How To Create Files of a Certain Size in Linux

Occasionally, you might need test files of exact size, to test transfer speeds for instance. Here is how

First, you may create a temporary folder and cd into it, but it's optional

mkdir tmp ; cd tmp

The dd way

Create a 10MB file

dd if=/dev/zero of=testfile_10MB_zero bs=1024 count=10240

10240+0 records in
10240+0 records out
10485760 bytes (10 MB, 10 MiB) copied, 0.0547554 s, 192 MB/s

dd if=/dev/random of=testfile_10MB_random bs=1024 count=10240

10240+0 records in
10240+0 records out
10485760 bytes (10 MB, 10 MiB) copied, 0.0926541 s, 113 MB/s

dd if=/dev/urandom of=testfile_10MB_urandom bs=1024 count=10240

10240+0 records in
10240+0 records out
10485760 bytes (10 MB, 10 MiB) copied, 0.087707 s, 120 MB/s

ls -la

total 30728
drwxrwxr-x  2 user group     4096 Feb 25 13:04 .
drwxr-x--- 24 user group     4096 Feb 25 13:02 ..
-rw-rw-r--  1 user group 10485760 Feb 25 13:04 testfile_10MB_random
-rw-rw-r--  1 user group 10485760 Feb 25 13:04 testfile_10MB_urandom
-rw-rw-r--  1 user group 10485760 Feb 25 13:04 testfile_10MB_zero

You should avoid using the /dev/zero when want to test transfer speeds over a compressed channel, like a rsync with -z option. The reason is, zerofiles will have a high compression ratio. here is the proof gzip the files gzip * list the results ls -l

total 20500
-rw-rw-r-- 1 user group 10487399 Feb 25 13:04 testfile_10MB_random.gz
-rw-rw-r-- 1 user group 10487400 Feb 25 13:04 testfile_10MB_urandom.gz
-rw-rw-r-- 1 user group    10227 Feb 25 13:04 testfile_10MB_zero.gz

or, human readable output ls -lh

total 21M
-rw-rw-r-- 1 user group 11M Feb 25 13:04 testfile_10MB_random.gz
-rw-rw-r-- 1 user group 11M Feb 25 13:04 testfile_10MB_urandom.gz
-rw-rw-r-- 1 user group 10K Feb 25 13:04 testfile_10MB_zero.gz

Other ways

Of course, there are many ways to accomplish this goal, here are few more

fallocate -l $((10*1024*1024)) fallocate_10MB truncate -s 10M truncate_10MB

Let's check the compression behavior of that files too.

gzip fallocate_10MB truncate_10MB

List the files, you can see it's basically the same, zero filled files

ls -lh fallocate_10MB.gz truncate_10MB.gz

-rw-rw-r-- 1 user group 10K Feb 25 13:15 fallocate_10MB.gz
-rw-rw-r-- 1 user group 10K Feb 25 13:15 truncate_10MB.gz

Head command

head -c 10MB /dev/urandom > head_10MB_urandom && ls -la head_10MB_urandom

-rw-rw-r-- 1 user group 10000000 Feb 25 13:29 head_10MB_urandom

head -c 10MB /dev/zero > head_10MB_zero && ls -la head_10MB_zero

-rw-rw-r-- 1 user group 10000000 Feb 25 13:30 head_10MB_zero

check the man pages