How to generate random number in Bash?

Question

How to generate a random number within a range in Bash?

Answer 1

Use $RANDOM. It's often useful in combination with simple shell arithmetic. For instance, to generate a random number between 1 and 10:

$ echo $((1 + RANDOM % 10))
3

The actual generator is in variables.c, the function brand(). Older versions were a simple linear generator. Version 4.0 of bash uses a generator with a citation to a 1985 paper, which presumably means it's a decent source of pseudorandom numbers. I wouldn't use it for a simulation (and certainly not for crypto), but it's probably adequate for basic scripting tasks.

If you're doing something that requires serious random numbers you can use /dev/random or /dev/urandom if they're available:

$ dd if=/dev/urandom count=4 bs=1 | od -t d

Answer 2

I like this trick:

echo ${RANDOM:0:1} # random number between 1 and 9
echo ${RANDOM:0:2} # random number between 1 and 99

...

Answer 3

Please see $RANDOM:

$RANDOM is an internal Bash function (not a constant) that returns a pseudorandom integer in the range 0 - 32767. It should not be used to generate an encryption key.

Answer 4

Reading from /dev/random or /dev/urandom character special files is the way to go.

These devices return truly random numbers when read and are designed to help application software choose secure keys for encryption. Such random numbers are extracted from an entropy pool that is contributed by various random events. {LDD3, Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman]

These two files are interface to kernel randomization, in particular

void get_random_bytes_arch(void* buf, int nbytes)

which draws truly random bytes from hardware if such function is by hardware implemented (usually is), or it draws from entropy pool (comprised of timings between events like mouse and keyboard interrupts and other interrupts that are registered with SA_SAMPLE_RANDOM).

dd if=/dev/urandom count=4 bs=1 | od -t d

This works, but writes unneeded output from dd to stdout. The command below gives just the integer I need. I can even get specified number of random bits as I need by adjustment of the bitmask given to arithmetic expansion:

me@mymachine:~/$ x=$(head -c 1 /dev/urandom > tmp && hexdump 
                         -d tmp | head -n 1 | cut -c13-15) && echo $(( 10#$x & 127 ))

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Answer 5

There is $RANDOM. I don't know exactly how it works. But it works. For testing, you can do :

echo $RANDOM

Answer 6

If you are using a linux system you can get a random number out of /dev/random or /dev/urandom. Be carefull /dev/random will block if there are not enough random numbers available. If you need speed over randomness use /dev/urandom.

These "files" will be filled with random numbers generated by the operating system. It depends on the implementation of /dev/random on your system if you get true or pseudo random numbers. True random numbers are generated with help form noise gathered from device drivers like mouse, hard drive, network.

You can get random numbers from the file with dd