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Comment by amstan | Hacker News Reader

Missing (probably because of the date of the article): `mv --exchange` aka renameat2+RENAME_EXCHANGE. It atomically swaps 2 file paths.

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I tried using this a while back and found it was not widely available. You need coreutils version 9.1 or later for this, many distros do not ship this.

I made https://github.com/rubenvannieuwpoort/atomic-exchange for my usecase.

Title says Unix, renameat2 is Linux-only.

>Title says Unix,

You're misinterpreting the title. The author didn't intend "Unix" to literally mean only the official AT&T/TheOpenGroup UNIX® System to the exclusion of Linux.

The first sentence of "UNIX-like" makes that clear : >This is a catalog of things UNIX-like/POSIX-compliant operating systems can do atomically,

Further down, he then mentions some Linux specifics : >fcntl(fd, F_GETLK, &lock), fcntl(fd, F_SETLK, &lock), and fcntl(fd, F_SETLKW, &lock) . [...] There is a “mandatory locking” mode but Linux’s implementation is unreliable as it’s subject to a race condition.

But I also don't think the auther meant Things you can do in Linux but not Unix

>But I also don't think the auther meant Things you can do in Linux but not Unix

I wasn't claiming that. I just thought the ggp had a useful comment about renameat2() which led to gp's "correction" which wasn't 100% accurate.

IBM z/OS UNIX also has renameat2(). It doesn't have the Linux specific flag RENAME_EXCHANGE.

https://www.ibm.com/docs/en/zos/3.1.0?topic=functions-rename...

pedantic but z/OS isn't a unix, it can just pretend to be one enough for the open group to call it one. IBM has a unix still, AIX.

In recent versions, z/OS has been copying lots of Linux-specific APIs (e.g. unshare [0]) in order to support the z/OS port of Kubernetes.

If Kubernetes starts using renameat2(RENAME_EXCHANGE), they could very plausibly add it.

[0] https://www.ibm.com/docs/en/zos/3.2.0?topic=csd-unshare-bpx1...

Bit rot alert: Linux doesn't even have mandatory file locks these days.

Linux-specific open file description locks could be brought up in a modern version of TFA though.

Sounds like the key term then is probably this:

> POSIX-compliant

Which, FWIW, doesn't mean Linux. AFAIK there is no Linux distro that's fully compliant, even before you worry about the specifics of whether it's certified as compliant.

EulerOS was certified UNIX some years ago.

Huh, TIL. Thanks.

AFAIK you don't even want to be POSIX-compliant unless having a sticker means more to you than being reasonable. Most projects knowingly steer away from compliance (and certifying compliance is probably also expensive)

>POSIX-compliant Which, FWIW, doesn't mean Linux. AFAIK there is no Linux distro that's fully compliant

I read author's use of "POSIX-compliant" as a loose and fuzzy family category rather than an exhaustive and authoritative reference on 100% strict compliance. Therefore, the author mentioning non-100%-compliant Linux is ok.

There seems to be 2 different expectations and interpretations of what the article is about.

- (1) article is attempting to be a strict intersection of all Unix-like systems that conform to official UNIX POSIX API. I didn't think this was a reasonable interpretation since we can't be sure the author actually verified/tested other POSIX-like systems such as FreeBSD, HP-UX, IBM AIX, etc.

- (2) article is a looser union of operating systems and can also include idiosyncracies of certain systems like Linux that the author is familiar with that don't apply to all other UNIX systems. I think some readers don't realize that all the author's citations to man pages point to Linux specific urls at : https://linux.die.net/man/

The ggp's (amstan) additional comment about renameat2(,,,,RENAME_EXCHANGE) is useful info and is consistent with interpretation (2).

If the author really didn't want Linux to be lumped in with "POSIX-like", it seems he would avoid linux.die.net and instead point to something more of a UNIX standard such as: https://unix.org/apis.html

[0] Intersection vs Union: https://en.wikipedia.org/wiki/Set_(mathematics)#Intersection

The slash is read as "OR" in this case.

As in: Unix-like OR POSIX-compliant

In that light, it's probably fine to not nitpick over certifications here.

Except POSIX doesn't specify some of them as happening atomically.

Many people write UNIX/POSIX without ever reading what it says.

They aren’t misinterpreting the title, the title is incorrect.

>, the title is incorrect.

Differing philosophies of how to interpret titles. Prescriptive vs Descriptive language.[0]

There can be different usages of the word "Unix":

#1: Unix is a UNIX(tm) System V descendent. More emphasis that the kernel needs to be UNIX. In this strict definition, you get the common reminder that "Linux is not a Unix!"

#2: "Unix" as a loose generic term for a family of o/s that looks/feels like Unix. This perspective includes using an o/s that has userland Unix utilities like cat/grep/awk. Sometimes deliberately styled as asterisk "*nix" or a suffix-qualifier "Unix-like" but often just written as a naked "Unix".

A Prescriptivist says the author's title is "incorrect". On the other hand, a Descriptivist looks at the whole content of the article -- notices the text has a lot of Linux specific info such as fcntl(,F_GETLEASE/F_SETLEASE), and every hyperlink to a man page url points to https://linux.die.net/man/ , etc -- and thus determines that the author is using "Unix"(#2) in the looser way that can include some Linux idiosyncrasies.

"Unix" instead of "*nix" as a generic term for Linux is not uncommon. Another example article where the authors use the so-called incorrect "Unix" in the title even though it's mostly discussing Linux CUPS instead of Solaris : https://www.evilsocket.net/2024/09/26/Attacking-UNIX-systems...

[0] https://en.wikipedia.org/wiki/Linguistic_prescription

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