Problem generating random string in Azure Cloud Shell Bash

Question

I am trying to generate a random string of a certain length in Bash. This works in every other Bash shell I could test (Ubuntu 18.04 LTS and Debian 10)

$ cat /dev/urandom | tr -dc 'a-z0-9' | fold -w 10 | head -n 1
n8a4wxvb01
$

cat /dev/urandom – endless stream of bytes

tr -dc 'a-z0-9' – only return lowercase characters and digits

fold -w 10 – break at 10 characters

head -n 1 – return the first line of the stream

In the Azure Cloud Shell Bash, this hangs. It returns the random string but never exits. The call to head -n 1 never exits after printing the value.

I have a work around but it seems backwards

head -n 10 /dev/urandom | tr -dc 'a-z0-9' | fold -w 10 | head -n 1

Answer 1

Hello @James Pansarasa ,

IMHO, this issue is not specific to Cloud Shell as such, and can happen with a *nix terminal.

As you already found, this command should work on your machine and the Cloud Shell:

head /dev/urandom | tr -dc a-z0-9 | fold -w 10 | head -n 1

Here are some possible explanations:

• https://superuser.com/questions/637860/why-does-cat-dev-urandom-break-your-terminal
• https://www.networkworld.com/article/3208389/unix-how-random-is-random.html
• https://www.unix.com/unix-for-advanced-and-expert-users/271649-cmd-cat-dev-urandom-not-closing-cleanly.html