A tale of two path separators
alexwlchan.net52 points by dbaupp 4 days ago
52 points by dbaupp 4 days ago
Old Macs (which I grew up with) had even more baroque path handling than mentioned in the blog post:
Double colon (::) meant the same as .. on Unix/DOS, that is "go up one level". So you have to be careful when concatenating paths to not get double separators.
Paths starting with : were relative. If a path didn't start with the separator, the first component was the volume name (disk partition). Again, quite unlike Unix.
Also, remember it was common to have spaces in names on Mac, even the default harddrive on Macs was named "Macintosh HD". So an absolute path like "Macintosh HD:Programs:MacWrite" would have been common. (I grew up with Macs in Swedish, so I'm back translating the names here, could be that the names were slightly different in English.)
Fun things is that I encountered that for the first time when using Clozure CL (https://ccl.clozure.com/) which quotes colons when converting paths to string even on Linux:
$ cat <<'EOF' >x.lisp
heredoc> (require :uiop)
heredoc> (let ((p (make-pathname :name "foo:bar")))
heredoc> (format t "~@{~A~%~}" (namestring p) (uiop:native-namestring p)))
heredoc> EOF
$ ccl -b -Q -l x.lisp </dev/null
foo\:bar
foo:bar
$ sbcl --script x.lisp
foo:bar
foo:barCurrent macOS Finder let's you name files with a slash in them, rendered as a : in the terminal.
Isn't that just what the original article that we all commented on described? I don't understand what you are trying to add to the conversation here.
I think that the slash might be the rendering though? If you
$ touch "foo:bar"
So who is lying, how is it stored in the directory entry in APFS itself?
It took me a long time to understand why colon wasn't a valid character for file names on Mac and I still find the colon separator to be the least visible these days. Finder can display paths with the forward slash separator (defaults write com.apple.finder _FXShowPosixPathInTitle -bool YES), and yet forward slash may be used in a file name created through Finder as noted in the post, while colon cannot (which is not addressed), but creating a file in the terminal named with a colon is possible and the shell will escape it correctly in use. This file then shows up with a slash in place of the colon when viewed in Finder, and conversely the file with a slash in the name shows up in Terminal with a colon!
You still have to worry about colons on Linux; while they're valid in file and folder names, they prevent folders from being put on PATH and understood properly.
(Other characters of course cause usability problems and are potentially even a security vulnerability depending on the terminal. But they're still "valid".)
Another Windows oddity: each drive letter has its own current directory. D: doesn't mean the root of D:, it means "wherever you last were on D:". Same with C:foo, which is relative to C:'s current directory. DOS baggage that's still around.
And you need `cd /d` to switch drives. This was how I rendered a Windows computer non-bootable for the first time. Ran Command Prompt as admin (because I was logged in as a user that didn’t have write access to D:\backups), and it starts in a rather important directory, then:
C:\WINDOWS\system32>cd D:\backups\some-huge-directory
C:\WINDOWS\system32>del /s *
I'm curious how much of this behaviour is still intentional design vs. just inertia. Are the modern filesystems still constrained by these older choices, or is it mostly for compatibility?
Filesystems usually do not see path separators at all, it's something handled at VFS level
But they do see / or : in file names, and the interesting question is which one it is today on which filesystem.
The filesystems of macOS are particularly opinionated, much more than most Unices which tend toward "anything is allowed [and usually preserved] except \0 and /".
macOS supports case-insensitivity[0] and performs unicode normalization[1] on filenames, and decomposes name data to an extent that the question "what does the fs see" is a bit moot.
With that said, the internal storage of filenames in APFS are a nul-terminated UTF-8 string[2], with (i'm pretty sure) colons as colons, which the Finder displays as slashes.
[0] if you make a file named "Makefile" then touch a file named "makefile", it'll touch the first file, instead of making a second file.
[1] if you make a file named "schön" (s-c-h-combining¨-o-n) and then search for (s-c-h-ö-n), you can find it, or vice versa. The particular normalization/canonicalization used is NFD.
[2] j_drec_key_t description in https://developer.apple.com/support/downloads/Apple-File-Sys...
> a file named "schön" (s-c-h-combining¨-o-n)
Combining marks come after the character they modify, btw. (Presumably thanks to support from things like harfbuzz, modern systems will happily put two dots above an h.)
I was expecting the story of the magical ¥ path separator
Would you mind sharing a related link? I'm finding it to be impossible to search "¥ path separator", the ¥ is stripped out by like every search engine even when quoted
The short version is, the DOS-era Japanese character sets had replaced \ with ¥, and modern Windows still shows paths as C:¥WINDOWS if you're using the Japanese locale.
Now imagine operating system, that has no directories (and no path separators) or no filesystem at all.
Like CP/M, DOS v1, RT-11 etc.
VMS used:
node::device:[dir1.dir2.dir3]filename.extension;version
The versioning was cool as long as you remembered to clean them up.