How can I populate a file with random data?

Question

How can I create a new file and fill it with 1 Gigabyte worth of random data? I need this to test some software.

I would prefer to use /dev/random or /dev/urandom.

score 340 · Accepted Answer · edited May 03 '17 at 13:36

340

On most unices:

head -c 1G </dev/urandom >myfile

If your head doesn't understand the G suffix you can specify the size in bytes:

head -c 1073741824 </dev/urandom >myfile

If your head doesn't understand the -c option (it's common but not POSIX; you probably have OpenBSD):

dd bs=1024 count=1048576 </dev/urandom >myfile

Do not use /dev/random on Linux, use /dev/urandom.

edited May 03 '17 at 13:36

Chris

103

answered Mar 07 '12 at 01:20

Gilles 'SO- stop being evil'

829,060

2

Interesting that head can read /dev/urandom, but tail cannot. – Stefan Lasiewski Mar 12 '12 at 23:24
34

@StefanLasiewski tail first tries to go to the end of the input file, which takes forever (literally). – Gilles 'SO- stop being evil' Mar 12 '12 at 23:25
Ah, like 'tail /dev/infinity`, if there was such a device. – Stefan Lasiewski Mar 12 '12 at 23:42
7

@StefanLasiewski You have /dev/zero as well, if you don't like variety. – Gilles 'SO- stop being evil' Mar 12 '12 at 23:55
If head don’t understand G, $((1<<30)) in Bash is easier to remember. – Franklin Yu Aug 01 '20 at 22:20
1

Correction to my last comment: Arithmetic Expansion is POSIX so it doesn’t have to be Bash. – Franklin Yu Aug 03 '20 at 02:29
2

No need for that first < file redirect, this works: head -c 1G /dev/urandom >myfile – agc Aug 28 '20 at 12:41
@agc, no need to pass /dev/urandom to head for it to open it as the shell can open it by itself with < /dev/urandom, saving that extra effort. – Stéphane Chazelas Oct 25 '20 at 17:25

score 55 · Answer 2 · edited Jun 08 '16 at 03:30

55

Assuming that pseudo-random data is sufficient, dd if=/dev/urandom of=target-file bs=1M count=1000 will do what you want.

dd(1) will read blocks of data from an input file and write them to an output file. The command line language is a little quirky, but it is one of those really useful tools worth mastering the basics of.

In this case if is input file, of is output file, bs is "block size" - and I used the GNU extension to set the size more conveniently. (You can also use 1048576 if your dd doesn't have GNU extension.) count is the number of blocks to read from if and write to of.

/dev/urandom is a better choice than /dev/random becuase, on Linux, it will fall back to strong pseudo-random data rather than blocking when genuinely random data is exhausted.

You may also want to look at http://www.random.org/ as another path to getting some random data without having to generate it yourself.

edited Jun 08 '16 at 03:30

Haozhun

103

answered Mar 07 '12 at 00:29

Daniel Pittman

7,674

4

A note -- unit specifications like 1M are not available in every standard dd variant. If your version of dd is affected, use bs=1048576. – Chris Down Mar 07 '12 at 00:39
4

The example command will create a file of count 1,000,000 blocks each of size 1 MB. That's about 1 TB (1M x 1MB), not 1 GB (which would be 1K x 1MB). As an aside, up to a point, increasing block size and reducing the block count tends to yield better throughput for a given amount of output data. – user Mar 07 '12 at 10:45
1

GB vs GiB. for dd 1M==1024*1024, use 1MB and count=1000 if you want 1 GB or 1M and count=1024 if you want 1 GiB. Or use 1GB or 1G and count=1. – 12431234123412341234123 Sep 07 '20 at 16:50

score 11 · Answer 3 · 2020-05-30T23:06:22.617

An alternative to using /dev/random/ and/or /dev/urandom with an excellent Cryptographically Secure PseudoRandom Number Generator (CSPRNG) is given by openssl. You can even choose the algorithm and encrypting method in such rare cases that that is needed:

size=$( echo 1G | numfmt --from=iec )         # 1 G ==> 1073741824

openssl rand -out myfile "$size"

If you need base64:

openssl rand -base64 -out myfile "$size"

Read man rand for the details.

score 6 · Answer 4 · answered May 08 '18 at 14:29

6

while true;do head /dev/urandom | tr -dc A-Za-z0-9;done | head -c 5000K | tee  5000kb

Used this to generate 5MB random character data. If you need different size, change the -c value of head, change the outfile name, execute and wait until the execution completes.

answered May 08 '18 at 14:29

James Jithin

205

1

Use base64 to increase recall rate (from 62 / 256 to 62 / 64). – iBug Mar 12 '20 at 04:26

score 2 · Answer 5 · answered Oct 25 '20 at 16:37

The solution doesn't use random but I use man pages to create sizable data. This does not however guarantee the max size so we might have to loop through a few cats to get the desired size,

man curl > /tmp/man-curl

Or if the size required is larger,

for i in {1..10}; do man curl >> /tmp/man-curl; done

Luca Etienne Schulz · Answer 6 · 2022-03-22T14:00:04.187

The UNIX tool jot can do exactly that. No need to pipe data from /dev/urandom into a file with dd.

jot -r -c $FILESIZE > $FILE

The flag -r generates random data based on arc4random(3), -c generates a char based on the ASCII table. Other data can be specified based on printf format types with the -w flag.

By default jot returns to stdout.

Edit:

Another use case of jot could be to generate random passwords:

$ jot -rcs '\'\'' 128 33 126
wFv/Fb7F}iO'|!cGy>?8yyez2$f,vy5vYqa;?s$-wBlVU^QX0|y~M.VeRq7}Jh!l/0'CcveuLTL]_1@P_yV#P)h8VZA\~,s{|69P{YwlL?;U{_yC3$\m*:H2V66RpdJ^
$ jot -rcs '\'\'' 128 33 126
X`&kh3[Z2'ej)w`7z'YXl~VG_MA@^jk6+STuB@.C/i\u)&g1|Y}%8;r>o>e5$UcyX]Q(F=XH6C@-BOTQ!<}CqI1Ff],7#;kb!GpZ^Ai85NaA,ya|YF+EuH]{FUgC0G7;

This one? https://lib.rs/crates/jot. Since it was not available in my system's software repository (Fedora). So cargo install jot? — Pysis, Jul 17 '23 at 15:13

How can I populate a file with random data?

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