EDX Linux Foundation Ch 9：User Environment Section 5 File Permissions

File Ownership

0.File Permission Modes

rwx:	rwx:	rwx
u:	g:	o

rwx

Files have three kinds of permissions: read (r), write (w), execute (x). These are generally represented as in rwx.

ugo

u:user/owner
g:group
0:others

1.chmod

$ ls -l a_file
-rw-rw-r-- 1 coop coop 1601 Mar 9 15:04 a_file
$ chmod uo+x,g-w a_file
$ ls -l a_file
-rwxr--r-x 1 coop coop 1601 Mar 9 15:04 a_file

This kind of syntax can be difficult to type and remember

so one often uses a shorthand which lets you set all the permissions in one step.

4 if read permission is desired.
2 if write permission is desired.
1 if execute permission is desired.
Thus 7 means read/write/execute, 6 means read/write, and 5 means read/execute.

When you apply this to the chmod command you have to give three digits for each degree of freedom, such as in

$ chmod 755 a_file
$ ls -l a_file
-rwxr-xr-x 1 coop coop 1601 Mar 9 15:04 a_file

2.chown

有三個檔案file-1, file-2, temp

$ ls -l
total 4
-rw-rw-r--. 1 bob bob 0 Mar 16 19:04 file-1
-rw-rw-r--. 1 bob bob 0 Mar 16 19:04 file-2
drwxrwxr-x. 2 bob bob 4096 Mar 16 19:04 temp

對file-1下chown改變檔案擁有者為root

$ sudo chown root file-1
[sudo] password for bob:

下ls -a看檔案完整的訊息，發現檔案擁有者已經改變成root。

$ ls -l
total 4
-rw-rw-r--. 1 root bob 0 Mar 16 19:04 file-1
-rw-rw-r--. 1 bob bob 0 Mar 16 19:04 file-2
drwxrwxr-x. 2 bob bob 4096 Mar 16 19:04 temp

3.chgrp

續上面的例子：這次對file-2下chgrp改變檔案的群組。

$ sudo chgrp bin file-2
$ ls -l
total 4
-rw-rw-r--. 1 root bob 0 Mar 16 19:04 file-1
-rw-rw-r--. 1 bob bin 0 Mar 16 19:04 file-2
drwxrwxr-x. 2 bob bob 4096 Mar 16 19:04 temp