Create random strings in Bash

Aug 03, 2024

I manage a number of systems on which web and therefore API services in any form are prohibited for security reasons. However, long-running processes have to be started and monitored from outside quite often and it should be possible to retrieve the output of the processes as well. To implement these requirements, I use an unprivileged SSH account on the systems mentioned at the beginning, which does not have an interactive shell available and which I have reduced to a single, permitted bash script, that is executed directly after authentication via public key. This script now only offers the following actions:

Start process
Transmit process status
Transmit process output

In order to be able to monitor the processes, I generate unique IDs at startup that allows me to retrieve the status and results of each process. There are many ways to generate such IDs in Bash scripts and I would like to briefly present my two favorites here:

1. Generate with openssl

The first method I prefer generates a 20-character hexadecimal random string. It is therefore limited to the following characters: a-f 0-9. However, depending on the application and length, this limitation is not a problem at all. It should be noted that each "character" is always represented by an output of 8 bits in length, i.e. character combinations 00 to ff. Accordingly, when setting the desired length, an even number must be used and then divided by two. Here we want 20 characters and therefore use a length of 10.

$ openssl rand -hex 10eec3da9e849fdc383d75

2. Read from /dev/urandom

The second way I like, generates a 20-character random string by using the available characters -_#% A-Z a-z 0-9. Please note that a minus sign must be escaped. This is why we precede it with a backslash when defining our filter using the tr command.

$ cat /dev/urandom | tr -dc '\-_#%A-Za-z0-9' | head -c20NK%eezihVNC6qbz0-cFH

As mentioned at the beginning, there is a wealth of methods for generating random strings using Bash. In my opinion, care should be taken to ensure that the random factor is secured. For this reason, I use openssl or /dev/urandom.

Have fun generating!