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Along with the usual read, write, and execute file permissions, Linux files have another set of attributes that control other characteristics of the file. Here’s how to see them and change them.

Permissions and Attributes

In Linux, who can access a file and what they can do with it is controlled by a user-centric set of permissions. Whether you can read the contents of a file, write new data into the file, or execute a file if it is a script or a program, is all governed by that set of permissions. The permissions are applied to the file, but they define the restrictions and capabilities for different categories of user.

There are permissions for the owner of the file, for the group of the file, and for others—that is, users who are not in the first two categories. You can use the ls command with the -l (long listing) option to see the permissions on a file or directory.

To change the permissions, you use the chmod command. At least, you can if you have write permissions for the file, or if you’re the root user.

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We can see that file permissions are user-centric because they award or remove permissions at the user level. By contrast, the attributes of a file are file system-centric. Like permissions, they’re set on the file or directory. But once they’re set, they’re the same for all users.

Attributes are a separate collection of settings from permissions. Attributes control characteristics such as immutability and other file system-level behaviors. To see the attributes of a file or directory we use the lsattr command. To set the attributes we use the chattr command.

Permissions and attributes are stored inside inodes. An inode is a file system structure that holds information about file system objects such as files and directories. A file’s location on the hard drive, its creation date, its permissions, and its attributes are all stored within its inode.

Because different file systems have different underlying structures and capabilities, attributes can behave differently—or be completely ignored—by some file systems. In this article, we’re using ext4 which is the default file system for many Linux distributions.

Looking at a File’s Attributes

The chattr and lsattr commands will already be present on your computer so there’s no need to install anything.

To check the attributes on the files in the current directory, use lsattr:

lsattr

Listing file attributes for all files in a directory

The dashed lines are placeholders for attributes that are not set. The only attribute that is set is the e (extents) attribute. This shows that the file system inodes are using—or will use if required—extents to point to all portions of the file on the hard drive.

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If the file is held in one contiguous sequence of hard drive blocks, its inode only has to record the first and last blocks used to store the file. If the file is fragmented, the inode has to record the number of the first and last block of each piece of the file. These pairs of hard drive block numbers are called extents.

This is the list of the most commonly used attributes.

  • a: Append only. A file with this attribute can only be appended to. It can still be written to, but only at the end of the file. It is not possible to overwrite any of the existing data within the file.
  • c: Compressed. The file is automatically compressed on the hard drive and uncompressed when it is read. Data written to the files is compressed before it is written to the hard drive.
  • A: No atime updates. The atime is a value in an inode that records the last time a file was accessed.
  • C: No copy-on-write. If two processes request access to a file, they can be given pointers to the same file. They are only given their own unique copy of the file if they try to write to the file, making it unique to that process.
  • d: No dump. The Linux dump command is used to write copies of entire file systems to backup media. This attribute makes dump ignore the file. It is excluded from the backup.
  • D: Synchronous directory updates. When this attribute is turned on for a directory, all changes to that directory are written synchronously—that is, immediately—on the hard drive. Data operations can be buffered.
  • e: Extent format. The e attribute indicates that the file system is using extents to map the location of the file on the hard drive. You cannot change this with chattr. It is a function of the operation of the file system.
  • i: Immutable. An immutable file cannot be modified, including renaming and deleting. The root user is the only person who can set or unset this attribute.
  • s: Secure deletion. When a file with this attribute set is deleted, the hard drive blocks that held the file data are overwritten with bytes containing zeroes. Note that this is not honored by the ext4 file system.
  • S: Synchronous updates. Changes to a file with its S attribute set are written to the file synchronously.
  • u: Deleting a file that has its u attribute set causes a copy of the file to be made. This can be beneficial to file recovery if the file was removed in error.

Changing a File’s Attributes

The chattr command lets us change the attributes of a file or directory. We can use the + (set) and - (unset) operators to apply or remove an attribute, similar to the chmod command and permissions.

The chattr command also has an = (set only) operator. This sets the attributes of a file or directory to only the attributes that are specified in the command. That is, all attributes not listed on the command line are unset.

Setting the Append Only Attribute

Let’s set the append-only attribute on a text file and see how it affects what we can do with the file.

sudo chattr +a text-file.txt

Setting the append only attribute on a text file

We can check to see that the append-only bit has been set by using lsattr :

lsattr text-file.txt

Listing the attributes for a text file

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The letter “a” indicates the attribute has been set. Let’s try to overwrite the file. Redirecting output to a file with a single angle bracket  “>” replaces all content in the file with the redirected output.

