我喜欢这个技巧:

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

...

使用随机的美元。它通常与简单的shell算法结合使用很有用。例如，要生成1到10(包括)之间的随机数:

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

实际的生成器在variables.c中，即函数brand()。旧版本是一个简单的线性生成器。bash 4.0版本使用了一个引用1985年论文的生成器，这可能意味着它是一个不错的伪随机数来源。我不会将它用于模拟(当然也不会用于加密)，但它可能足以用于基本的脚本任务。

如果你在做一些需要大量随机数的事情，你可以使用/dev/random或/dev/urandom(如果它们可用的话):

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

一个bash函数，使用perl生成n位的随机数。指定数字数或n个0的模板。

rand() {
  perl -E '$ARGV[0]||=""; $ARGV[0]=int($ARGV[0])||length($ARGV[0]); say join "", int(rand(9)+1)*($ARGV[0]?1:0), map { int(rand(10)) } (0..($ARGV[0]||0)-2)' $1
}

用法:

$ rand 3
381
$ rand 000
728

调用rand n的演示，n在0到15之间:

$ for n in {0..15}; do printf "%02d: %s\n" $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

演示调用rand n，对于n，一个长度在0到15之间的0模板

$ for n in {0..15}; do printf "%15s :%02d: %s\n" $(printf "%0${n}d" 0) $n $(rand $(printf "%0${n}d" 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

没有模块化的纯Bash随机数

lowerRange=10   # inclusive
upperRange=20   # exclusive

randomNumber=$(( RANDOM * ( upperRange - lowerRange) / 32767 + lowerRange ))

我已经采取了一些这些想法，并做出了一个函数，应该执行迅速，如果大量的随机数是必需的。

如果你需要很多随机数，调用od的代价是很高的。相反，我只调用它一次，并存储来自/dev/urandom的1024个随机数。当调用rand时，返回最后一个随机数并按比例缩放。然后从缓存中删除它。当缓存为空时，再读取1024个随机数。

例子:

rand 10; echo $RET

返回RET中0到9(含9)之间的随机数。

declare -ia RANDCACHE
declare -i RET RAWRAND=$(( (1<<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 ]] && { 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<100000; c++ )); do
  rand 10
  BIN[RET]+=1  # add to bin to check distribution
done

for (( c=0; c<10; c++ )); do
  printf "%d %d\n" $c ${BIN[c]} 
done

更新:这并不适用于所有n。如果使用小n，也会浪费随机比特。注意(在这种情况下)一个32位随机数有足够的熵容纳9个0到9之间的随机数(10*9=1,000,000,000 <= 2*32)，我们可以从每个32个随机源值中提取多个随机数。

#!/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 ]] && { 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 ]] && {  # 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<100000; c++ )); do
  randBetween 10
  BIN[RET]+=1
done

for c in ${BIN[*]}; do
  echo $c
done

0到9之间的随机数。

echo $((RANDOM%10))