3 ms·

Right. You should always test the command first. If the data is critical, use a temporary file instead. I usually use this in scripts so I don’t have to deal wi

Right. You should always test the command first. If the data is critical, use a temporary file instead. I usually use this in scripts so I don’t have to deal with cleanup.

sedatk 4y ago
> If the data is critical, use a temporary file instead Use a temporary file always. Sponge process may be interrupted, and you end up with a half-complete /etc/passwd in return.
- rcoveson 4y ago
  Couldn't `mv` or `cp` from the temp file to `/etc/passwd` be interrupted as well? I think the only way to do it atomically is a temporary file on the same filesystem as `/etc`, followed by a rename. On most systems `/tmp` will be a different filesystem from `/etc`.
  - AdamJacobMuller 4y ago
    mv can't, or, more correctly the rename system call can not. rename is an atomic operation from any modern filesystem's perspective, you're not writing new data, you're simply changing the name of the existing file, it either succeeds or fails. Keep in mind that if you're doing this, mv (the command line tool) as opposed to the `rename` system call, falls back to copying if the source and destination files are on different filesystems since you can not really mv a file across filesystems! In order to have truly atomic writes you need to: open a new file on the same filesystem as your destination file write contents call fsync call rename call sync (if you care about the file rename itself never being reverted). This is some very naive golang code (from when I barely knew golang) for doing this which has been running in production since I wrote it without a single issue: https://github.com/AdamJacobMuller/atomicxt/blob/master/file.go https://github.com/AdamJacobMuller/atomicxt/blob/master/file...
    - JJMcJ 4y ago
      Not clear on need for fsync and sync. Are those for networked like NTFS or just as security against crashes. Logically on a single system there would be no effect assuming error free filesystem operation. Unless I'm missing something.
      - AnssiH 4y ago
        Without the fsync() before rename(), on system crash, you can end up with the rename having been executed but the data of the new file not yet written to stable storage, losing the data. ext4 on Linux (since 2009) special-cases rename() when overwriting an existing file so that it works safely even without fsync() (https://lwn.net/Articles/322823/ https://lwn.net/Articles/322823/), but that is not guaranteed by all other implementations and filesystems. The sync() at the end is indeed not needed for the atomicity, it just allows you to know that after its completion the rename will not "roll back" anymore on a crash. IIRC you can also use fsync() on the parent directory to achieve this, avoiding sync() / syncfs().
        AdamJacobMuller 4y ago
        > ext4 on Linux (since 2009) special-cases rename This is interesting. The linked git entry (https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?id=dbc85aa9f11d8c13c15527d43a3def8d7beffdc8 https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.g...) from the LWN article says "Notice: this object is not reachable from any branch." Did this never get merged because I definitely saw this issue in production well after 2009. I guess it either got changed, or, a different patch applied but perhaps this https://github.com/torvalds/linux/blob/master/fs/ext4/namei.c#L3765-L3766 https://github.com/torvalds/linux/blob/master/fs/ext4/namei.... does it?
