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?






shell io-redirection stdout stdin







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  • 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
















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?






shell io-redirection stdout stdin







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














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?






shell io-redirection stdout stdin







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




shell io-redirection stdout stdin






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Nov 23 at 3:43









Jeff Schaller

38.5k1053125




38.5k1053125










asked Nov 22 at 19:23









HappyFace

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 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
1






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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 at 21:06












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











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active

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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 at 21:06












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












  • >; ? mind blown.
    – glenn jackman
    Nov 23 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 at 20:04

























answered Nov 22 at 19:31









thrig

24.2k22956




24.2k22956












  • 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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