How can I set full permissions to a user in a specified dir in linux?
Depends what you mean 'full permissions'. If you want a user to have full read and write access to all files and directories in that directory, then this will help:
chown -R username directory chmod -R u+rX directory
The first command makes the user own the directory.
The second command gives them full read and access permissions. The
r gives read permission, the
X gives 'execute' permission to directories, and not files.
If you do not wish to change the existing permissions of the directory, yet would like to give a user (or multiple users or groups) permissions to the contents of the directory, you can use ACLs. Some filesystems (ext3) require the acl flag on mount to enable ACLs. Often, just using groups is sufficient, but ACLs can be more flexible.
Look at the setfacl and getfacl commands for more information.