Linux has very fine grained access control. You can set permissions for the user (owner of the file) the group and also others
-rw-rw-r-- 1 kbaddi kbaddi
r read - 4
w write - 2
x eXecute - 1
- None - 0
rwx = 7
rw- = 6
r-- = 4
r-x = 5
--- = 0
chmod 750 <Filename>
755
chmod 755 <filename>
chmod 400 pem
chmod 644 <filename>
chmod 750 <filename>
chmod to change permissions of either the user, group or others
`chmod 666 <filename>`
chown to change owner of a file
`sudo chown <newowner> <filename`
chgrp to change the group of a file
`sudo chgrp <newgroup> <filename>`
chmod 640