We’ve preloaded the text file with some lorem ipsum placeholder text.

cat text-file.txt

Placeholder text in a text file

We’ll redirect the output from ls into the file:

ls -l > text-file.txt
sudo ls -l > text-file.txt

Trying to overwrite an append-only text file

The operation is not permitted, even if we use the sudo command.

If we use two angle brackets  “>>” to redirect output it is appended to the existing data in the file. That should be acceptable to our append-only text file.

sudo ls -l >> text-file.txt

Redirecting output to the end of a text file

We’re returned to the command prompt without any error messages. Let’s peek inside the file to see what has happened.

cat text-file.txt

Examining the contents of a text file

The redirected output from ls has been added to the end of the file.

New data appended to an append only text file

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Although we can append data to the file, that is the only change we can make to it. We can’t delete it and neither can root.

rm text-file.txt
sudo rm text-file.txt

Failing to delete an append only text file

Setting the Immutable Attribute

If you want to protect a file that will never have new data added to it, you can set the immutable attribute. This prevents all changes to the file, including appending data.

sudo chattr +i second-file.txt
lsattr second-file.txt

Setting the immutable attribute on a text file

We can see the “i” indicating the immutable attribute has been set. Having made our file immutable, even the root user can’t rename it (mv), delete it (rm), or add data to it.

sudo mv second-file.txt new-name.txt
sudo rm second-file.txt
sudo ls -l >> second-file.txt

An immutable file resisting change

Don’t Rely on Secure Deletion on ext4

As we pointed out, some operating systems do not support all of the attributes. The secure delete attribute is not honored by the ext family of file systems, including ext4. Don’t rely on this for the secure deletion of files.

It’s easy to see that this doesn’t work in ext4. We’ll set the s (secure deletion) attribute on a text file.

sudo chattr +s third-file.txt

Setting the secure deletion attribute on a text file

What we’re going to do is find out the inode that holds the metadata about this file. The inode holds the first hard drive block occupied by the file. The file contains some lorem ipsum placeholder text.

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We’ll read that block directly from the hard drive to verify we’re reading the correct hard drive location. We’ll delete the file and then read that same hard dive block once more. If the secure deletion attribute is being honored, we should read zeroed bytes.

We can find the inode of the file by using the hdparm command with the --fibmap (file block map) option.

sudo hdparm --fibmap third-file.txt

Finding the inode of a file

The first hard drive block is 18100656. We’ll use the dd command to read it.

The options are:

  • if=/dev/sda: Read from the first hard drive on this computer.
  • bs=512: Use a hard drive block size of 512 bytes.
  • skip=18100656: Skip all blocks before block 18100656. In other words, start reading at block 18100656.
  • count=1: Read one block of data.
sudo dd if=/dev/sda bs=512 skip=18100656 count=1

Reading the first hard drive block of a file

As expected we see the lorem ipsum placeholder text. We’re reading the correct block on the hard drive.

Now we’ll delete the file.

rm third-file.txt

If we read that same hard drive block, we can still see the data.

sudo dd if=/dev/sda bs=512 skip=18100656 count=1

Reading the data from a hard drive block used by a deleted file

Again, don’t depend on this for secure deletion on ext4.There are better methods available to delete files so that they can’t be recovered.

RELATED: How to Securely Delete Files on Linux

Useful, But Use With Caution

Setting the attributes of files can make them impervious to accidental disaster. If you can’t delete or overwrite a file, it is pretty safe.

You might think you’d like to apply them to system files and make your Linux installation more secure. But system files need to be replaced periodically as updates are issued or upgrades are applied. For that reason, it’s safest to only use these attributes on files of your own creation.

RELATED: How to Secure Your Linux Server with fail2ban

Profile Photo for Dave McKay Dave McKay
Dave McKay first used computers when punched paper tape was in vogue, and he has been programming ever since. After over 30 years in the IT industry, he is now a full-time technology journalist. During his career, he has worked as a freelance programmer, manager of an international software development team, an IT services project manager, and, most recently, as a Data Protection Officer. His writing has been published by  howtogeek.com, cloudsavvyit.com, itenterpriser.com, and opensource.com. Dave is a Linux evangelist and open source advocate.
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