Being a Debian user, I always like to experiment with the latest packages, and so I have many third party debian repositories listed in the sources.list file. Whenever I am adding it for the first time, I usually get the error:

“The following signatures couldn’t be verified because the public key is not available”

On googling multiple solutions can be found to add a public key using apt-key, but most of them has 3 or 4 commands, including downloading the key, move it to a file and then import. But the one line command that does the same is as below:

apt-key adv --keyserver keyserver_url --recv-keys key_id

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Change permissions of files and directories separately in GNU/Linux using command line.
For Directories:

find location -type d -print -exec chmod 755 {} \;

For Files:

find location -type f -print -exec chmod 644 {} \;

Conditional permission changing

find location -type d -perm 777 -print -exec chmod 755 {} \;

Changes all directories(including sub-directories) with 777 permission to 755