File Permissions and the chmod/chgrp/chown commands

古城微笑少年丶 2022-06-06 01:46 161阅读 0赞

The **chmod/chgrp/chown** commands are used to change the permissions/ownership of files and/or directories. Linux is often used as a multi-user system and it is not desirable that all users have access to all files and directories.

**For** **eg. :** On a multi-user environment in a corporate office using a central server running linux , it might be required the accounts documents be shared between employees of the acccounts department . At the same time, it might be undesirable and indeed dangerous if anyone having access to the server is able to read/edit them. It is for such situations that Linux has a 3X3 permission system.

There are 3 levels of security for a file :

*  Read Permission : Permission to read a file (r)
 *  Write Permission : Permission to edit a file (w)
 *  Execute Permission : Permission to execute a file if it is executable(x)

and 3 different levels for a directory :

*  Enter Permission : Permission to Enter into the Directory
 *  Show Entry : Permission to see the contents of the Directory
 *  Write Entry : Permission to make a new file or subdirectory in

the Directory

For granting the above permissions, users are divided into 3 different sets

*  User : The owner of the file/directory - mostly the person who created the file/directory
 *  Group : Linux users can be divided in groups and one user can be a member of more than one group. A Group denotes all users who are members of group(s) to which the owner of a file/directory belongs
 *  Others : All users not in the group(s) of the owner.

**For eg :**

A user level **r/w/x** permission means only the owner can read,write and execute the file  
A group level **r/w/x** permission means only the members of group(s) to which the owner belongs can read, write and execute the file  
An other level **r/w/x** permission means Everyone can read/write/execute the file.

### **The chmod Command** ###

The chmod command is used to change the permissions of files/directories in linux. It’s syntax is as follows :

> chmod -R/c/f/v \[u / g / o / a\] \[+ / - / =\] \[rwxXstugo..\]

**For eg.:**  if u want to give all users in the group of the owner just read permission to a file called foo.txt, the command is

> chmod g+r /home/aarjav/foo.txt

here g stands for group

*  **+** stands for giving permission
 *  (as against - for taking permission away)
 *  **r** stands for read permission.

so **g+r** means ?give group read permission?. All users for the owners group now have read permission to foo.txt

Now if they misbehave and u want to take their read permission away.

The command is the same as above, just substituting the + sign with a minus sign

> chmod g-r /home/aarjav/foo.txt

As shown the general format of the command is

> chmod -R/c/f/v \[u / g / o / a\] \[+ / - / =\] \[rwxXstugo\]

here

*  u : user
 *  g: group
 *  o : others
 *  a : all

> **plus:**      give permission  
> **minus:**   take permission away  
> **equal:**    cause the permissions given to be the only permissions of the file

*  r : read permission
 *  w: write permission
 *  x : execute permission
 *  X: execute only if it is a directory or already has execute permission for some user
 *  s : set user or group ID on execution
 *  t : save program text on swap device
 *  u : the permissions that the user who owns the file has for it
 *  g : the permissions that the owner?s group has for a file
 *  o : the permissions that users not in the owner?s group have for it (X, s, t, u, g and o are not required for common tasks)

the initial options -R/c/f/v are explained as follows :

*  \-c : Displays names of only those files whose permissions are being changed ( –changes can also be used instead of -c )
 *  \-f : Suppresses display of error messages when a file?s permissions cannot be changed ( –silent of –quiet can also be used instead of -f )
 *  \-R: Recursively changes the permission of all files in all subdirectories of the directory whose permissions are being changed ( –recursive can also be used )
 *  \-v : Displays results of all permission changes ( –verbose can also be used )

### **The chown command** ###

The chown command is used to change the user and/or group which owns one or more files or directories. Its general format is :

> chown \[-Rcfv\] \[username\]\[:.\]\[groupname\] foo.txt

The flags used above are same as those used in the chmod command . The following are the different ways in which this command can be used :

*  The username followed by a dot or colon followed by a groupname changes both the user and group ownerships to those specified.
 *  The username followed by a dot or colon and no groupname changes the user ownership as specified and changes the group ownership to the specified user’s login group.
 *  If the colon or dot and groupname are specified without a username, then only the groupownership is changed. This is effectively the same as The chgrp command. If the username is not followed by a dot or a colon, then only the user ownership is changed.

### **The chgrp command** ###

The chgrp command is used to change the group ownership of one or more files or directories. Its general syntax is :

> chgrp \[-Rcfv\] groupname foo.txt

The flags used here are also the same as those in the chmod command. The changes in ownership are applied to the groupname and the filename specified.

[http://www.freeos.com/articles/4440?page=3][http_www.freeos.com_articles_4440_page_3]

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