Generate random passwords in shell with one special character - bash

Question

I have the following code:
</dev/urandom tr -dc 'A-Za-z0-9##$%&_+=' | head -c 16
which is randomly generating passwords perfectly.
I want two changes:
It should only contain one special character listed above
It should choose a random length
I tried with length = $(($RANDOM%8+9))
then putting length as
</dev/urandom tr -dc 'A-Za-z0-9##$%&_+=' | head -c$length
but got no positive result.

Answer 1

#! /bin/bash
chars='##$%&_+='
{ </dev/urandom LC_ALL=C grep -ao '[A-Za-z0-9]' \
| head -n$((RANDOM % 8 + 9))
echo ${chars:$((RANDOM % ${#chars})):1} # Random special char.
} \
| shuf \
| tr -d '\n'
LC_ALL=C prevents characters like ř from appearing.
grep -o outputs just the matching substring, i.e. a single character.
shuf shuffles the lines. I originally used sort -R, but it kept the same characters together (ff1#22MvbcAA).

Answer 2

## Objective: Generating Random Password:
function random_password () {
[[ ${#1} -gt 0 ]] && { local length=${1}; } || { local length=16; }
export DEFAULT_PASSWORDLENGTH=${length};
export LC_CTYPE=C;
local random="$(
tr -cd "[:graph:]" < /dev/urandom \
| head -c ${length} \
| sed -e 's|\`|~|g' \
-e 's|\$(|\\$(|g';
)";
echo -e "${random}";
return 0;
}; alias random-password='random_password';
$ random-password 32 ;
)W#j*deZ2#eMuhU4TODO&eu&r)&.#~3F
# Warning: Do not consider these other options
# date +%s | sha256sum | base64 | head -c 32 | xargs -0;
# Output: MGFjNDlhMTE2ZWJjOTI4OGI4ZTFiZmEz
# dd if=/dev/urandom count=200 bs=1 2>/dev/null \
# | tr -cd "[:graph:]" \
# | cut -c-${length} \
# | xargs -0;
# Output: AuS*D=!wkHR.4DZ_la