what is mean by File and Folder Permission : –
For many users of Linux, getting used to file permissions and ownership can be a bit of a challenge. It is commonly assumed, to get into this level of usage, the command line is a must. Although there is always far more power and flexibility to be had, running seemingly complicated command isn’t always necessity. With the help of some of the most user-friendly desktop interfaces available, you can get away with little to no command line usage. Even with file permission and ownership.
That’s right, much to the surprise of many a new user, managing files and folders can be done from within the file managers. But before we get to the GUI, it’s always best to have a solid understanding of what it’s doing. So, we’ll start with the command line first.
Command line: File permissions
This lesson will cover the following commands:
- chmod – modify file access rights
- su – temporarily become the superuser
- chown – change file ownership
- chgrp – change a file’s group ownership
Linux uses the same permissions scheme as Unix. Each file and directory on your system is assigned access rights for the owner of the file, the members of a group of related users, and everybody else. Rights can be assigned to read a file, to write a file, and to execute a file (i.e., run the file as a program).
To see the permission settings for a file, we can use the ls command as follows:[me@linuxbox me]$ ls -l some_file
-rw-rw-r– 1 me me 1097374 Sep 26 18:48 some_file
We can determine a lot from examining the results of this command:
- The file “some_file” is owned by user “me”
- User “me” has the right to read and write this file
- The file is owned by the group “me”
- Members of the group “me” can also read and write this file
- Everybody else can read this file
Let’s try another example. We will look at the bash program which is located in the /bin directory:[me@linuxbox me]$ ls -l /bin/bash
-rwxr-xr-x 1 root root 316848 Feb 27 2000 /bin/bash
Here we can see:
- The file “/bin/bash” is owned by user “root”
- The superuser has the right to read, write, and execute this file
- The file is owned by the group “root”
- Members of the group “root” can also read and execute this file
- Everybody else can read and execute this file
In the diagram below, we see how the first portion of the listing is interpreted. It consists of a character indicating the file type, followed by three sets of three characters that convey the reading, writing and execution permission for the owner, group, and everybody else.
The chmod command is used to change the permissions of a file or directory. To use it, you specify the desired permission settings and the file or files that you wish to modify. There are two ways to specify the permissions, but I am only going to teach one way.
It is easy to think of the permission settings as a series of bits (which is how the computer thinks about them). Here’s how it works:
rwx rwx rwx = 111 111 111
rw- rw- rw- = 110 110 110
rwx — — = 111 000 000
and so on…
rwx = 111 in binary = 7
rw- = 110 in binary = 6
r-x = 101 in binary = 5
r– = 100 in binary = 4
Now, if you represent each of the three sets of permissions (owner, group, and other) as a single digit, you have a pretty convenient way of expressing the possible permissions settings. For example, if we wanted to set some_file to have read and write permission for the owner, but wanted to keep the file private from others, we would:[me@linuxbox me]$ chmod 600 some_file
Here is a table of numbers that covers all the common settings. The ones beginning with “7” are used with programs (since they enable execution) and the rest are for other kinds of files.
|777||(rwxrwxrwx) No restrictions on permissions. Anybody may do anything. Generally not a desirable setting.|
|755||(rwxr-xr-x) The file’s owner may read, write, and execute the file. All others may read and execute the file. This setting is common for programs that are used by all users.|
|700||(rwx——) The file’s owner may read, write, and execute the file. Nobody else has any rights. This setting is useful for programs that only the owner may use and must be kept private from others.|
|666||(rw-rw-rw-) All users may read and write the file.|
|644||(rw-r–r–) The owner may read and write a file, while all others may only read the file. A common setting for data files that everybody may read, but only the owner may change.|
|600||(rw——-) The owner may read and write a file. All others have no rights. A common setting for data files that the owner wants to keep private.|
The chmod command can also be used to control the access permissions for directories. In most ways, the permissions scheme for directories works the same way as they do with files. However, the execution permission is used in a different way. It provides control for access to file listing and other things. Here are some useful settings for directories:
|777||(rwxrwxrwx) No restrictions on permissions. Anybody may list files, create new files in the directory and delete files in the directory. Generally not a good setting.|
|755||(rwxr-xr-x) The directory owner has full access. All others may list the directory, but cannot create files nor delete them. This setting is common for directories that you wish to share with other users.|
|700||(rwx——) The directory owner has full access. Nobody else has any rights. This setting is useful for directories that only the owner may use and must be kept private from others.|
Becoming the superuser for a short while
It is often useful to become the superuser to perform important system administration tasks, but as you have been warned (and not just by me!), you should not stay logged on as the superuser. In most distributions, there is a program that can give you temporary access to the superuser’s privileges. This program is called su (short for substitute user) and can be used in those cases when you need to be the superuser for a small number of tasks. To become the superuser, simply type the su command. You will be prompted for the superuser’s password:[me@linuxbox me]$ su
After executing the su command, you have a new shell session as the superuser. To exit the superuser session, type exit and you will return to your previous session.
