Linux allows for very good security by allowing the owner of a file to choose who has access to that file and what kind of access they have. The owner of a file is usually the person who created the file or who has been set as the owner and can choose the access level of people belonging to a particular group and others outside that group. A group is simply a collection of users belonging to the same group users within a group all share the same access rights as all others in that group. Rather than giving each and every user independent access rights it is often easier to simply associate them with a group.
chmod
When you perform a listing using the -l switch you will see all kinds of letters and - signs before the individual files and directories.
e.g.
-rwxr--r--     1 root     root       1024 Jan 27  09:45 happy
lrwxrwxrwx   1 root     root       1024 Feb 13 09:45 myfile3
drwxr-xr--     1 root     root       1024 Feb 18 09:45 logs
What does it all mean?
Well the first 10 letters that look similar to this lrwxrwxrwx contain the permissions that can be controlled with the chmod command (the first letter you can't change as you will see for obvious reasons). The first letter (lrwxrwxrwx) is simply the type of file it is either a l for a link to another file, d for a directory or - for a file and is set by the linux operating system you can not manually change this letter (unless you change the file type of course). Next come the permissions for the owner (-rwxrwxrwx) followed by the permissions for the group (-rwxrwxrwx) and permissions for others (-rwxrwxrwx). The three letters stand for read, write and executable. If the letter is present then that particular permission is on and if there is a - in its place it is off. This means the owner of a file can alter and change the permissions to what ever they like for example -rwxr--r-- means that the file is readable, writable and executable for the owner but only readable for everyone else. Another example would be -rwxr-x--x this would mean that the file is readable, writable and executable by the owner but only readable and executable for everyone in the group and only executable to others outside the group. As you can imagine this ability to give or remove permissions can be very handy and adds to the security of linux. To change permissions on a file you need to be the owner and you can do it in two ways
The format for the chmod command is
chmod xyz file
with x representing the permissions for the owner followed by y the permissions for the group then z the permissions for all others. The file is the name of the relevant file being modified.
You can use a variety of methods to set the permissions for your files. The first is by using octal encoding where by numbers represent the rwx
letters
Read=4
Write=2
Execute=1
So if you wanted to set up the permission for the file myfile2 to be rwxrw-r-- you will need to add up the numbers for the owner being rwx (4+2+1=7), the group being rw (4+2=6) and others r (2) so the chmod command will be
chmod 762 myfile2
Though this may seem confusing at first you will learn it quite easily with practice and it will prove to be virtually fool proof over time.
You can also add permissions
e.g.
chmod +x file
This will make the file executable to all users.
You can also use the following
chmod go-rwx
This will remove all permissions from the group and other users making the file only readable, writable or executable by the owner and no one else.
chmod o+x file
This will add the executable permission to other users only.
You can also apply permissions to directory meaning you can restrict who can list the directory contents. You can allow them to view files in the directory but not list the directory contents. This and other permissions enable the owner of a file or directory to have complete control over its use enabling more security and privacy.