Hi I am trying to install a plugin for WordPress and I am being asked for FTP credentials. I had solved this problem with this command:

sudo chown -R myusername:www-data public_html

but now I'm having the same problem again, I don't understand why.

I use the following command

sudo chown -R username /var/www/yourdir/

I tend to use it from root(danger danger will robingson). BUT BE SURE TO TURN THE PERMS BACK as it might pose a security risk

A good way to check your file perms is

ls -l -h -a 

Also is your user a part of the group www-data ??

And have a look at the file perms list for the worpress folders on the wordpress site. GL

http://codex.wordpress.org/Changing_File_Permissions

That might help you in your quest