In some distributions, most notably Ubuntu, an alternate method is used. Rather than using su, these systems employ the sudo command instead. With sudo, one or more users are granted superuser privileges on an as needed basis. To execute a command as the superuser, the desired command is simply preceeded with the sudo command. After the command is entered, the user is prompted for the user’s password rather than the superuser’s:[me@linuxbox me]$ sudo some_command
Changing file ownership
You can change the owner of a file by using the chown command. Here’s an example: Suppose I wanted to change the owner of some_file from “me” to “you”. I could:[me@linuxbox me]$ su
[root@linuxbox me]# chown you some_file
[root@linuxbox me]# exit
Notice that in order to change the owner of a file, you must be the superuser. To do this, our example employed the su command, then we executed chown, and finally we typed exit to return to our previous session.
chown works the same way on directories as it does on files.
Changing group ownership
The group ownership of a file or directory may be changed with chgrp. This command is used like this:[me@linuxbox me]$ chgrp new_group some_file
In the example above, we changed the group ownership of some_file from its previous group to “new_group”. You must be the owner of the file or directory to perform a chgrp.
Neither command is difficult to use. It is important, however, that you understand the only user that can actually modify the permissions or ownership of a file is either the current owner or the root user. So, if you are user Bethany, you cannot make changes to files and folders owned by Jacob without the help of root (or sudo). For example:
A new folder was created on a data partition called /DATA/SHARE. Both users Bethany and Jacob need read and write access to this folder. There are a number of ways this can be done (one of which would be to join the users to a special group – we’ll go over managing groups in another post). If Bethany and Jacob are the only users on the system (and you know your network is safe – very important), you can change the permissions of the folder to give them access. One way to do this would be to issue the command:
sudo chmod -R ugo+rw /DATA/SHARE
The breakdown of the above command looks like:
- sudo – this is used to gain admin rights for the command on any system that makes use of sudo (otherwise you’d have to ‘su’ to root and run the above command without ‘sudo’)
- chmod – the command to modify permissions
- -R – this modifies the permission of the parent folder and the child objects within
- ugo+rw – this gives User, Group, and Other read and write access.
As you can probably surmise, this command opens wide the SHARE folder such that anyone on the system can have access to that folder. As I mentioned earlier, a more secure method would be to use groups. But we’re just using this for the purpose of demonstration.
The breakdown of permissions looks like this:
- u – user
- g – group
- o – other
The ‘other’ entry is the dangerous one, as it effectively gives everyone permission for the folder/file. The permissions you can give to a file or folder are:
- r – read
- w – write
- x – execute
Using the -R switch is important. If you have a number of sub-folders and files within the SHARE directory, and you want the permissions to apply from the parent object (the containing folder) to the child objects (the sub-folders and files), you must use the -R (recursive) switch so the same permissions are applied all the way to the deepest folder, contained within the parent.
Command line: File ownership
Changing the ownership of a file or folder is equally as simple. Say Jacob moved a folder for Bethany into the SHARE directory – but Jacob still has ownership. This can be changed with a simple command:
sudo chown -R bethany /DATA/SHARE
Let’s break this down.
- sudo – admin rights must be used since we are dealing with a folder that belongs to another user
- chown – the command for changing ownership
- -R – the recursive switch to make sure all child objects get the same ownership changes
- bethany – the new owner of the folder
- /DATA/SHARE – the directory to be modified
Should Bethany send the folder back to Jacob, the ownership would need to again be changed (again, this will be simplified with the use of groups).
GUI: File permissions
I’m going to demonstrate changing file permissions using the Nautilus file manager on an Ubuntu 13.10 system.
Let’s say you need to allow everyone to gain read/write permissions to the folder TEST. To do this, within the Nautilus file manager, follow these steps:
- Open Nautilus
- Navigate to the target file or folder
- Right click the file or folder
- Select Properties
- Click on the Permissions tab
- Click on the Access files in the Others section
- Select “Create and delete files”
- Click Change Permissions for Enclosed Files
- In the resulting window, Select Read and Write under Files and Create and delete files under Folders (Figure A)
- Click Change
- Click Close.
The trick comes when you need to change the permissions of a folder which does not belong to you. It can be done, but Nautilus must be started with admin access. To do this, follow these steps:
- Open up a terminal window
- Issue the command sudo -i
- Issue the command nautilus
The sudo -i command gives you persistent access to sudo, until you enter the exit command to remove that access. Once Nautilus is open, you can change the permissions of the folder or file as described above – even if you are not the owner of the folder or file.
NOTE: If you’re using a distribution that doesn’t use sudo, alter the above instructions to:
- Open up a terminal window
- Issue the command su
- Type your root password and hit Enter
- Issue the command
After you’ve completed the task, close the Nautilus window and then the terminal window.
GUI: Change ownership
Changing the ownership of a file or folder will most often require the use of admin rights. So for this, you’ll need to start Nautilus in the method described above.
For changing ownership of a folder or file through Nautilus, do the following:
- In the Nautilus window (opened with admin rights), locate the folder or file in question
- Right click the folder (or file)
- Click on the Permissions tab
- Select the new owner from the Owner drop-down (below)
- Click Close.