The format of a symbolic mode is +-=[ASacdisu].
The operator `+' causes the selected attributes to be added to the existing attributes of the files; `-' causes them to be removed; and `=' causes them to be the only attributes that the files have.
The letters `ASacdisu' select the new attributes for the files: don't update atime (A), synchronous updates (S), append only (a), compressed (c), immutable (i), no dump (d), secure deletion (s), and undeletable (u).
A file with the `a' attribute set can only be open in append mode for writing. Only the superuser can set or clear this attribute.
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.
A file with the `d' attribute set is not candidate for backup when the dump(8) program is run.
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. Only the superuser can set or clear this attribute.
When a file with the `s' attribute set is deleted, its blocks are zeroed and written back to the disk.
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.
When a file with the `u' attribute set is deleted, its contents are saved. This allows the user to ask for its undeletion.