Linux permissions write only

Write access without read access

Is it possible for a user to have a write access to a file and not be able to read it? How is it possible? I tried the following commands:

debianbox@debian:~/posix/io$ touch filetest debianbox@debian:~/posix/io$ ls -l filetest -rw-r--r-- 1 debianbox debianbox 0 14 oct. 03:10 filetest debianbox@debian:~/posix/io$ echo "Hello World" > filetest debianbox@debian:~/posix/io$ cat filetest Hello World debianbox@debian:~/posix/io$ chmod u-r filetest debianbox@debian:~/posix/io$ cat filetest cat: filetest: Permission forbidden debianbox@debian:~/posix/io$ 

As you can see here, I have write but not read access on this file. How can this be possible? Is this considered as a bug? If not, in what situation would this be useful?

3 Answers 3

The main reason to allow write access without read access is that it simplifies the management of permissions, both inside the kernel and in user programs. There are two permissions, one for reading and one for writing, and they are managed independently. This is not a bug since the documented behavior coincides with the actual behavior and there is no good reason to require a different behavior.

Having write permissions without read permissions doesn’t make much sense for regular files. It does make sense for various special files.

• Some systems allow append-only files. This is useful for log files, for example. It can make sense to allow many users to create log entries, but not to allow them to erase or overwrite existing entries (hence: write permission, but append-only attribute), nor to allow them to read others’ entries (hence: no read permission).
• A program may be allowed to write to a named pipe without being allowed to read from it.
• Some devices are write-only. For example, a sound output device connected to a loudspeaker but no microphone should have write permission but no read permission.
• There are various special filesystems where reading or writing to a file has an immediate effect instead of retrieving or adding data to storage. For example, under Linux, there are various files under /proc and /sys that allow user space programs to send commands to the kernel by writing to a particular file. If that command doesn’t provide any feedback, the special file is made write-only.

It is not a bug, it’s a feature TM (Also, just a consequence of universal unix approach to permisions).

Apart from dropbox-like behavior in case of directories (as described by BillThor), write-only access is necessary for some special (pseudo-) files under /proc and /sys . Such files are used to set some driver or kernel properties or trigger a system action. You cannot read them, because they are used only for one-way signaling — you can only echo some text/data to them. To find such files, you can use

find /proc/[^0-9]* /sys -perm /222 ! -perm /444 

Notice that, since these files are used for advanced system configuration (potentially dangerous), only root has write access to them (in most cases).

No this is not a bug. However, I don’t see it commonly applied to files.

I have most often seen write only access on dropbox directories. Users can add files to the directory, but can’t see which files are exist.

For a regular text file write only access would be appropriate for dropbox type access.

Setting write only access for yourself would not be terribly useful, but not allowing it would complicate permissions code.

EDIT: Dropbox files are not likely to be useful. However, it might be useful for non-root logs as it would make it more difficult to overwrite log entries. If you can read the file, it is much easier to identify where to write a replacement log entry. However, I don’t know anyone who sets their logs up like this. It is common to use remote logging to prevent local modification of log entries.

Setting rules on which permission combinations are permissible, could lead to preventing unforeseen useful permissions. Many combinations make more sense on the group or world level than the owner level. Any attempts to prevent access by the owner can be easily overridden. However, they can be useful to force sober second though.

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File Permissions in Linux / Unix: How to Read, Write & Change?

Linux is a clone of UNIX, the multi-user operating system which can be accessed by many users simultaneously. Linux can also be used in mainframes and servers without any modifications. But this raises security concerns as an unsolicited or malign user can corrupt, change or remove crucial data. For effective security, Linux divides authorization into 2 levels.

In this Linux file commands tutorial, you will learn-

The concept of Linux File permission and ownership is crucial in Linux. Here, we will explain Linux permissions and ownership and will discuss both of them. Let us start with the Ownership.

Click here if the video is not accessible

Linux File Ownership

Every file and directory on your Unix/Linux system is assigned 3 types of owner, given below.

User

A user is the owner of the file. By default, the person who created a file becomes its owner. Hence, a user is also sometimes called an owner.

Group

A user- group can contain multiple users. All users belonging to a group will have the same Linux group permissions access to the file. Suppose you have a project where a number of people require access to a file. Instead of manually assigning permissions to each user, you could add all users to a group, and assign group permission to file such that only this group members and no one else can read or modify the files.

Other

Any other user who has access to a file. This person has neither created the file, nor he belongs to a usergroup who could own the file. Practically, it means everybody else. Hence, when you set the permission for others, it is also referred as set permissions for the world.

