How can I populate a file with random data?

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.

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.

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.

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

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/iu)&g1|Y}%8;r>o>e5$UcyX]Q(F=XH6C@-BOTQ!<}CqI1Ff],7#;kb!GpZ^Ai85NaA,ya|YF+EuH]{FUgC0G7;