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理解并使用文件权限

在 Linux 和 Unix 的世界中,一切东西都是一个文件。文件夹是文件,文档是文件以及设备也是文件。设备通常称其为结点; 不管怎么样,他们仍然是文件。系统中所有的这些文件都由权限来允许或禁止他人读 ,修改或执行。 If the file is of type Directory then it restricts different actions than files and device nodes. The super user "root" has the ability to access any file on the system. Each file has access restrictions with permissions, user restrictions with owner/group association. Permissions are referred to as bits. To change or edit files that are owned by root, sudo must be used - please see RootSudo for details. If the owner read & execute bit are on, then the permissions are:

-r-x------

There are three types of access restrictions:

Permission Action chmod option
read (view) r or 4
write (edit) w or 2
execute (execute) x or 1

There are also three types of user restrictions:

User ls output
owner -rwx------
group ----rwx---
other -------rwx

Directories have directory permissions. The directory permissions restrict different actions than with files or device nodes.

Permission Action chmod option
read (view contents, i.e. ls command) r or 4
write (create or remove files from dir) w or 2
execute (cd into directory) x or 1
  • read restricts or allows viewing the directories contents, i.e. ls command
  • write restricts or allows creating new files or deleting files in the directory. (Caution: write access for a directory allows deleting of files in the directory even if the user does not have write permissions for the file!)
  • execute restricts or allows changing into the directory, i.e. cd command

Permissions in Action

[email protected]:/home/user$ ls -l /etc/hosts
-rw-r--r-- 1 root root 288 2005-11-13 19:24 /etc/hosts
[email protected]:/home/user$

Using the example above we have the file "/etc/hosts" which is owned by the user root and belongs to the root group. What are the permissions from the above /etc/hosts ls output?

-rw-r--r--

owner = Read & Write (rw-)
group = Read (r--)
other = Read (r--)

Changing Permissions

The command to use when modifying permissions is chmod. There are two ways to modify permissions, with numbers or with letters. Using letters is easier to understand for most people. When modifying permissions be careful not to create security problems. Some files are configured to have very restrictive permissions to prevent unauthorized access. For example, the /etc/shadow file (file that stores all local user passwords) does not have permissions for regular users to read or otherwise access.

[email protected]:/home/user# ls -l /etc/shadow
-rw-r----- 1 root shadow 869 2005-11-08 13:16 /etc/shadow
[email protected]:/home/user#

Permissions:
owner = Read & Write (rw-)
group = Read (r--)
other = None (---)

Ownership:
owner = root
group = shadow

chmod with Letters

Usage: chmod {options} filename
Options Definition
u owner
g group
o other
x execute
w write
r read
+ add permission
- remove permission
= set permission

Here are a few examples of chmod usage with letters (try these out on your system). First create some empty files:

[email protected]:/home/user$ touch file1 file2 file3 file4
[email protected]:/home/user$ ls -l
total 0
-rw-r--r-- 1 user user 0 Nov 19 20:13 file1
-rw-r--r-- 1 user user 0 Nov 19 20:13 file2
-rw-r--r-- 1 user user 0 Nov 19 20:13 file3
-rw-r--r-- 1 user user 0 Nov 19 20:13 file4

Add owner execute bit:

[email protected]:/home/user$ chmod u+x file1
[email protected]:/home/user$ ls -l file1
-rwxr--r-- 1 user user 0 Nov 19 20:13 file1

Add other write & execute bit:

[email protected]:/home/user$ chmod o+wx file2
[email protected]:/home/user$ ls -l file2
-rw-r--rwx 1 user user 0 Nov 19 20:13 file2

Remove group read bit:

[email protected]:/home/user$ chmod g-r file3
[email protected]:/home/user$ ls -l file3
-rw----r-- 1 user user 0 Nov 19 20:13 file3

Add read, write and execute to everyone:

[email protected]:/home/user$ chmod ugo+rwx file4
[email protected]:/home/user$ ls -l file4
-rwxrwxrwx 1 user user 0 Nov 19 20:13 file4
[email protected]:/home/user$

chmod with Numbers

Usage: chmod {options} filename
Options Definition
#-- owner
-#- group
--# other
1 execute
2 write
4 read

Owner, Group and Other is represented by three numbers. To get the value for the options determine the type of access needed for the file then add. For example if you want a file that has -rw-rw-rwx permissions you will use the following:

Owner Group Other
read & write read & write read, write & execute
4+2=6 4+2=6 4+2+1=7
[email protected]:/home/user$ chmod 667 filename

Another example if you want a file that has --w-r-x--x permissions you will use the following:

Owner Group Other
write read & execute execute
2 4+1=5 1
[email protected]:/home/user$ chmod 251 filename

Here are a few examples of chmod usage with numbers (try these out on your system). First create some empty files:

[email protected]:/home/user$ touch file1 file2 file3 file4
[email protected]:/home/user$ ls -l
total 0
-rw-r--r-- 1 user user 0 Nov 19 20:13 file1
-rw-r--r-- 1 user user 0 Nov 19 20:13 file2
-rw-r--r-- 1 user user 0 Nov 19 20:13 file3
-rw-r--r-- 1 user user 0 Nov 19 20:13 file4

