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Removing old kernels

check the kernel you’re using:

$ uname -r

4.4.0-104-generic

Do NOT delete that kernel!

 

List the old kernels:

$ sudo dpkg --list 'linux-image*'|awk '{ if ($1=="ii") print $2}'|grep -v `uname -r`

linux-image-4.4.0-103-generic
linux-image-extra-4.4.0-103-generic

 

Remove the old kernels one by one:

$ sudo apt-get purge linux-image-3.19.0-25-generic

 

Remove the packages that are not needed anymore:

$ sudo apt-get autoremove

 

Update the grub kernel list:

$ sudo update-grub

 

References :
Safest way to clean up boot partition – Ubuntu 14.04LTS-x64

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Changing directory or file permissions or ownerships

Recursively change ownership of directories or files

find /www -type d -exec chown root:root {} \;

find /www -type -f -exec chown root:root {} \;

1: find directories under /www and execute the chown command on each directory found and change their ownership to root
2: find files under /www and execute the chown command on each directory found and change their ownership to root

 

Recursively change permissions for a directory or file

find /www -type d -exec chmod 644 {} \;

find /www -type f -exec chmod 644 {} \;