How does sponge (from moreutils) work?












1















sponge can “soak up” stdin and write it atomically to a file, enabling one to do cat f|sponge a. I want to know how exactly it accomplishes this. How does it know when the input is finished?










share|improve this question




















  • 2





    What do you mean? The same way every other program knows (e.g. cat f | wc or cat f | grep foo or whatever), why would you expect sponge to be special?

    – terdon
    Nov 22 '18 at 19:31
















1















sponge can “soak up” stdin and write it atomically to a file, enabling one to do cat f|sponge a. I want to know how exactly it accomplishes this. How does it know when the input is finished?










share|improve this question




















  • 2





    What do you mean? The same way every other program knows (e.g. cat f | wc or cat f | grep foo or whatever), why would you expect sponge to be special?

    – terdon
    Nov 22 '18 at 19:31














1












1








1


1






sponge can “soak up” stdin and write it atomically to a file, enabling one to do cat f|sponge a. I want to know how exactly it accomplishes this. How does it know when the input is finished?










share|improve this question
















sponge can “soak up” stdin and write it atomically to a file, enabling one to do cat f|sponge a. I want to know how exactly it accomplishes this. How does it know when the input is finished?







shell io-redirection stdout stdin






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Nov 23 '18 at 3:43









Jeff Schaller

40.1k1054126




40.1k1054126










asked Nov 22 '18 at 19:23









HappyFaceHappyFace

31811




31811








  • 2





    What do you mean? The same way every other program knows (e.g. cat f | wc or cat f | grep foo or whatever), why would you expect sponge to be special?

    – terdon
    Nov 22 '18 at 19:31














  • 2





    What do you mean? The same way every other program knows (e.g. cat f | wc or cat f | grep foo or whatever), why would you expect sponge to be special?

    – terdon
    Nov 22 '18 at 19:31








2




2





What do you mean? The same way every other program knows (e.g. cat f | wc or cat f | grep foo or whatever), why would you expect sponge to be special?

– terdon
Nov 22 '18 at 19:31





What do you mean? The same way every other program knows (e.g. cat f | wc or cat f | grep foo or whatever), why would you expect sponge to be special?

– terdon
Nov 22 '18 at 19:31










1 Answer
1






active

oldest

votes


















3














strace or similar will show the system calls used by sponge, which is probably to write(2) the input read(2) from standard input out to a temporary file, and then to rename(2) that temporary file to the desired output filename when the input ends. The input ends when a read(2) call fails or returns 0 (which indicates end-of-file) at which point sponge can do the rename.






share|improve this answer


























  • And when the rename() fails with EXDEV when /tmp is on a different file system, it ends up copying the data again into the destination file. You can avoid that by setting TMPDIR to $(dirname target-file) or use ksh93's >; operator instead of sponge which does that automatically (and also doesn't override the target file if the redirected command failed).

    – Stéphane Chazelas
    Nov 22 '18 at 21:06













  • >; ? mind blown.

    – glenn jackman
    Nov 23 '18 at 16:40











Your Answer








StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "106"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});


}
});














draft saved

draft discarded


















StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f483524%2fhow-does-sponge-from-moreutils-work%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown

























1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









3














strace or similar will show the system calls used by sponge, which is probably to write(2) the input read(2) from standard input out to a temporary file, and then to rename(2) that temporary file to the desired output filename when the input ends. The input ends when a read(2) call fails or returns 0 (which indicates end-of-file) at which point sponge can do the rename.






share|improve this answer


























  • And when the rename() fails with EXDEV when /tmp is on a different file system, it ends up copying the data again into the destination file. You can avoid that by setting TMPDIR to $(dirname target-file) or use ksh93's >; operator instead of sponge which does that automatically (and also doesn't override the target file if the redirected command failed).

    – Stéphane Chazelas
    Nov 22 '18 at 21:06













  • >; ? mind blown.

    – glenn jackman
    Nov 23 '18 at 16:40
















3














strace or similar will show the system calls used by sponge, which is probably to write(2) the input read(2) from standard input out to a temporary file, and then to rename(2) that temporary file to the desired output filename when the input ends. The input ends when a read(2) call fails or returns 0 (which indicates end-of-file) at which point sponge can do the rename.






share|improve this answer


























  • And when the rename() fails with EXDEV when /tmp is on a different file system, it ends up copying the data again into the destination file. You can avoid that by setting TMPDIR to $(dirname target-file) or use ksh93's >; operator instead of sponge which does that automatically (and also doesn't override the target file if the redirected command failed).

    – Stéphane Chazelas
    Nov 22 '18 at 21:06













  • >; ? mind blown.

    – glenn jackman
    Nov 23 '18 at 16:40














3












3








3







strace or similar will show the system calls used by sponge, which is probably to write(2) the input read(2) from standard input out to a temporary file, and then to rename(2) that temporary file to the desired output filename when the input ends. The input ends when a read(2) call fails or returns 0 (which indicates end-of-file) at which point sponge can do the rename.






share|improve this answer















strace or similar will show the system calls used by sponge, which is probably to write(2) the input read(2) from standard input out to a temporary file, and then to rename(2) that temporary file to the desired output filename when the input ends. The input ends when a read(2) call fails or returns 0 (which indicates end-of-file) at which point sponge can do the rename.







share|improve this answer














share|improve this answer



share|improve this answer








edited Nov 22 '18 at 20:04

























answered Nov 22 '18 at 19:31









thrigthrig

24.6k23056




24.6k23056













  • And when the rename() fails with EXDEV when /tmp is on a different file system, it ends up copying the data again into the destination file. You can avoid that by setting TMPDIR to $(dirname target-file) or use ksh93's >; operator instead of sponge which does that automatically (and also doesn't override the target file if the redirected command failed).

    – Stéphane Chazelas
    Nov 22 '18 at 21:06













  • >; ? mind blown.

    – glenn jackman
    Nov 23 '18 at 16:40



















  • And when the rename() fails with EXDEV when /tmp is on a different file system, it ends up copying the data again into the destination file. You can avoid that by setting TMPDIR to $(dirname target-file) or use ksh93's >; operator instead of sponge which does that automatically (and also doesn't override the target file if the redirected command failed).

    – Stéphane Chazelas
    Nov 22 '18 at 21:06













  • >; ? mind blown.

    – glenn jackman
    Nov 23 '18 at 16:40

















And when the rename() fails with EXDEV when /tmp is on a different file system, it ends up copying the data again into the destination file. You can avoid that by setting TMPDIR to $(dirname target-file) or use ksh93's >; operator instead of sponge which does that automatically (and also doesn't override the target file if the redirected command failed).

– Stéphane Chazelas
Nov 22 '18 at 21:06







And when the rename() fails with EXDEV when /tmp is on a different file system, it ends up copying the data again into the destination file. You can avoid that by setting TMPDIR to $(dirname target-file) or use ksh93's >; operator instead of sponge which does that automatically (and also doesn't override the target file if the redirected command failed).

– Stéphane Chazelas
Nov 22 '18 at 21:06















>; ? mind blown.

– glenn jackman
Nov 23 '18 at 16:40





>; ? mind blown.

– glenn jackman
Nov 23 '18 at 16:40


















draft saved

draft discarded




















































Thanks for contributing an answer to Unix & Linux Stack Exchange!


  • Please be sure to answer the question. Provide details and share your research!

But avoid



  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.


To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f483524%2fhow-does-sponge-from-moreutils-work%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown







Popular posts from this blog

404 Error Contact Form 7 ajax form submitting

How to know if a Active Directory user can login interactively

TypeError: fit_transform() missing 1 required positional argument: 'X'