How to set a short password on Ubuntu?

Use following command in Terminal:

sudo passwd <user>

Replace <user> with the username whose password you wish to change.

This works because passwd suppresses all checks for length or entropy when you use it as the root user.

Warning: if the target user has an encrypted home directory, this will cause problems! (see comments below)

By default, Ubuntu requires a minimum password length of 6 characters, as well as some basic entropy checks. These values are controlled in the file /etc/pam.d/common-password, which is outlined below.

password        [success=2 default=ignore]      pam_unix.so obscure sha512

If you would like to adjust the minimum length to 4 characters, add the appropriate variable (minlen=4) to the end of the line. The modification is outlined below.

password        [success=2 default=ignore]      pam_unix.so obscure sha512 minlen=4

Source.

Bring up a terminal and edit /etc/pam.d/common-password

Change this line:

password    [success=1 default=ignore]  pam_unix.so obscure sha512

to:

password    [success=1 default=ignore]  pam_unix.so obscure sha512 minlen=4

Password also need a certain amount of complexity, as specified by the obscure parameter above.

password    [success=1 default=ignore]  pam_unix.so minlen=2 sha512

removes that check also.

This all presupposes that you think this is wise.

See man pam_unix

These work on my system.

To set up a simple password, I tried the simple sudo passwd username method, but it failed on my Ubuntu Server 12.04 LTS.

So I tried to remove the obscure option from /etc/pam.d/common-passwd config file, but it still failed.

So I also removed the obscure option from /usr/share/pam-configs/unix config file. And then it worked :-)

I do agree that it should be simplier, when acting as su to set up a weak password, whatever the reason why one wants to do it! A warning saying "weak password, confirm?" would be perfect...