How to generate random number in Bash

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How to generate a random number within a range in Bash

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I like this trick   echo   RANDOM 0 1    random number between 1 and 9 echo   RANDOM 0 2    random number between 1 and 99

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I wrote several articles on this   https   linuxconfig org generating-random-numbers-in-bash-with-examples https   linuxconfig org random-entropy-in-bash https   www cloudsavvyit com 7572 how-to-generate-better-random-numbers-at-the-bash-command-line     RANDOM   date   s N   cut -b10-19    echo      RANDOM   113   13     The above will give a number between 13 and 113  with reasonable random entropy

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There is  RANDOM  I don t know exactly how it works  But it works  For testing  you can do    echo  RANDOM

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Generate random number in the range of 0 to n  signed 16-bit integer   Result set in  RAND variable  For example      bin bash  random         local range   1 -1       RAND  od -t uI -N 4  dev urandom   awk   print  2        let  RAND  RAND   range 1      n 10 while       n - 1    -ge  0     do     random 500     echo   RAND  done

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Use  RANDOM  It s often useful in combination with simple shell arithmetic  For instance  to generate a random number between 1 and 10  inclusive     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

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What about   perl -e  print int rand 10    n

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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

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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

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Try this from your shell     od -A n -t d -N 1  dev urandom   Here  -t d specifies that the output format should be signed decimal  -N 1 says to read one byte from  dev urandom

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I have taken a few of these ideas and made a function that should perform quickly if lots of random numbers are required   calling od is expensive if you need lots of random numbers  Instead I call it once and store 1024 random numbers from  dev urandom  When rand is called  the last random number is returned and scaled  It is then removed from cache  When cache is empty  another 1024 random numbers is read   Example   rand 10  echo  RET   Returns a random number in RET between 0 and 9 inclusive   declare -ia RANDCACHE declare -i RET RAWRAND      1 lt  lt 32 -1     function rand       pick a random number from 0 to N-1  Max N is 2 32   local -i N  1         RANDCACHE     -eq 0     amp  amp    RANDCACHE     od -An -tu4 -N1024  dev urandom          refill cache   RET      RANDCACHE -1  N 1  RAWRAND       pull last random number and scale   unset RANDCACHE    RANDCACHE    -1        pop read random number       test by generating a lot of random numbers  then effectively place them in bins and count how many are in each bin   declare -i c  declare -ia BIN  for    c 0  c lt 100000  c       do   rand 10   BIN RET   1    add to bin to check distribution done  for    c 0  c lt 10  c       do   printf   d  d n   c   BIN c    done   UPDATE  That does not work so well for all N  It also wastes random bits if used with small N  Noting that  in this case  a 32 bit random number has enough entropy for 9 random numbers between 0 and 9  10 9 1 000 000 000  lt   2 32  we can extract multiple random numbers from each 32 random source value      bin bash  declare -ia RCACHE  declare -i RET               return value declare -i ENT 2             keep track of unused entropy as 2  entropy  declare -i RND RANDOM ENT    a store for unused entropy - start with 1 bit  declare -i BYTES 4           size of unsigned random bytes returned by od declare -i BITS 8 BYTES      size of random data returned by od in bits declare -i CACHE 16          number of random numbers to cache declare -i MAX 2  BITS       quantum of entropy per cached random number declare -i c  function rand       pick a random number from 0 to 2 BITS-1         RCACHE     -eq 0     amp  amp    RCACHE     od -An -tu BYTES -N CACHE  dev urandom          refill cache - could use  dev random if CACHE is small   RET   RCACHE -1                  pull last random number and scale   unset RCACHE    RCACHE    -1     pop read random number     function randBetween      local -i N  1      ENT -lt N     amp  amp       not enough entropy to supply ln N  ln 2  bits     rand  RND RET         get more random bits     ENT MAX               reset entropy       RET RND N    random number to return   RND RND N    remaining randomness   ENT ENT N    remaining entropy     declare -ia BIN  for    c 0  c lt 100000  c       do   randBetween 10   BIN RET   1 done  for c in   BIN      do   echo  c done

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you can also get random number from awk  awk  BEGIN        seed    srand      for  i 1 i lt  1000 i          print int 1   rand     100

