Generating random strings in Bash

I needed to generate some random strings in a Bash script, here’s how I ended up doing it.

As with all Bash tricks, one line gets the job done:

$ tr -dc [:alnum:] < /dev/urandom | head -c "${len}"

This command gets random bits from /dev/urandom and deletes (-d) all characters that are not (-c) in a letters and digits set ([:alnum:]). The head command is to limit the output to $len bytes (characters in this case).

Before getting to this solution, I used a more pedestrian script:

alphabet="abcdefghijklmnopqrstuvwxyz1234567890"
len=5

str=""
for i in $(seq 1 ${len}); do
	char="${alphabet:${RANDOM} % ${#alphabet}:1}"
	str="${str}${char}"
done

We first define all characters to choose from, then pick one randomly as many times as the length of the final string.

The trick is picking the random character. Here we use substring expansion (${variable:offset:length}) to select only one character from our alphabet (the length is 1). The offset is calculated as a random number (${RANDOM}) modulo our alphabet length (${#alphabet}).

In substring expansions, both offset and length are arithmetic expressions, that’s why we can calculate RANDOM % #alphabet in there.