Add owner execute bit:

[email protected]:/home/user$ chmod 744 file1
[email protected]:/home/user$ ls -l file1
-rwxr--r-- 1 user user 0 Nov 19 20:13 file1

Add other write & execute bit:

[email protected]:/home/user$ chmod 647 file2
[email protected]:/home/user$ ls -l file2
-rw-r--rwx 1 user user 0 Nov 19 20:13 file2

Remove group read bit:

[email protected]:/home/user$ chmod 604 file3
[email protected]:/home/user$ ls -l file3
-rw----r-- 1 user user 0 Nov 19 20:13 file3

Add read, write and execute to everyone:

[email protected]:/home/user$ chmod 777 file4
[email protected]:/home/user$ ls -l file4
-rwxrwxrwx 1 user user 0 Nov 19 20:13 file4
[email protected]:/home/user$

chmod with sudo

Changing permissions on files that you do not have ownership of: (Note that changing permissions the wrong way on the wrong files can quickly mess up your system a great deal! Please be careful when using sudo!)

[email protected]:/home/user$ ls -l /usr/local/bin/somefile
-rw-r--r-- 1 root root 550 2005-11-13 19:45 /usr/local/bin/somefile
[email protected]:/home/user$

[email protected]:/home/user$ sudo chmod o+x /usr/local/bin/somefile

[email protected]:/home/user$ ls -l /usr/local/bin/somefile
-rw-r--r-x 1 root root 550 2005-11-13 19:45 /usr/local/bin/somefile
[email protected]:/home/user$

Recursive Permission Changes

To change the permissions of multiple files and directories with one command. Please note the warning in the chmod with sudo section and the Warning with Recursive chmod section.

Recursive chmod with -R and sudo

To change all the permissions of each file and folder under a specified directory at once, use sudo chmod with -R

[email protected]:/home/user$ sudo chmod 777 -R /path/to/someDirectory
[email protected]:/home/user$ ls -l
total 3
-rwxrwxrwx 1 user user 0 Nov 19 20:13 file1
drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder
-rwxrwxrwx 1 user user 0 Nov 19 20:13 file2

Recursive chmod using find, pipemill, and sudo

To assign reasonably secure permissions to files and folders/directories, it's common to give files a permission of 644, and directories a 755 permission, since chmod -R assigns to both. Use sudo, the find command, and a pipemill to chmod as in the following examples. To change permission of only files under a specified directory.

[email protected]:/home/user$ sudo find /path/to/someDirectory -type f | while read var1; do sudo chmod 644 "$var1"; done
[email protected]:/home/user$ ls -l
total 3
-rw-r--r-- 1 user user 0 Nov 19 20:13 file1
drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder
-rw-r--r-- 1 user user 0 Nov 19 20:13 file2

To change permission of only directories under a specified directory (including that directory):

[email protected]:/home/user$ sudo find /path/to/someDirectory -type d | while read var1; do sudo chmod 755 "$var1"; done
[email protected]:/home/user$ ls -l
total 3
-rw-r--r-- 1 user user 0 Nov 19 20:13 file1
drwxr--r-- 2 user user 4096 Nov 19 20:13 folder
-rw-r--r-- 1 user user 0 Nov 19 20:13 file2

Warning with Recursive chmod

WARNING: Although it's been said, it's worth mentioning in context of a gotcha typo. Please note, Recursively deleting or chown-ing files are extremely dangerous. You will not be the first, nor the last, person to add one too many spaces into the command. This example will hose your system:

[email protected]:/home/user$ sudo chmod -R / home/john/Desktop/tempfiles

Note the space between the first / and home. You have been warned.

Changing the File Owner and Group

A file's owner can be changed using the chown command. For example, to change the foobar file's owner to tux:

[email protected]:/home/user$ sudo chown tux foobar

To change the foobar file's group to penguins, you could use either chgrp or chown with special syntax:

[email protected]:/home/user$ sudo chgrp penguins foobar
[email protected]:/home/user$ sudo chown :penguins foobar

Finally, to change the foobar file's owner to tux and the group to penguins with a single command, the syntax would be:

[email protected]:/home/user$ sudo chown tux:penguins foobar

<!> Note that, by default, you must use sudo to change a file's owner or group.

For more information

  • man chmod
  • man chown
  • man chgrp

Changing Permissions For Volumes With umask

This section has been moved: VolumePermissions

ACLs

Posix ACLs are a way of achieving a finer granularity of permissions than is possible with the standard Unix file permissions. To enable Posix ACLs, install the acl package

sudo apt-get install acl

Documentation can then be found in the online man pages:

  • man acl
  • man setfacl
  • man getfacl

The Eiciel package allows GUI access to ACLs through the Nautilus file manager.

ToDo

  • sticky bit
  • umask (add file and directory umask section, with specific focus on security)
  • * Suggestion: I often use find instead of chmod -R, because it's easier to differentiate between files and directories that way. Yes, I know about the 'X' permission, but I don't trust it.