How does sponge (from moreutils) work?

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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?










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    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 at 19:31














up vote
1
down vote

favorite
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 at 19:31












up vote
1
down vote

favorite
1









up vote
1
down vote

favorite
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






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edited Nov 23 at 3:43









Jeff Schaller

36.9k1052121




36.9k1052121










asked Nov 22 at 19:23









HappyFace

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27111







  • 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 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 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 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 at 19:31










1 Answer
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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.






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  • 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 at 21:06











  • >; ? mind blown.
    – glenn jackman
    Nov 23 at 16:40










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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
3
down vote



accepted










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 at 21:06











  • >; ? mind blown.
    – glenn jackman
    Nov 23 at 16:40














up vote
3
down vote



accepted










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 at 21:06











  • >; ? mind blown.
    – glenn jackman
    Nov 23 at 16:40












up vote
3
down vote



accepted







up vote
3
down vote



accepted






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 at 20:04

























answered Nov 22 at 19:31









thrig

23.7k12955




23.7k12955











  • 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 at 21:06











  • >; ? mind blown.
    – glenn jackman
    Nov 23 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 at 21:06











  • >; ? mind blown.
    – glenn jackman
    Nov 23 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 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 at 21:06













>; ? mind blown.
– glenn jackman
Nov 23 at 16:40




>; ? mind blown.
– glenn jackman
Nov 23 at 16:40

















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