Usage
The chmod command options are specified like this:
$ chmod [options] mode file1 ...
To view what the permissions currently are, type:
$ ls -l
Options
The chmod command has a number of command line options that affect its behavior. The most common options are:
● -R: Changes the modes of directories and files recursively
● -v: Verbose mode; lists all files as they are being processed
To the chmod utility, all permissions and special modes are represented by its mode parameter. One way to adjust the mode of files or directories is to specify a symbolic mode. The symbolic mode is composed of three components, which are combined to form a single string of text:
$ chmod [references][operator][modes] file1 ...
The references (or classes) are used to distinguish the users to whom the permissions apply. If no references are specified it defaults to "all". They are represented by one or more of the following letters:
Reference
Class
Description
u
user
the owner of the file
g
group
users who are members of the file's group
o
others
users who are not the owner of the file or members of the group
a
all
all three of the above, is the same as ugo
The chmod program uses an operator to specify how the modes of a file should be adjusted. The following operators are accepted:
Operator
Description
+
adds the specified modes to the specified classes
-
removes the specified modes from the specified classes
=
the modes specified are to be made the exact modes for the specified classes
The modes indicate which permissions are to be granted or taken away from the specified classes. There are three basic modes which correspond to the basic permissions:
Mode
Name
Description
r
read
read a file or list a directory's contents
w
write
write to a file or directory
x
execute
execute a file or recurse a directory tree
X
special execute
which is not a permission in itself but rather can be used instead of x. It applies execute permissions to directories regardless of their current permissions and applies execute permissions to a file which already has at least 1 execute permission bit already set (either user, group or other). It is only really useful when used with '+' and usually in combination with the -R option for giving group or other access to a big directory tree without setting execute permission on normal files (such as text files), which would normally happen if you just used "chmod -R a+rx .", whereas with 'X' you can do "chmod -R a+rX ." instead
s
setuid/gid
t
sticky
The combination of these three components produces a string that is understood by the chmod command. Multiple changes can be specified by separating multiple symbolic modes with commas.
String mode examples
For example, the following command would be used to add the read and write permissions to the user and group classes of a file or directory named sample:
$ chmod ug+rw sample
$ ls -ld sample
drw-rw---- 2 unixguy unixguy 96 Dec 8 12:53 sample
This command removes all permissions, and allows no one to read, write, or execute the file named sample.
$ chmod a-rwx sample
$ ls -l sample
---------- 2 unixguy unixguy 96 Dec 8 12:53 sample
The following command changes the permissions for the user and the group to read and execute only (no write permission) on sample .
Sample file permissions before command
$ ls -ld sample
drw-rw---- 2 unixguy unixguy 96 Dec 8 12:53 sample
$ chmod ug=rx sample
$ ls -ld sample
dr-xr-x--- 2 unixguy unixguy 96 Dec 8 12:53 sample
Octal numbers
The chmod command also accepts three- and four-digit octal numbers representing modes. See the article mentioned above for more. Using a four-digit octal number to set the modes of a file or directory named sample would look something like this:
$ chmod 0664 sample
Assuming that the setuid, setgid and sticky bits are not set, this is equivalent to:
$ chmod 664 sample
or
$ chmod +r,-x,ug+w sample
Special modes
The chmod command is also capable of changing the additional permissions or special modes of a file or directory. The symbolic modes use s to represent the setuid and setgid modes, and t to represent the sticky mode. The modes are only applied to the appropriate classes, regardless of whether or not other classes are specified.
Most operating systems support the specification of special modes using octal modes, but some do not. On these systems, only the symbolic modes can be used.
Examples
● chmod +r file – read is added for all
● chmod -x file – execute permission is removed for all
● chmod u=rw,go= file – read and write is set for the owner, all permissions are cleared for the group and others
● chmod +rw file – change the permissions of the file file to read and write for all.
● chmod -R u+w,go-w docs/ – change the permissions of the directory docs and all its contents to add write access for the user, and deny write access for everybody else.
● chmod 0 file – removes all privileges for all
● chmod 666 file – ADDS! read and write access for the owner, the group, and all others.
● chmod 0755 file – equivalent to u=rwx (4+2+1),go=rx (4+1 & 4+1). The 0 specifies no special modes.
● chmod 4755 file – the 4 specifies set user ID.
● find path/ -type d -exec chmod a-x {} \; – sets a-x for all directories in tree starting from path/ (use '-type f' to match files only).
● find path/ -type d -exec chmod 777 {} \; allows directory browsing ls for example if you've reset permissions for Samba write access.
● chmod -R u+rwX,g-rwx,o-rwx
● chmod -R a-x+X
Change Ownership chown
As I already mentioned earlier, the owner is the file's creator; therefore, the file is owned by a user and the user is owned by a group. This basically means the file is also owned by the group. A regular user is not able to take ownership of a file created by a different user, but the administrator (root) can and there after can reassign permissions.
This is much simpler; all you have to do is be root
[root@server2 collections]# chown user1 afile
[root@server2 collections]#ls –l
total
rw-r--r-- 1 user1 root 6 Aug 17 09:09 afile
-rw-r--r-- 1 root root 27 Aug 17 09:46 file2
-rw-r-- r-- 1 root root 14 Aug 17 09:46 myfile
drwxr-xr-x 2 root root 4096 Aug 17 10:34 another/
[root@server2 collections]#
As you can see the new owner now is user1, even though user1 belongs to group root.
How can you change the group?
Look at the following example
I will add user1 to group user1:
root@server2 collections]#chown user1.user1 afile
Now I will do the ls –l
[root@server2 collections]#ls –l total
rw-r--r-- 1 user1 user1 6 Aug 17 09:09 afile
-rw-r--r-- 1 root root 27 Aug 17 09:46 file2
-rw-r-- r-- 1 root root 14 Aug 17 09:46 myfile
drwxr-xr-x 2 root root 4096 Aug 17 10:34 another/
[root@server2 collections]#
Now afile belongs to User1, User1 can now do anything to the file.
Note: An owner can be from any group, and still can have full control of the specified file if proper permission is granted.
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