        AnssiH 4y ago
        The patch just got rebased, here's the one that was actually applied in master for v2.6.30: https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?id=8750c6d5fcbd3342b3d908d157f81d345c5325a7 https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.g... And yes, the code you highlighted is exactly this special-case in its current form. The mount option "noauto_da_alloc" can be used to disable these software-not-calling-fsync safety features.
  - sedatk 4y ago
    You can use `/etc/passwd.new` as a temporary file to avoid the problems you mentioned. In the worst case, you'll have an orphaned passwd.new file, but /etc/passwd is guaranteed to remain intact.
- einpoklum 4y ago
  Probably not. If it's implemented responsibly, it will internally: 1. Write to a temporary file 2. Do the equivalent of mv tmpfile originalfile so it will either succeed or do nothing
  - sedatk 4y ago
    "Responsibly" is subjective here. I could argue that responsible thing to do is to use as little resources as possible, and in that case, directly overwriting the file would be the "responsible" thing to do.
    - einpoklum 4y ago
      > I could argue that responsible thing to do is to use as little resources as possible No, you couldn't, because a sponge is intentionally using more resources: It soaks up as much want as it can. And the program is intended to soak up all of the output. Otherwise it would be `cat`.
      - sedatk 4y ago
        Your example proves my point: what's responsible is subjective. It's meaningless to talk about doing the "responsible" thing.
Quekid5 4y ago
This is why I usually just use a temporary directory and do a quick git init . git add . git commit -m "wip" ... and proceed from there. So many ways to screw up ad hoc data processing using shell and the above can be a life saver. (Along with committing along the way, ofc.) EDIT: Doesn't work if you have huuuuge files, obviously... but you should perhaps be using different tools for that anyway.
- chungy 4y ago
  You might like to try using src, a simple single-file VC: http://www.catb.org/esr/src/ http://www.catb.org/esr/src/
  - ElectricalUnion 4y ago
    I guess if you want something single-file that resembles git (now thinking better, not sure if a requirement at all), you can also try Fossil ( https://www2.fossil-scm.org https://www2.fossil-scm.org ).
- wombatpm 4y ago
  This is why I read HN. You never know when a brilliant idea will appear. Thank you. I never thought of doing this for temporary work
antihero 4y ago
Why not write it to a different file
- dredmorbius 4y ago
  cmd < somefile | somefile.tmp && mv somefile.tmp somefile Will read from somefile, and only replace the source if (and when) the pipleline exits successfully. Mind that this may still bite in interesting ways. But less frequently. You can also invoke tempfile(1) which is strongly recommended in scripts. https://www.unix.com/man-page/Linux/1/tempfile/ https://www.unix.com/man-page/Linux/1/tempfile/
  - HellsMaddy 4y ago
    I was wondering what the difference between tempfile and mktemp was. At the bottom of the tempfile man page, it says: > tempfile is deprecated; you should use mktemp(1) instead.
    - dredmorbius 4y ago
      I ... need to revisit that. Though I suspect you're right.
suction 4y ago
How do you know you've tested all cases?