Now, the big question arises how does Linux distinguish between these three user types so that a user ‘A’ cannot affect a file which contains some other user ‘B’s’ vital information/data. It is like you do not want your colleague, who works on your Linux computer, to view your images. This is where Permissions set in, and they define user behavior.

Let us understand the Permission system on Linux.

Linux File Permissions

Every file and directory in your UNIX/Linux system has following 3 permissions defined for all the 3 owners discussed above.

• Read: This permission give you the authority to open and read a file. Read permission on a directory gives you the ability to lists its content.
• Write: The write permission gives you the authority to modify the contents of a file. The write permission on a directory gives you the authority to add, remove and rename files stored in the directory. Consider a scenario where you have to write permission on file but do not have write permission on the directory where the file is stored. You will be able to modify the file contents. But you will not be able to rename, move or remove the file from the directory.
• Execute: In Windows, an executable program usually has an extension “.exe” and which you can easily run. In Unix/Linux, you cannot run a program unless the execute permission is set. If the execute permission is not set, you might still be able to see/modify the program code(provided read & write permissions are set), but not run it.

Let’s see file permissions in Linux with examples:

ls – l on terminal gives

Here, we have highlighted ‘-rw-rw-r–‘and this weird looking code is the one that tells us about the Unix permissions given to the owner, user group and the world.

Here, the first ‘–‘ implies that we have selected a file.p>

Else, if it were a directory, d would have been shown.

The characters are pretty easy to remember.

r = read permission
w = write permission
x = execute permission
– = no permission

Let us look at it this way.

The first part of the code is ‘rw-‘. This suggests that the owner ‘Home’ can:

• Read the file
• Write or edit the file
• He cannot execute the file since the execute bit is set to ‘-‘.

By design, many Linux distributions like Fedora, CentOS, Ubuntu, etc. will add users to a group of the same group name as the user name. Thus, a user ‘tom’ is added to a group named ‘tom’.

The second part is ‘rw-‘. It for the user group ‘Home’ and group-members can:

The third part is for the world which means any user. It says ‘r–‘. This means the user can only:

Changing file/directory permissions in Linux Using ‘chmod’ command

Say you do not want your colleague to see your personal images. This can be achieved by changing file permissions.

We can use the ‘chmod’ command which stands for ‘change mode’. Using the command, we can set permissions (read, write, execute) on a file/directory for the owner, group and the world.

chmod permissions filename

There are 2 ways to use the command –

Absolute(Numeric) Mode in Linux

In this mode, file permissions are not represented as characters but a three-digit octal number.

The table below gives numbers for all for permissions types.

Number	Permission Type	Symbol
0	No Permission	—
1	Execute	–x
2	Write	-w-
3	Execute + Write	-wx
4	Read	r–
5	Read + Execute	r-x
6	Read +Write	rw-
7	Read + Write +Execute	rwx

Let’s see the chmod permissions command in action.

In the above-given terminal window, we have changed the permissions of the file ‘sample to ‘764’.

‘764’ absolute code says the following:

• Owner can read, write and execute
• Usergroup can read and write
• World can only read

This is shown as ‘-rwxrw-r–

This is how you can change user permissions in Linux on file by assigning an absolute number.

Symbolic Mode in Linux

In the Absolute mode, you change permissions for all 3 owners. In the symbolic mode, you can modify permissions of a specific owner. It makes use of mathematical symbols to modify the Unix file permissions.

Operator	Description
+	Adds a permission to a file or directory
–	Removes the permission
=	Sets the permission and overrides the permissions set earlier.

The various owners are represented as –

User Denotations
u	user/owner
g	group
o	other
a	all

We will not be using permissions in numbers like 755 but characters like rwx. Let’s look into an example

Changing Ownership and Group in Linux

For changing the ownership of a file/directory, you can use the following command:

In case you want to change the user as well as group for a file or directory use the command

In case you want to change group-owner only, use the command

chgrp group_name filename

‘chgrp’ stands for change group.

Tip

• The file /etc/group contains all the groups defined in the system
• You can use the command “groups” to find all the groups you are a member of

Summary:

• Linux being a multi-user system uses permissions and ownership for security.
• There are three user types on a Linux system viz. User, Group and Other
• Linux divides the file permissions into read, write and execute denoted by r,w, and x
• The permissions on a file can be changed by ‘chmod’ command which can be further divided into Absolute and Symbolic mode
• The ‘chown’ command can change the ownership of a file/directory. Use the following commands: chown user file or chown user:group file
• The ‘chgrp’ command can change the group ownership chrgrp group filename
• What does x – eXecuting a directory mean? A: Being allowed to “enter” a dir and gain possible access to sub-dirs.

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