chattr

chattr is the command in the Linux operating system that allows a user to set certain attributes of a file residing on a Linux file system. lsattr is the command that displays the attributes of a file.

Modern BSD-like systems, including OS X, have analogous chflags to set, but no command specifically meant to display them; specific options to the ls command are used instead.

The Solaris system has no commands specifically meant to manipulate them. chmod[1] and ls[2] are used instead.

Other Unices, in general, have no analogous commands. The similar-sounding commands chatr (from HP-UX) and lsattr (from AIX) exist but have unrelated functions.

Among other things, the chattr command is useful to make files immutable so that password files and certain system files cannot be erased during software upgrades.[3]

In Linux systems (chattr and lsattr)

File system support

The attributes chattr and lsattr manipulate were originally specific to the Second Extended Filesystem family (ext2, ext3, ext4), and are available as part of the e2fsprogs package.

However, the functionality has since been extended, fully or partially, to many other systems, including XFS, ReiserFS, JFS and OCFS2. The btrfs file system includes the attribute functionality, including C which turns off the write-on-copy built in feature of btrfs due to slower performance associated with CoW.

Even the originally targeted file systems miss some features, as pointed further in this article.

chattr description

The form of the chattr command is:

 chattr [-RVf] [-+=AacDdijsTtSu] [-v version] files...

lsattr description

The form of the lsattr command (gnu 1.41.3):

 lsattr [ -RVadv ] [ files...  ]

Attributes

Some attributes include:

File attributes on a Linux file system according to the chattr(1) Linux man page
Attribute lsattr flag chattr option Semantics and rationale
No atime updates A +A to set
-A to clear
  • When a file with the A attribute set is accessed, its atime record is not modified.
  • This avoids a certain amount of disk I/O for laptop systems.
Append only a +a to set
-a to clear[note 1]
  • A file with the a attribute set can only be open in append mode for writing.
Compressed c +c to set
-c to clear[note 2]
  • A file with the c attribute set is automatically compressed on the disk by the kernel.
  • A read from this file returns uncompressed data.
  • A write to this file compresses data before storing them on the disk.
Synchronous directory updates D +D to set
-D to clear
  • When a directory with the D attribute set is modified, the changes are written synchronously on the disk
  • This is equivalent to the dirsync mount option, applied to a subset of the files.
No dump d +d to set
-d to clear
  • A file with the d attribute set is not candidate for backup when the dump program is run.
Compression error E (unavailable)
  • The E attribute is used by the experimental compression patches to indicate that a compressed file has a compression error.
Extent format e (unavailable)
  • The e attribute indicates that the file is using extents for mapping the blocks on disk.
Huge file h (unavailable)
  • The h attribute indicates the file is storing its blocks in units of the filesystem blocksize instead of in units of sectors.
  • It means that the file is, or at one time was, larger than 2TB.
Indexed directory I (unavailable)
  • The I attribute is used by the htree program code to indicate that a directory is being indexed using hashed trees.
Immutable i +i to set
-i to clear[note 1]
  • A file with the i attribute cannot be modified.
  • It cannot be deleted or renamed, no link can be created to this file and no data can be written to the file.
  • When set, prevents, even the superuser, from erasing or changing the contents of the file.
Data journaling j +j to set
-j to clear[note 3]
  • A file with the j attribute has all of its data written to the ext3 journal before being written to the file itself, if the filesystem is mounted with the "data=ordered" or "data=writeback" options.
  • When the filesystem is mounted with the "data=journal" option all file data is already journaled, so this attribute has no effect.
Secure deletion s +s to set
-s to clear[note 2][note 4]
Synchronous updates S +S to set
-S to clear
  • When a file with the S attribute set is modified, the changes are written synchronously on the disk; this is equivalent to the 'sync' mount option applied to a subset of the files.
  • This is equivalent to the sync mount option, applied to a subset of the files.
Top of directory hierarchy T +T to set
-T to clear
  • A directory with the T attribute will be deemed to be the top of directory hierarchies for the purposes of the Orlov block allocator.
  • This is a hint to the block allocator used by ext3 and ext4 that the subdirectories under this directory are not related, and thus should be spread apart for allocation purposes.
  • For example: it is a very good idea to set the T attribute on the /home directory, so that /home/john and /home/mary are placed into separate block groups.
  • For directories where this attribute is not set, the Orlov block allocator will try to group subdirectories closer together where possible.
No tail-merging t +t to set
-t to clear
  • For those filesystems which support tail-merging, a file with the t attribute will not have a partial block fragment at the end of the file merged with other files.
  • This is necessary for applications such as LILO which read the filesystem directly, and which don't understand tail-merged files.
Undeletable u +u to set
-u to clear[note 2]
  • When a file with the u attribute set is deleted, its contents are saved.
  • This allows the user to ask for its undeletion.
Compression raw access X (unavailable)
  • The X attribute is used by the experimental compression patches to indicate that a raw contents of a compressed file can be accessed directly.
Compressed dirty file Z (unavailable)
  • The Z attribute is used by the experimental compression patches to indicate a compressed file is "dirty".
Version / generation number -v -v version
  • File's version/generation number.

Notes

  1. 1 2 Only the superuser or a process possessing the CAP_LINUX_IMMUTABLE capability can set or clear these attributes.
  2. 1 2 3 These attributes are not honored by the ext2 and ext3 filesystems as implemented in the current mainline Linux kernels.
  3. ↑ Only the superuser or a process possessing the CAP_SYS_RESOURCE capability can set or clear this attribute.
  4. ↑ This attribute is not honored by the ext4 filesystem as implemented in the current mainline Linux kernels as reported in Bug #17872.

Other attributes

Other attributes include:

  • no Copy-on-write (C) [4] The btrfs file system includes the use of the C attribute.

In BSD-like systems (chflags)

File system support

The chflags command is not specific to particular file systems. UFS on BSD systems, and HFS+, SMB, AFP, and FAT on OS X support least some flags.

chflags description

The form of the chflags command is:

 chflags [-R [-H | -L | -P]] flags file ...

Displaying

BSD-like systems, in general, have no default user-level command specifically meant to display the flags of a file. The ls command will do with either the -lo, or the -lO, depending on the system, flags passed.

Attributes

All attributes can be set or cleared by the super-user; some can also be set or cleared by the owner of the file. Some attributes include:

File attributes
Attribute ls flag chflags flag Owner-settable OS support Semantics and rationale
Archived arch arch, archived No All
Opaque opaque opaque Yes All Directory is opaque when viewed through a union mount
No dump nodump nodump Yes All
System append-only sappnd sappnd, sappend No All Existing data in the file can't be overwritten and the file cannot be truncated
System immutable schg schg, schange, simmutable No All File cannot be changed, renamed, moved, or removed
User append-only uappnd uappnd, uappend Yes All Existing data in the file can't be overwritten and the file cannot be truncated
User immutable uchg uchg, uchange, uimmutable Yes All Existing data in the file can't be overwritten
Hidden hidden hidden Yes OS X File is hidden by default in the GUI (but not in ls)

See also

Notes

  1. ↑ chmod(1) – illumos and OpenSolaris User Commands Reference Manual from latest Sun based OpenSolaris
  2. ↑ ls(1) – illumos and OpenSolaris User Commands Reference Manual from latest Sun based OpenSolaris
  3. ↑ chflags(1) – OpenBSD General Commands Manual
  4. ↑ E2fsprogs: add compress and cow support in chattr, lsattr, retrieved April 9, 2012

References

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