Who can read, write and execute what.
THE THREE SETS
Owner, group and others. Each has read, write and execute permissions.
Expressed numerically: read is 4, write is 2, execute is 1. Added together per set gives the familiar three-digit values.
THE COMMON VALUES
755 for directories: owner full, others read and enter 644 for files: owner read and write, others read 600 for files holding credentials: owner only 775 or 664 where a group needs write access
NEVER 777
It permits any process on the server to modify the file. If documentation tells you to use it, that documentation is wrong.
OWNERSHIP
Files should be owned by the user the application runs as. Wrong ownership after a restore or a copy is one of the most common causes of an application failing afterwards.
chown changes it, with -R to recurse.
SETTING THEM IN BULK
Use find to apply directory permissions and file permissions separately, rather than applying one value to everything.
CHECKING
ls -l shows permissions and ownership.