Right. You should always test the command first. If the data is critical, use a temporary file instead. I usually use this in scripts so I don’t have to deal with cleanup.

sedatk 4y ago

> If the data is critical, use a temporary file instead Use a temporary file always. Sponge process may be interrupted, and you end up with a half-complete /etc/passwd in return.

rcoveson 4y ago
Couldn't `mv` or `cp` from the temp file to `/etc/passwd` be interrupted as well? I think the only way to do it atomically is a temporary file on the same filesystem as `/etc`, followed by a rename. On most systems `/tmp` will be a different filesystem from `/etc`.
- AdamJacobMuller 4y ago
  mv can't, or, more correctly the rename system call can not. rename is an atomic operation from any modern filesystem's perspective, you're not writing new data, you're simply changing the name of the existing file, it either succeeds or fails. Keep in mind that if you're doing this, mv (the command line tool) as opposed to the `rename` system call, falls back to copying if the source and destination files are on different filesystems since you can not really mv a file across filesystems! In order to have truly atomic writes you need to: open a new file on the same filesystem as your destination file write contents call fsync call rename call sync (if you care about the file rename itself never being reverted). This is some very naive golang code (from when I barely knew golang) for doing this which has been running in production since I wrote it without a single issue: https://github.com/AdamJacobMuller/atomicxt/blob/master/file.go https://github.com/AdamJacobMuller/atomicxt/blob/master/file...
  - JJMcJ 4y ago
    Not clear on need for fsync and sync. Are those for networked like NTFS or just as security against crashes. Logically on a single system there would be no effect assuming error free filesystem operation. Unless I'm missing something.
    - AnssiH 4y ago
      Without the fsync() before rename(), on system crash, you can end up with the rename having been executed but the data of the new file not yet written to stable storage, losing the data. ext4 on Linux (since 2009) special-cases rename() when overwriting an existing file so that it works safely even without fsync() (https://lwn.net/Articles/322823/ https://lwn.net/Articles/322823/), but that is not guaranteed by all other implementations and filesystems. The sync() at the end is indeed not needed for the atomicity, it just allows you to know that after its completion the rename will not "roll back" anymore on a crash. IIRC you can also use fsync() on the parent directory to achieve this, avoiding sync() / syncfs().
      - AdamJacobMuller 4y ago
        > ext4 on Linux (since 2009) special-cases rename This is interesting. The linked git entry (https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?id=dbc85aa9f11d8c13c15527d43a3def8d7beffdc8 https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.g...) from the LWN article says "Notice: this object is not reachable from any branch." Did this never get merged because I definitely saw this issue in production well after 2009. I guess it either got changed, or, a different patch applied but perhaps this https://github.com/torvalds/linux/blob/master/fs/ext4/namei.c#L3765-L3766 https://github.com/torvalds/linux/blob/master/fs/ext4/namei.... does it?
        AnssiH 4y ago
        The patch just got rebased, here's the one that was actually applied in master for v2.6.30: https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?id=8750c6d5fcbd3342b3d908d157f81d345c5325a7 https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.g... And yes, the code you highlighted is exactly this special-case in its current form. The mount option "noauto_da_alloc" can be used to disable these software-not-calling-fsync safety features.
- sedatk 4y ago
  You can use `/etc/passwd.new` as a temporary file to avoid the problems you mentioned. In the worst case, you'll have an orphaned passwd.new file, but /etc/passwd is guaranteed to remain intact.
einpoklum 4y ago
Probably not. If it's implemented responsibly, it will internally: 1. Write to a temporary file 2. Do the equivalent of mv tmpfile originalfile so it will either succeed or do nothing
- sedatk 4y ago
  "Responsibly" is subjective here. I could argue that responsible thing to do is to use as little resources as possible, and in that case, directly overwriting the file would be the "responsible" thing to do.
  - einpoklum 4y ago
    > I could argue that responsible thing to do is to use as little resources as possible No, you couldn't, because a sponge is intentionally using more resources: It soaks up as much want as it can. And the program is intended to soak up all of the output. Otherwise it would be `cat`.
    - sedatk 4y ago
      Your example proves my point: what's responsible is subjective. It's meaningless to talk about doing the "responsible" thing.

Quekid5 4y ago

This is why I usually just use a temporary directory and do a quick git init . git add . git commit -m "wip" ... and proceed from there. So many ways to screw up ad hoc data processing using shell and the above can be a life saver. (Along with committing along the way, ofc.) EDIT: Doesn't work if you have huuuuge files, obviously... but you should perhaps be using different tools for that anyway.

chungy 4y ago
You might like to try using src, a simple single-file VC: http://www.catb.org/esr/src/ http://www.catb.org/esr/src/
- ElectricalUnion 4y ago
  I guess if you want something single-file that resembles git (now thinking better, not sure if a requirement at all), you can also try Fossil ( https://www2.fossil-scm.org https://www2.fossil-scm.org ).
wombatpm 4y ago
This is why I read HN. You never know when a brilliant idea will appear. Thank you. I never thought of doing this for temporary work

antihero 4y ago

Why not write it to a different file

dredmorbius 4y ago
cmd < somefile | somefile.tmp && mv somefile.tmp somefile Will read from somefile, and only replace the source if (and when) the pipleline exits successfully. Mind that this may still bite in interesting ways. But less frequently. You can also invoke tempfile(1) which is strongly recommended in scripts. https://www.unix.com/man-page/Linux/1/tempfile/ https://www.unix.com/man-page/Linux/1/tempfile/
- HellsMaddy 4y ago
  I was wondering what the difference between tempfile and mktemp was. At the bottom of the tempfile man page, it says: > tempfile is deprecated; you should use mktemp(1) instead.
  - dredmorbius 4y ago
    I ... need to revisit that. Though I suspect you're right.

suction 4y ago

How do you know you've tested all cases?