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I've been using $((1 + RANDOM % 1000)) to generate a random number.

Is it possible to do something similar but provide a seed?

So that given the same seed the same random number will always be output?

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2 Answers 2

Reset to default
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Assign a seed value to RANDOM

$ bash -c 'RANDOM=640; echo $RANDOM $RANDOM $RANDOM'
28612 27230 24923
$ bash -c 'RANDOM=640; echo $RANDOM $RANDOM $RANDOM'
28612 27230 24923
$ bash -c 'RANDOM=640; echo $RANDOM $RANDOM $RANDOM'
28612 27230 24923
$

Notice that single quotes are used; double quotes run afoul shell interpolation rules:

$ bash -c 'RANDOM=42; echo $RANDOM $RANDOM $RANDOM'
19081 17033 15269
$ RANDOM=42
$ bash -c "RANDOM=640; echo $RANDOM"
19081
$ bash -c "RANDOM=640; echo $RANDOM"
17033
$ bash -c "RANDOM=640; echo $RANDOM"
15269
$

because $RANDOM is being interpolated by the parent shell before the child bash -c ... process is run. Either use single quotes to turn off interpolation (as shown above) or otherwise prevent the interpolation:

$ RANDOM=42
$ SEED_FOR_MY_GAME=640
$ bash -c "RANDOM=$SEED_FOR_MY_GAME; echo \$RANDOM"
28612
$ bash -c "RANDOM=$SEED_FOR_MY_GAME; echo \$RANDOM"
28612
$

This feature of RANDOM is mentioned in the bash(1) manual

   RANDOM Each time this parameter is referenced, a random integer between
          0 and 32767 is generated.  The sequence of random numbers may be
          initialized by assigning a value to RANDOM.  If RANDOM is unset,
          it loses its special properties,  even  if  it  is  subsequently
          reset.
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  • If I run random=$(bash -c "RANDOM=640; echo $RANDOM") shouldn't it return the same each time? Or is it because the $(bash ... isn't treated as a new instance of bash? Commented Nov 17, 2017 at 17:47
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    doublequotes is probably not what you want there Commented Nov 17, 2017 at 17:55
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    Watch what happens if you do: set -x; FOO=42; bash -c "FOO=999; echo $FOO". The double-quotes are allowing the outer shell to replace $RANDOM with the outer shell's random; the inner bash shell is simply echoing an integer. Commented Nov 17, 2017 at 17:56
  • @thrig I need double quotes because in my case it is actually random=$(bash -c "RANDOM=$mac; echo $RANDOM") Commented Nov 17, 2017 at 17:56
  • Of course it should be bash -c 'RANDOM=42; echo "$RANDOM" "$RANDOM" "$RANDOM"'. Commented Nov 18, 2017 at 0:58
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I came here late, but I have to say something about "So that given the same seed the same random number will always be output?" I don't comprehend that.

If that's what you want, you do not want a random number. You can skip the seeding and use the "same random number" you wanted. The stuff about single and double quotes may be persiflage.

I would hesitate to say that some of the comments are mockery. The hesitation would take too long.

RANDOM=$(date +%s);echo $RANDOM; echo $RANDOM; echo $(($RANDOM%1000))

That's about as good as you can get for randomness, I think. But 420 comes up too many times.

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    There most definitely are valid use cases for predictable sequences of “random” numbers. Commented Jun 21, 2020 at 21:05
  • I apologize. After googling it, I suppose that has something to do with gaming, such as predicting "randomly" generated poker hands. Another link suggests using a predictable sequence of random numbers as a way to crash an application you are testing. And this: en.wikipedia.org/wiki/Predictable_serial_number_attack All of those uses seem a little shady to me. I'm leaving my answer as is, because "random" and "predicable" are mutually exclusive terms, and the word "random" is misused in those links and in this question. Commented Jun 22, 2020 at 13:24
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    @Wastrel it's used all the time. For example if you want to do an experiment which involves randomly splitting the data into groups (e.g. machine learning). If you provide a seed the person reading the experiment can reproduce the exact results. If you were just to do random with no seed they have to take your word for it, which is a problem when something needs to be peer reviewed. Commented Jun 23, 2020 at 21:10
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    @Philip Kirkbride Thank you. I have decided I don't understand it and don't understand what the OP wants. Why use a seed to make a random number when you can just tell him the random number? But I did write a script to generate predictable random numbers, if I have any idea what they are. Is a file of random numbers any help, if you can access any number or series of numbers on demand, or create other files that are different and do the same? Commented Jun 25, 2020 at 0:48

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