My-Computer:~ user$ echo hello
hello
My-Computer:~ user$

stringen () { cat /dev/urandom | env LC_CTYPE=C tr -cd 'a-Z-A-z-0-9' | head -c "$1" ; }

My-Computer:~ user$ stringen 18
^k6JPWVIUdEiudcWSMy-Computer:~ user$

variable=$(stringen 18)
echo $variable

testfunc() { echo hello; }    # a newline is printed
output=$(testfunc)            # $output does NOT contain a trailing newline
echo "$output"                # echo adds a newline
printf "%s" "$output"         # no newline

$ stringen() { cat /dev/urandom | env LC_CTYPE=C tr -cd 'a-Z-A-z-0-9' | head -c "$1" ; }

$ stringen 18
tr: range-endpoints of 'a-Z' are in reverse collating sequence order

$ stringen () { cat /dev/urandom | env LC_CTYPE=C tr -cd 'A-Za-z0-9' | head -c "$1" ; }
$ stringen 18
V068BrYGKIICyLI4QB
$