chattr Command in Linux with 5 Examples

Last updated: July 10, 2022 | Bobbin Zachariah

chattr (Change Attribute) is a command-line Linux utility that is used to change the file attributes. It can set and unset certain attributes on a file, which can control how these files are used and accessed. The chattr mostly works on ext2, ext3, ext4, and btrfs filesystems.

Common Use Cases:

  • Protect System Files: Set the immutable attribute on critical system files.
  • Secure Log Files: Make log files append-only to prevent tampering.
  • Data Integrity: Use synchronous updates for files needing immediate disk writing.

Syntax

The basic syntax for chattr command:

chattr [options] [operator][attributes] [files]

Options

  • -R: Recursively change attributes of directories and their contents.
  • -+= [attributes]: Add the specified attributes to the existing attributes of the files.
  • -=[attributes]: Remove the specified attributes from the existing attributes of the files.
  • -V: Verbose mode. Reports detailed information about what chattr is doing.

Attributes

  • A: No atime updates - when set, the Linux kernel will not update the access time when the file is accessed.
  • a: Append only - once set, the file can only be opened in append mode for writing.
  • C: No copy-on-write - this attribute disables copy-on-write for the file.
  • c: Compressed - the file is automatically compressed on the disk.
  • D: Synchronous directory updates - when a directory is modified, the changes are written synchronously on the disk.
  • d: No dump - when set, the file is not a candidate for backup when the dump program is run.
  • E: Extent format - indicates that the file is using extents for mapping the blocks on disk.
  • e: Extents are in use - used to indicate that the file uses extents for block mapping.
  • I: Immutable directory - prevents the directory from being deleted or renamed, and no files can be added or removed.
  • i: Immutable - when set, the file cannot be modified, deleted, or renamed, and no link can be created to this file.
  • j: Data journaling - ensures that all of its data is written to the journal before being written to the file itself, applicable for ext3 or ext4 filesystems.
  • s: Secure deletion - when a file is deleted, its blocks are zeroed and written back to the disk.
  • S: Synchronous updates - if the file is modified, the changes are written synchronously on the disk.
  • u: Undeletable - if a file is deleted, its contents are saved, allowing for undeletion.
  • X: Compressed dirty file - used to mark a compressed file that has been modified but not recompressed.
  • Z: Compressed file - indicates that the file is compressed.

chattr Examples

Let's check how to use chattr command with some examples.

1. Make file immutable

To set a file attribute we will use chattr command with  + operator followed by the attribute name.

Let's check with examples of how to set immutable attribute to a file. Only root or user with sudo privilege can set and remove immutable flag on a file.

A file with an immutable attribute:

  • Cannot be modified, deleted, renamed
  • No soft or hard link can be created by anyone including the root user.
  • No data can be written to the file

Let's create an empty file using touch command as follows:

$ touch file1

Now let's see how to list attributes of the file using lsattr command:

$ lsattr 
-------------e-- ./nagios-cookbook-tutorial
-------------e-- ./chefdk_1.3.40-1_amd64.deb
-------------e-- ./file1

Add some content to the file using echo command:

$ echo "Test of i attribute" > file1 
$ cat file1 
Test of i attribute

Now we can set immutable attribute using +i on the file named file1.

$ sudo chattr +i file1
$ lsattr 
-------------e-- ./nagios-cookbook-tutorial
-------------e-- ./chefdk_1.3.40-1_amd64.deb
----i--------e-- ./file1

In the following commands we will try append, delete both using normal user and sudo user (root):

$ echo "Try to edit after set i attribute" >> file1 
-bash: file1: Permission denied
$ sudo echo "Try to edit after set i attribute" >> file1 
-bash: file1: Permission denied
$ rm -f file1 
rm: cannot remove 'file1': Operation not permitted
$ sudo rm -f file1 
rm: cannot remove 'file1': Operation not permitted

We can observe that above all operations are not permitted.

2. Remove immutable

To remove any attribute from the file we have to use - operator followed by the attribute name.

In the following example, let us unset the immutable attribute from the file (file1).

$ sudo chattr -i file1 
$ lsattr file1
-------------e-- ./file1

You should be now able to do all normal operations on the file.

3. Recursive on directory

In order to secure the directory, we have to set attribute recursively (-R) using + operator.

The following command will set the immutable bit on the directory ('temp') recursively:

$ sudo chattr -R +i /temp 
$ lsattr -d temp
----i--------e-- temp/

The above command will recursively make /temp directory undeletable in Linux.

To unset, you have use  - operator followed by i attribute.

$ sudo chattr -R -i linoxide/

4. Append to file

It is possible to allow everyone to just append data on a file without changing or modifying already entered data with the a attribute.

It means that you can only add content on the current file without modifying data already present.

The following examples set append atrribute to the file (file2).

$ sudo chattr +a file2

5. Make file undeletable

chattr command allows making files in Linux undeletable by any user (not even by root user). This mostly helps to secure important files such as /etc/passwd or /etc/shadow.

To make a file undeletable:

sudo chattr +i /path-to-filename

For example to make the password and shadow file undeletable.

$ sudo chattr +i /etc/passwd
$ sudo chattr +i /etc/shadow

Remember this will disable user account creation. But when you will try to create a new system user, you will get an error message saying 'cannot open /etc/passwd'. If you try to change the password, you will be prompt to enter a new password but when you will log in next time, the new password will not appear to be valid, you will have to use the old password to log in.

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