Setting directories to 0755 and files to 0644 is straight forward using chmod(1). However, sometimes batch processing or in other words the permissions of all files/(sub)folders in a given location should be changed. There are basically two approaches to do so. 1.) using a list of file paths and pipe the path to chmod. The other option is to use chmod directly.
find(1) is commonly used to descend into a directory recursively. To apply chmod on folders/directories (-type d) a command to change permissions recursively would look like this:
find /path -type d -exec chmod 0755 {} \;
find /path -type d -print0 | xargs -0 chmod 0755
This command can be used for files (-type f) accordingly:
find /path -type f print0 | xargs -0 chmod 0644
find /path -type f -exec chmod 0644 {} \;
The second option is to use chmod directly with its -R argument which applies changes recursively.
chmod -R a=r-wx,u=wr,a+X /path
a refers to all which means user, group and other their base permissions are set to 0444 or r-wx which means ‘read only’.
u refers to user and it allows the user to read and write when set to 6 or wr.
If a file path is a directory it is made executable (+X). In combination this leads to 0755 for directories and 0644 for files.
One more thing. What do these numbers (0755/0644) actually mean?
The first number is set to 0 if not specified otherwise (0755 is the same as 755). Other meanings of the first numbers are 1 to enable the “sticky bit”, 2 to set group ID on execution and 4 to set user ID on execution.
The more interesting numbers are the remaining three. The first number stands for user, the second for group and the third digit stands for others.
4 allows to read, 2 to write and 1 to execute. These numbers are simply add up and e.g. read, write, execute results in a 7 whereas 6 allows to read and write only and 4 sets permissions to read only.