Generate a random string of characters with Bash.

random.bash

# This reads a stream of random bytes from the system and drops input that isn’t
# a valid encoding for a letter or a number. It stops after 8 characters.
#
# Setting LC_CTYPE=C is important: Modern systems will (hopefully) be set to
# UTF-8, but that makes tr throw errors for illegal byte sequences. Usually, we
# want that kind of sane behavior — but here, we don’t.
LC_CTYPE=C tr -d -c '[:alnum:]' </dev/urandom | head -c 8

# This saves a 15-character alphanumeric string in a variable:
random=$(LC_CTYPE=C tr -d -c '[:alnum:]' </dev/urandom | head -c 15)

# Instead of letters and numbers, this reads all printable characters. We can
# use it to make a 25-character password:
password=$(LC_CTYPE=C tr -d -c '[:print:]' </dev/urandom | head -c 25)