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Random branching of a program or yes no  1 0  true false output   if    RANDOM -gt 16383     then                16383   32767 2      echo var true 1 yes go hither else      echo var false 0 no go thither fi   of if you lazy to remember 16383   if    RANDOM   2     then      echo  yes  else      echo  no  fi

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Wanted to use  dev urandom without dd and od function roll     local modulus   1 -6   echo     1   0x  env LC CTYPE C tr -dc  0-9a-fA-F   lt   dev urandom   head -c5      modulus        Testing   roll 5   roll 12 12  Just how random is it     echo  quot count roll percentage quot   i 0  while    i -lt 10000    do roll  i    i 1    done   sort   uniq -c   awk   print  0   1 10000 100  quot   quot       column -t count  roll  percentage 1625   1     16 25  1665   2     16 65  1646   3     16 46  1720   4     17 2  1694   5     16 94  1650   6     16 5

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Maybe I am a bit too late  but what about using jot to generate a random number within a range in Bash   jot -r -p 3 1 0 1   This generates a random  -r  number with 3 decimal places precision  -p   In this particular case  you ll get one number between 0 and 1  1 0 1   You can also print sequential data  The source of the random number  according to the manual  is      Random numbers are obtained through arc4random 3  when no seed is specified  and through        random 3  when a seed is given

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bash 5 1 introduces a new variable  SRANDOM  which gets its random data from the system s entropy engine and so is not linear and cannot be reseeded to get an identical random sequence  This variable can be used as a substitute for RANDOM for generating more random numbers    echo    1   SRANDOM   10   4

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Random number between 0 and 9 inclusive   echo    RANDOM 10

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Generate random 3-digit number This is great for creating sample data  Example  put all testing data in a directory called  quot test-create-volume-123 quot   then after your test is done  zap the entire directory  By generating exactly three digits  you don t have weird sorting issues  printf   02d n     1   RANDOM   100    This scales down  e g  to one digit  printf   01d n     1   RANDOM   10    It scales up  but only to four digits  See above as to why

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Based on the great answers of  Nelson   Barun and  Robert  here is a Bash script that generates random numbers    Can generate how many digits you want  each digit is separately generated by  dev urandom which is much better than Bash s built-in  RANDOM      usr bin env bash  digits 10  rand   od -A n -t d -N 2  dev urandom  tr -d      num    rand   10   while      num  -lt  digits    do   rand   od -A n -t d -N 1  dev urandom  tr -d        num    num    rand   10    done echo  num

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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  gt  tmp  amp  amp  hexdump                           -d tmp   head -n 1   cut -c13-15   amp  amp  echo     10  x  amp  127

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A bash function that uses perl to generate a random number of n digits  Specify either the number of digits or a template of n 0s  rand       perl -E   ARGV 0     quot  quot    ARGV 0  int  ARGV 0    length  ARGV 0    say join  quot  quot   int rand 9  1    ARGV 0  1 0   map   int rand 10      0    ARGV 0   0 -2    1    Usage    rand 3 381   rand 000 728  Demonstration of calling rand n  for n between 0 and 15    for n in  0  15   do printf  quot  02d   s n quot   n   rand  n   done 00  0 01  3 02  98 03  139 04  1712 05  49296 06  426697 07  2431421 08  82727795 09  445682186 10  6368501779 11  51029574113 12  602518591108 13  5839716875073 14  87572173490132 15  546889624135868  Demonstration of calling rand n  for n a template of 0s between length 0 and 15   for n in  0  15   do printf  quot  15s   02d   s n quot    printf  quot  0  n d quot  0   n   rand   printf  quot  0  n d quot  0    done               0  00  0               0  01  0              00  02  70             000  03  201            0000  04  9751           00000  05  62237          000000  06  262860         0000000  07  1365194        00000000  08  83953419       000000000  09  838521776      0000000000  10  2355011586     00000000000  11  95040136057    000000000000  12  511889225898   0000000000000  13  7441263049018  00000000000000  14  11895209107156 000000000000000  15  863219624761093

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You can also use shuf  available in coreutils   shuf -i 1-100000 -n 1

[bash] How to generate random number in Bash?

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