Create a File of Specific Size in Linux

Sometimes, I would need to create files of size 1MB, 100MB, 250MB, 1.5GB, 10GB or 100GB. There are different quick ways to create files of varying sizes in Linux. Most of these steps work on macOS too.

How to create a file of a specific size in Linux, and macOS too

You may also have questions like this:

• How do I create a file of size 1GB?
• How do I create a 10GB file?
• What is the fastest way to create a 5GB file?
• Are there one-liner commands to create file of specific sizes?
• Do you have a Python script to create a 100GB file?

Create file of 150MB using dd

Using dd, this command will create a file bigfile.txt of size 150MB.

dd if=/dev/zero of=bigfile.txt bs=150M count=1

Output:

$ dd if=/dev/zero of=bigfile.txt bs=150M count=1
1+0 records in
1+0 records out
157286400 bytes (157 MB, 150 MiB) copied, 0.158732 s, 991 MB/s

You can also use this command:

dd if=/dev/zero of=bigfile.txt bs=15M count=10

Output:

$ dd if=/dev/zero of=bigfile.txt bs=15M count=10
10+0 records in
10+0 records out
157286400 bytes (157 MB, 150 MiB) copied, 0.147535 s, 1.1 GB/s

$ ls -alh bigfile.txt 
-rw-r--r-- 1 arul arul 150M Aug  4 23:49 bigfile.txt

Create file of 250MB using truncate

The truncate command can be used to extend or shrink a file to the desired size.

truncate -s 250M bigfile.txt

Output:

$ truncate -s 250M bigfile.txt

$ ls -alh bigfile.txt 
-rw-r--r-- 1 arul arul 250M Aug  4 23:52 bigfile.txt

Create file of 300MB using fallocate

Using fallocate, the disk space of a file is either preallocated or deallocated. This is done by allocating and marking blocks as uninitialized without actually writing to them. This way, the file is generated much faster than if it were created by writing data into it.

fallocate -l 300M bigfile.txt

Output:

$ fallocate -l 300M bigfile.txt

$ ls -alh bigfile.txt 
-rw-r--r-- 1 arul arul 300M Aug  4 23:56 bigfile.txt

Create file of 375MB using head

We can also create a file using the head command by reading the first few lines of any text given to it as an input, and writing the text to standard output.

head -c 375MB /dev/zero > bigfile.txt

Output:

$ head -c 375MB /dev/zero > bigfile.txt

$ ls -alh bigfile.txt 
-rw-r--r-- 1 arul arul 358M Aug  4 23:58 bigfile.txt

$ ls -al bigfile.txt 
-rw-r--r-- 1 arul arul 375000000 Aug  4 23:58 bigfile.txt

The file generated using head is substantially smaller because it actually counts the bytes.

• 375000000 bytes = 366210.9375 kilobytes
• 366210.9375 kilobytes = 357.6278 MB, which rounds to 358MB

Using Python to create a 100GB file or of any specific file size

You can create a file using Python as well. Let us create a file of size 100 GB using Python.

This Python program generate_bigfile.py will do the trick.

generate_bigfile.py

import os

with open('bigfile100gb.txt', 'w') as fp:
    os.truncate(fp.fileno(), 1024 * 1024 * 1024 * 100)

Output:

$ python generate_bigfile.py 

$ ls -alh bigfile100gb.txt 
-rw-r--r-- 1 arul arul 100G Aug  5 00:26 bigfile100gb.txt

Conclusion

If you were able to create a file with random characters this blog post was useful, let me know which command you used. Thanks.

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