Seven kinds of file

Level: 201 · working knowledge

One line: A path names one of seven kinds of thing — regular file, directory, symbolic link, FIFO, socket, character device, block device — ls -l shows which in its first character, fs::metadata answers three of them and FileTypeExt the other four, and File::open on a directory succeeds, so the check that matters is on the first read.

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

What it has to cover

• The seven, with the ls -l letter for each (-, d, l, p, s, c, b) and one example path on a Mac and on Linux — /dev/zero is a character device on both, /dev/disk0 a block device on one
• FileType ↗: is_file, is_dir, is_symlink, and why the three are not exhaustive; std::os::unix::fs::FileTypeExt ↗ for is_fifo, is_socket, is_char_device, is_block_device
• metadata follows a symlink and symlink_metadata does not — measured for the inspector page: /tmp on macOS is a directory to the first and a link to the second
• File::open on a directory returns Ok on Unix and the first read fails with ErrorKind::IsADirectory — the reason a tool checks the kind before reading, or handles the read error, and never trusts the open
• What each kind does to a reader: a FIFO blocks until a writer arrives, /dev/zero never ends, a socket refuses open outright
• Size, permissions and modification time from the same Metadata, and which of them a pipe does not have
• What is not a kind: an .app bundle is a directory, a hard link is a regular file with two names, and a Windows junction is a reparse point that is_symlink reports as a link

The trap it exists for

if path.exists() { File::open(path)? } passes for a directory, a FIFO and a device, and the failure moves to wherever the first read happens — often inside a library, with a message that names none of them. exists() answers one question; file_type() answers the one you meant.

See also

• Opening a file — the open that succeeds on a directory
• Missing is not empty — the two answers that both look like nothing there
• Path and PathBuf — the name, before it is any of the seven
• Writing a file inspector — why a tool needs all seven answers
• File type is four questions ↗ — the inode is question one; file, the mode bits and the extension are the other three

Po polsku

Ścieżka nazywa jedną z siedmiu rzeczy — zwykły plik, katalog, dowiązanie symboliczne, kolejkę FIFO, gniazdo, urządzenie znakowe albo urządzenie blokowe — a ls -l pokazuje którą w pierwszym znaku wiersza (-, d, l, p, s, c, b). fs::metadata(ścieżka)?.file_type() odpowiada na trzy z tych pytań (is_file, is_dir, is_symlink), a uniksowa cecha FileTypeExt na pozostałe cztery; metadata podąża za dowiązaniem, symlink_metadata nie, więc /tmp na Macu jest katalogiem dla pierwszej i dowiązaniem dla drugiej. Rzecz, dla której ta strona istnieje: File::open na katalogu kończy się powodzeniem w Uniksie, a dopiero pierwszy read zwraca IsADirectory — więc sprawdzenie path.exists() przed otwarciem niczego nie chroni, a błąd przenosi się tam, gdzie ktoś pierwszy raz czyta, często w cudzej bibliotece i z komunikatem, który nie nazywa żadnego z siedmiu rodzajów.

Szukaj po polsku: rodzaje plików w Uniksie · dowiązanie symboliczne a twarde · kolejka FIFO · rust fs::metadata file_type · rust FileTypeExt is_fifo · rust IsADirectory