As stated in
man mysqldump: see 220.127.116.11. End-User Guidelines for Password Security in the MySQL reference manual.
An option file is the safest bet, not least according to the above reference. Giving it in plaintext in crontab is not good, not least since the process command line by default is visible through
ps for other users. The same actually applies for environment variables as explained in the reference.
Relevant portion of the MySQL reference manual:
Store your password in an option file. For example, on Unix, you can list your password in the
[client] section of the
.my.cnf file in your home directory:
To keep the password safe, the file should not be accessible to anyone but yourself. To ensure this, set the file access mode to
600. For example:
shell> chmod 600 .my.cnf
To name from the command line a specific option file containing the password, use the
--defaults-file=file_name option, where
file_name is the full path name to the file. For example:
shell> mysql --defaults-file=/home/francis/mysql-opts
Section 18.104.22.168, “Using Option Files”, discusses option files in more detail.
Also see http://stackoverflow.com/q/10725209.