Issue
I was experimenting with the $RANDOM variable in a Unix shell and noticed something peculiar. I ran the following command, which reads $RANDOM in a loop 100k times and then pipes the output to "uniq" to find the duplicates.
$ for i in {1..100000}; do echo $RANDOM; done | uniq -d
I ran the command above 7 times, and the same two numbers (4455 and 4117) were repeated all 7 times. The screenshot below shows the command line output.
kali@kali:~% for i in {1..100000}; do echo $RANDOM; done | uniq -d
4455
4117
kali@kali:~% for i in {1..100000}; do echo $RANDOM; done | uniq -d
4455
4117
kali@kali:~% for i in {1..100000}; do echo $RANDOM; done | uniq -d
4455
4117
kali@kali:~% for i in {1..100000}; do echo $RANDOM; done | uniq -d
4455
4117
kali@kali:~% for i in {1..100000}; do echo $RANDOM; done | uniq -d
4455
4117
kali@kali:~% for i in {1..100000}; do echo $RANDOM; done | uniq -d
4455
4117
kali@kali:~% for i in {1..100000}; do echo $RANDOM; done | uniq -d
4455
4117
See: https://i.stack.imgur.com/5bpEe.png
I also opened another terminal window and repeated the process. In the second terminal, the numbers were different, but repeated from in a similar fashion. This makes me wonder about when the entropy of the $RANDOM variable, and how it is seeded.
My guess is that it is re-seeded whenever bash is invoked, but I was wondering if anyone has any info about why the same values are repeated when I repeat the command in a single terminal window.
Solution
This is due to a zsh bug / "behaviour" for RANDOM in subshells. This bug doesn't appear in bash.
echo $RANDOM # changes at every run
echo `echo $RANDOM` # always return the same value until you call the first line
Because RANDOM is seeded by its last value, but in a subshell the value obtained is not updated in the main shell.
In man zshparam:
RANDOM <S>
A pseudo-random integer from 0 to 32767, newly generated each
time this parameter is referenced. The random number generator
can be seeded by assigning a numeric value to RANDOM.
The values of RANDOM form an intentionally-repeatable
pseudo-random sequence; subshells that reference RANDOM will
result in identical pseudo-random values unless the value of
RANDOM is referenced or seeded in the parent shell in between
subshell invocations.
There is even crazier because calling uniq creates a subshell
for i in {1..10}; do echo $RANDOM; done # changes at every run
for i in {1..10}; do echo $RANDOM; done | uniq # always the same 10 numbers
Source : Debian bug report 828180
Answered By - lolesque Answer Checked By - Willingham (WPSolving Volunteer)