Mrthdrb

In some rare cases, you can't use 770 and 660, like @MagenX says, 755 and 644 could also be the permission you need. (Some Fast-CGI users I guess)

So in that case, you run:

find . -type d -exec chmod 755 ; && find . -type f -exec chmod 644 ; && chmod u+x bin/magento

That seems to fix it, but if Magento generates new files or directories, those will have the permission 770 and 660 again. You can edit those default chmod values in the following files:

/vendor/magento/framework/Filesystem/DriverInterface.php 
(WRITEABLE_DIRECTORY_MODE and WRITEABLE_FILE_MODE)

/lib/internal/Cm/Cache/Backend/File.php 
(directory_mode and file_mode)

After these changes, run the first command again and after that, newly generated files shouldn't be an issue anymore.

Note: Editing files like this is never a good idea, but I suspect these chmod options are going to be configurable in the future, so I took the easy way.

sudo find . -type d -exec chmod 770 ; && sudo find . -type f -exec chmod 660 ; && sudo chmod u+x bin/magento

For local development
set pub and var to 775 or 777

If you are using SELinux ̣(e.g. CentOS) try:

sudo chcon -R -t httpd_sys_rw_content_t var

(from magento root folder of course)

You can use the method like the Magento documentation recommends:

find . -type f -exec chmod 664 ;
find . -type d -exec chmod 775 ;
find var pub/static pub/media app/etc -type f -exec chmod g+w ;
find var pub/static pub/media app/etc -type d -exec chmod g+ws ;
chmod u+x bin/magento

In Magento Community recommend

We recommend setting the permissions as follows:

All directories have 770 permissions.
770 permissions give full control (that is, read/write/execute) to the owner and to the group and no permissions to anyone else.

All files have 660 permissions.
660 permissions mean the owner and the group can read and write but other users have no permissions.

For more ref this link http://devdocs.magento.com/guides/v2.0/install-gde/install/file-system-perms.html

no special permissions, you just make sure that files owned by user who runs php or in case of cPanel - cpanel user, and same group

so your command is good:

find . -type f -exec chmod -c 644 ; && find . -type d -exec chmod -c 755 ;

also

chown -R user:user *

Similar issue also for sharing hosting:
Magento 2: Recommended File Permission for shared hosting

devdocs magento for permisions:
http://devdocs.magento.com/guides/v2.0/install-gde/prereq/file-system-perms.html

and for prod:
http://devdocs.magento.com/guides/v2.0/config-guide/prod/prod_file-sys-perms.html

Put this content inside your root folder, then execute it with the console.

Dont forget to chmod +x yourfile.sh.

#!/bin/sh
chmod 2775 -R .

chmod 777 -R ./pub/

chmod 777 -R ./var/

find . -type f -exec chmod 400 ;
find . -type d -exec chmod 500 ;
find var/ -type f -exec chmod 600 ;
find media/ -type f -exec chmod 600 ;
find var/ -type d -exec chmod 700 ;
find media/ -type d -exec chmod 700 ;
chmod 700 includes
chmod 600 includes/config.php

Also detailed information is available on, http://devdocs.magento.com/guides/m1x/install/installer-privileges_after.html

i solved in this way for CentOS Linux 7.4.1708 on Plesk Onyx 17.8.11

find . -type f -exec chmod 644 ; && find . -type d -exec chmod 755 ; && find ./var -type d -exec chmod 777 ; && find ./pub/media -type d -exec chmod 777 ; && find ./pub/static -type d -exec chmod 777 ; && chmod 777 ./app/etc && chmod 644 ./app/etc/*.xml

chown -R :<web server group> .