Linux

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 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:

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

For more details about this command, refer truncate man pages.

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:

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:

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

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:

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

Refer man pages for further details about head command.

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 certain size as well.

To create a file with size 5MB, run:

Sample output:

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:

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

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.

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