Change all files and folders permissions of a directory to 644/755

The easiest way is to do:

chmod -R u+rwX,go+rX,go-w /path/to/dir

which basically means:

to change file modes -Recursively by giving:

• user: read, write and eXecute permissions,
• group and other users: read and eXecute permissions, but not -write permission.

Please note that X will make a directory executable, but not a file, unless it's already searchable/executable.

+X - make a directory or file searchable/executable by everyone if it is already searchable/executable by anyone.

Please check man chmod for more details.

See also: How to chmod all directories except files (recursively)? at SU

One approach could be using find:

for directories

find /desired_location -type d -print0 | xargs -0 chmod 0755

for files

find /desired_location -type f -print0 | xargs -0 chmod 0644