# 1、example
```bash
cat /dev/urandom | head -1 | md5sum | head -c 6
# 随机纯数字（20位为例）：
head /dev/urandom | tr -dc 0-9 | head -c 20
# 随机小写字母+数字（20位为例）：
head /dev/urandom | tr -dc a-z0-9 | head -c 20
# 随机大小写字母+数字（20位为例）：
head /dev/urandom | tr -dc A-Za-z0-9 | head -c 20
# 3rd_session
head -n 80 /dev/urandom | tr -dc A-Za-z0-9 | head -c 22
cat /dev/urandom | od -x | tr -d  ' ' | head -n 1
```

# 2、熵
```bash
# 查看系统熵池的容量
cat /proc/sys/kernel/random/poolsize
# 查看系统熵池中拥有的熵数
cat /proc/sys/kernel/random/entropy_avail 
# 查看从熵池中读取熵的阀值，当 entropy_avail 中的值少于这个阀值,这读取 /dev/random 会被阻塞
cat /proc/sys/kernel/random/read_wakeup_threshold
# tomcat启动慢
if [[ "$JAVA_OPTS" != *-Djava.security.egd=* ]]; then
 JAVA_OPTS="$JAVA_OPTS -Djava.security.egd=file:/dev/./urandom"
fi
JAVA_OPTS="$JAVA_OPTS -Djava.security.egd=file:/dev/./urandom"
```

# 3、参考
* [Myths about /dev/urandom](https://www.2uo.de/myths-about-urandom/)
* [How To Safely Generate A Random Number](https://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers/)
* [在虚拟机上使用hasged作为熵源是否合适](https://security.stackexchange.com/questions/34523/is-it-appropriate-to-use-haveged-as-a-source-of-entropy-on-virtual-machines)
* [使用Haveged为云服务器设置熵](https://www.digitalocean.com/community/tutorials/how-to-setup-additional-entropy-for-cloud-servers-using-haveged)