Стиковый бит в файловых системах Unix

UNIX file system permission
the filesystem concept
In computing, the sticky bit is a user ownership access right flag that can be assigned to files and directories on Unix like systems. There are two definitions: one for files, one for directories. For files, particularly executables, superuser could tag these as to be retained in main memory, even when their need ends, to minimize swapping that would occur when another need arises, and the file now has to be reloaded from relatively slow secondary memory. This function has become obsolete due to swapping optimization. For directories, when a directory's sticky bit is set, the filesystem treats the files in such directories in a special way so only the file's owner, the directory's owner, or root user can rename or delete the file. Without the sticky bit set, any user with write and execute permissions for the directory can rename or delete contained files, regardless of the file's owner. Typically this is set on the /tmp directory to prevent ordinary users from deleting or moving other users' files. The modern function of the sticky bit refers to directories, and protects directories and their content from being hijacked by non owners; this is found in most modern Unix like systems. Files in a shared directory such as /tmp belong to individual owners, and non owners may not delete, overwrite or rename them.

The sticky bit was introduced in the Fifth Edition of Unix (in 1974) for use with pure executable files. When set, it instructed the operating system to retain the text segment of the program in swap space after the process exited. This speeds up subsequent executions by allowing the kernel to make a single operation of moving the program from swap to real memory. Thus, frequently used programs like editors would load noticeably faster. One notable problem with "stickied" programs was replacing the executable (for instance, during patching); to do so required removing the sticky bit from the executable, executing the program and exiting to flush the cache, replacing the binary executable, and then restoring the sticky bit. Subsequently, this behavior became operative only in HP UX and UnixWare. Solaris appears to have abandoned this in 2005. The 4.4 Lite release of BSD retained the old sticky bit behavior, but it has been subsequently dropped from OpenBSD (as of release 3.7) and FreeBSD (as of release 2.2.1). No version of Linux has ever supported this traditional behavior; Linux performs caching of executable files in the same way as all files, so re executing the program to flush the cache is not necessary.