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0 pointsquotemstr1y ago0 comments

The thing is, though, that PATH being a userspace concept is a contingent detail, an accident of history, not something inherent to the concept of an operating system. You can imagine a kernel that does path searches. Why not?

There's a difference between something being a certain way because it has to be that way in order to implement the semantics of the system (e.g. interrupt handlers being a privilege transition) and something being a certain way as a result of an arbitrary implementation choice.

OSes differ on these implementation choices all the time. For example,

* in Linux, the kernel is responsible for accepting a list of execve(2) argument-words and passing them to the exec-ed process with word boundaries intact. On Windows, the kernel passes a single string instead and programs chop that string up into argument words in userspace, in libc

* in Linux, the kernel provides a 32-bit system call API for 32-bit programs running on 64-bit kernels; on Windows, the kernel provides only a 64-bit system call API and it's a userspace program that does long-mode switching and system call argument translation

* on Windows, window handles (HWNDs, via user32.dll) in IPC message passing (ALPC, in ntoskrnl) are implemented in the kernel, whereas the corresponding concepts on most Linux systems are pure user-space constructs

And that's not even getting into weirder OSes! Someone familiar with operating systems in general can nevertheless be surprised at how a particular OS chooses to implement this or that feature.

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amiga3861y ago· 3 in thread

* in Linux, the kernel is responsible for accepting a list of execve(2) argument-words

Yes it does, but the more surprising thing is (coming from AmigaOS with its dos.library function ReadArgs()) that the shell does this. The shell is also responsible for argument expansion - madness!

On AmigaOS, when you type "delete foo#? force", the shell passes the entire command line to the delete command. The delete command calls ReadArgs() with a template (FILE/M/A, ALL/S, QUlET/S, FORCE/S), and the standard OS function parses it into lists of files, flags, keyword arguments, etc. The "file" passed is "foo#?", and the command uses MatchFirst()/MatchNext() to do file pattern matching.

Every command (that uses ReadArgs() and didn't plump for "standard C" parsing) has the same behaviour: running the command with "?" gives you the template, which tells you how to use it. Args are parsed consistently across all programs.

Then you get "standard C", which because K&R and main(), ignores this standard Amiga parsing function and just does naive splits. Across multiple Amiga C compilers, quoting rules are inconsistent. Amiga C compilers have to produce an executable, and it knows it'll be called with a full command line, so the executable itself has to break that into words before it can call main(), and it's up to each compiler writer how they're going to do that. Urgh.

In unix-land, it's up to the shell to parse the command line, and pass only the words... hence why the shell naturally does all the filename globbing, and why you have gotchas like when these two commands are sometimes the same and sometimes they're not:

    find . -name foo*
    find . -name 'foo*'

Then we have Windows, which is like Amiga C programs - it's being passed a full command string and will have its C runtime parse it for main() to consume. There's a vague expectation that it'll do quoting "like COMMAND", which itself has very odd quoting rules. At least, most people are all using the same C compiler on Windows, so it's mostly only MSVCRT's implementation so it's mostly consistent.

AlienRobot1y ago

Username checks out.

I think one of the most surprising things I learned about bash is that you can do this:

    touch ./-rf
    rm *

And now you have rm -rf'd. :)

teo_zero1y ago

We should use "--" more, but who has all this time to waste? :)

PhilipRoman1y ago

Indeed, always prefer ./* to *

I often wish there was a convenient way of doing such an operation in the shell: if path start with "/", leave it, otherwise prepend "./"

2 more replies

Animats1y ago

> The thing is, though, that PATH being a userspace concept is a contingent detail, an accident of history, not something inherent to the concept of an operating system. You can imagine a kernel that does path searches. Why not?

Right. You can't be sure that someone didn't stick $PATH expansion into glibc, or something. Because someone did.

QNX gets program loading entirely out of the kernel. When QNX is booted, initial programs and .so files in the boot image are loaded into memory. That's how things get started. Disk drivers, etc. come in that way, assuming the system has a disk.

Calling "exec.." or ".. spawn" merely links to a .so file that knows how to open and read an executable image. Program loading is done entirely by userspace code. Tiny microkernel. The "exec.." functions do not use the PATH variable.[1]

However, "posix_spawn" does read the PATH environment variable, in both QNX [2] and Linux.[3] Linux, for historical reasons, tends not to use "spawn" as much, but those are the defined semantics for it. QNX normally uses "spawn", because it lacks the legacy that encouraged fork/exec type process startup. "posix_spawn" is apparently faster in modern Linux, especially when the parent process is large, but there's a lot of fork/exec legacy code out there.

"posix_spawn" comes from FreeBSD in 2009, but I think the QNX implementation precedes that, because QNX's architecture favors "spawn" over "exec.." It may go back to UCLA Locus.

Windows has different program startup semantics. Someone from Windows land can address that. MacOS has a built in search path if you don't have a PATH variable.[5]

[1] https://www.qnx.com/developers/docs/8.0/com.qnx.doc.neutrino...

[2] https://www.qnx.com/developers/docs/8.0/com.qnx.doc.neutrino...

[3] https://www.man7.org/linux/man-pages/man3/posix_spawn.3.html

[4] https://www.whexy.com/posts/fork

[5] https://developer.apple.com/library/archive/documentation/Sy...

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