Security – How to Trigger System Self Destruct with Password

In regard to your update:

When a process is started it has a dedicated area of memory where arguments are stored and a int which tells how many arguments was passed.

MEMORY
argc    2
argv[0] program_name
argv[1] foo
argv[2] bar

MySQL check if password was passed on command line by -p, and if it was copy it to a new variable that is not visible, then overwrite that region of memory with x'es.

In simple terms e.g.:

argc 2
argv[1] -p
argv[2] p4ssw0rd

new_var = copy(argv[2]);
argv[2] = "xxxxx";

You can find it e.g. in client/mysqladmin.cc of the source code:

  case 'p':
      ...
      opt_password=my_strdup(argument,MYF(MY_FAE));
      while (*argument) 
          *argument++= 'x';     /* Destroy argument */

When ps run it reads the memory region of the arguments, (argv[N]), and thus it is xxxx.

For a very short while the password is visible, but only for a few CPU cycles.

You can update the MySQL password using the special --init-file option and procedure. C.5.4.1.2. Resetting the Root Password: Unix Systems

mysqld_safe --init-file=/home/me/mysql-init &

Edit:

As @Gilles say, you can echo, printf or use here document from a script.

You can also add this to .my.cnf of your home directory or in a (temporary) file and use the --defaults-extra-file option. (Believe you have to add that option early on the command line.) optionally also include user. Also note the extra in the option name unless you want to use only that file as configuration:

[client]
user=foo
password='password!'

shell> chmod 400 my_tmp.cnf
shell> mysql --defaults-extra-file=my_tmp.conf -...

Optionally the [client] grouping makes mysqld skip the configuration.

One can also use MYSQL_PWD environment variable, but that should never be used as you can list environment, in many ps implementations by ps -e, in the /proc/<PID>/environ file on Linux etc.

tr '\0' '\n' < /proc/<PID>/environ

More on the topic here.

You might also want to have a look at the MySQL Configuration Utility which enables you to store password in a encrypted file in your home directory – .mylogin.cnf.

If you really want to go this path and there's no passwd parameter, you can use this Expect script:

#!/usr/bin/env expect -f
set old_timeout $timeout
set timeout -1

stty -echo
send_user "Current password: "
expect_user -re "(.*)\n"
set old_password $expect_out(1,string)

stty echo
send_user "\nNew password: "
expect_user -re "(.*)\n"
set new_password $expect_out(1,string)

set timeout $old_timeout
spawn passwd
expect "password:"
send "$old_password\r"
expect "password:"
send "$new_password\r"
expect "password:"
send "$new_password\r"
expect eof

How it works:

[dave@hal9000 ~]$ ./passwd.tcl
Current password: 
New password: bowman
spawn passwd
Changing password for user dave.
Changing password for dave.
(current) UNIX password: 
New password: 
Retype new password: 
passwd: all authentication tokens updated successfully.

This shell script might also work (tested on Fedora 20 with bash-4.2.47-2 and passwd-0.79-2):

#!/bin/sh
stty -echo
echo -n "Current password: "
read old_password

stty echo
echo
echo -n "New password: "
read new_password

passwd << EOF
$old_password
$new_password
$new_password
EOF

How it works:

[dave@hal9000 ~]$ ./passwd.sh
Current password: 
New password: bowman
Changing password for user dave.
Changing password for dave.
(current) UNIX password: New password: Retype new password: passwd: all authentication tokens updated successfully.