why doesn't tar preserve file permissions?

Understanding Tar's Permission Handling: Why Your Files Might Lose Permissions

Explore the nuances of how the tar command handles file permissions, common pitfalls, and best practices to ensure your archived files retain their original attributes.

The tar command is a fundamental utility in Linux and Unix-like systems for archiving files. It's widely used for backups, software distribution, and packaging. However, users often encounter situations where files extracted from a tar archive don't retain their original permissions. This article delves into the reasons behind this behavior, how tar interacts with file permissions, and strategies to ensure your file attributes are preserved.

How Tar Handles Permissions by Default

By default, tar attempts to preserve as many file attributes as possible, including permissions, ownership, and timestamps. When creating an archive, tar records these attributes. When extracting, it tries to restore them. However, several factors can interfere with this process, leading to unexpected permission changes. The most common reason for permissions not being preserved is the user's permissions on the system where the extraction is performed, or specific tar options being omitted.

flowchart TD
    A[Create Tar Archive] --> B{"Record File Attributes (Permissions, Ownership)"}
    B --> C[Archive Created]
    C --> D[Extract Tar Archive]
    D --> E{"Attempt to Restore Attributes"}
    E --> F{"User Permissions (root vs. non-root)"}
    E --> G{"Tar Options Used (--no-same-owner, --no-same-permissions)"}
    E --> H{"Filesystem Capabilities (e.g., FAT32)"}
    F --> I{Permissions Restored?}
    G --> I
    H --> I
    I --> J[Success: Permissions Preserved]
    I --> K[Failure: Permissions Modified/Lost]

Flowchart illustrating tar's permission handling process during archive creation and extraction.

Common Reasons for Permission Loss

Several scenarios can lead to permissions not being preserved correctly when using tar:

1. Non-root User Extraction: If you extract files as a non-root user, you typically cannot set arbitrary ownership (user and group) or special permissions (like SUID/SGID bits) that belong to other users. The extracted files will usually inherit the ownership of the user performing the extraction, and permissions might be masked by the user's umask.

2. --no-same-owner / --no-same-permissions Options: These tar options explicitly tell tar not to restore ownership or permissions, respectively. While useful in specific contexts (e.g., extracting a package where you want files owned by the current user), they are a common cause of permission loss if used unintentionally.

3. Filesystem Limitations: Some filesystems, particularly older ones or those designed for cross-platform compatibility (like FAT32 or exFAT), do not support Unix-style permissions. When files are extracted to such a filesystem, their permissions will be lost or translated to a generic set.

4. Umask Settings: The umask setting of the user extracting the archive can restrict the permissions that are set on new files. If the umask is set to 022, for example, newly created files will not have write permissions for group or others, even if the archive specifies them.

# Create an archive with a file having specific permissions
echo "test content" > my_file.txt
chmod 755 my_file.txt
ls -l my_file.txt
tar -cvf archive.tar my_file.txt

# Extract as non-root user (simulated)
rm my_file.txt
# If current user is not root, permissions might change
tar -xvf archive.tar
ls -l my_file.txt

# Extract with --no-same-permissions
rm my_file.txt
tar -xvf archive.tar --no-same-permissions
ls -l my_file.txt

Demonstrating permission changes during tar extraction with and without specific options.

Ensuring Permission Preservation

To maximize the chances of preserving file permissions and ownership, follow these guidelines:

• Extract as Root: Whenever possible, extract archives containing sensitive permissions or ownership as the root user. This grants tar the necessary privileges to restore all attributes accurately.
• Use the -p or --preserve-permissions Option: While tar usually tries to preserve permissions by default, explicitly using -p or --preserve-permissions during extraction can reinforce this behavior. This is particularly useful if you suspect a tar version or environment might be behaving unexpectedly.
• Use the -P or --absolute-names Option (for absolute paths): If your archive contains files with absolute paths (e.g., /etc/passwd), tar will, by default, strip the leading / to prevent overwriting system files. Using -P allows tar to extract files to their absolute paths, which might be necessary for certain system backups, but use with extreme caution.
• Avoid --no-same-owner / --no-same-permissions: Unless you specifically intend to change ownership or permissions, ensure these options are not used during extraction.
• Verify Filesystem Compatibility: Ensure the target filesystem supports Unix-style permissions (e.g., ext4, XFS, Btrfs). If you're extracting to a non-Unix filesystem, expect permission loss.

# Create a file with specific permissions and ownership
sudo touch /tmp/root_owned_file
sudo chmod 640 /tmp/root_owned_file
sudo chown root:root /tmp/root_owned_file
ls -l /tmp/root_owned_file

# Archive it
sudo tar -cvf /tmp/archive_with_root.tar /tmp/root_owned_file

# Extract as a regular user (permissions/ownership will likely change)
rm /tmp/root_owned_file
tar -xvf /tmp/archive_with_root.tar
ls -l /tmp/root_owned_file

# Extract as root (permissions/ownership preserved)
sudo rm /tmp/root_owned_file
sudo tar -xpvf /tmp/archive_with_root.tar
ls -l /tmp/root_owned_file

Example demonstrating the importance of extracting as root to preserve ownership and permissions.