How does sponge (from moreutils) work?
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
edited Nov 23 at 3:43
Jeff Schaller
38.5k1053125
asked Nov 22 at 19:23
HappyFace
31811
add a comment |
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
edited Nov 23 at 3:43
Jeff Schaller
38.5k1053125
asked Nov 22 at 19:23
HappyFace
31811
2
What do you mean? The same way every other program knows (e.g.cat f | wcorcat f | grep fooor whatever), why would you expectspongeto be special?
– terdon♦
Nov 22 at 19:31
add a comment |
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
edited Nov 23 at 3:43
Jeff Schaller
38.5k1053125
asked Nov 22 at 19:23
HappyFace
31811
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
edited Nov 23 at 3:43
Jeff Schaller
38.5k1053125
asked Nov 22 at 19:23
HappyFace
31811
edited Nov 23 at 3:43
Jeff Schaller
38.5k1053125
asked Nov 22 at 19:23
HappyFace
31811
edited Nov 23 at 3:43
Jeff Schaller
38.5k1053125
edited Nov 23 at 3:43
Jeff Schaller
38.5k1053125
edited Nov 23 at 3:43
Jeff Schaller
38.5k1053125
38.5k1053125
asked Nov 22 at 19:23
HappyFace
31811
asked Nov 22 at 19:23
HappyFace
31811
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 | wcorcat f | grep fooor whatever), why would you expectspongeto be special?
– terdon♦
Nov 22 at 19:31
add a comment |
2
What do you mean? The same way every other program knows (e.g.cat f | wcorcat f | grep fooor whatever), why would you expectspongeto 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
add a comment |
1 Answer
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oldest
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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.
answered Nov 22 at 19:31
thrig
24.2k22956
And when therename()fails with EXDEV when/tmpis on a different file system, it ends up copying the data again into the destination file. You can avoid that by settingTMPDIRto$(dirname target-file)or useksh93's>;operator instead ofspongewhich does that automatically (and also doesn't override the target file if the redirected command failed).
– Stéphane Chazelas
Nov 22 at 21:06
>;? mind blown.
– glenn jackman
Nov 23 at 16:40
add a comment |
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strace or similar will show the system calls used by sponge, which is probably to write(2) the input read(2) from standard input out to a temporary file, and then to rename(2) that temporary file to the desired output filename when the input ends. The input ends when a read(2) call fails or returns 0 (which indicates end-of-file) at which point sponge can do the rename.
answered Nov 22 at 19:31
thrig
24.2k22956
And when therename()fails with EXDEV when/tmpis on a different file system, it ends up copying the data again into the destination file. You can avoid that by settingTMPDIRto$(dirname target-file)or useksh93's>;operator instead ofspongewhich 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
add a comment |
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.
answered Nov 22 at 19:31
thrig
24.2k22956
And when therename()fails with EXDEV when/tmpis on a different file system, it ends up copying the data again into the destination file. You can avoid that by settingTMPDIRto$(dirname target-file)or useksh93's>;operator instead ofspongewhich does that automatically (and also doesn't override the target file if the redirected command failed).
– Stéphane Chazelas
Nov 22 at 21:06
>;? mind blown.
– glenn jackman
Nov 23 at 16:40
add a comment |
strace or similar will show the system calls used by sponge, which is probably to write(2) the input read(2) from standard input out to a temporary file, and then to rename(2) that temporary file to the desired output filename when the input ends. The input ends when a read(2) call fails or returns 0 (which indicates end-of-file) at which point sponge can do the rename.
answered Nov 22 at 19:31
thrig
24.2k22956
strace or similar will show the system calls used by sponge, which is probably to write(2) the input read(2) from standard input out to a temporary file, and then to rename(2) that temporary file to the desired output filename when the input ends. The input ends when a read(2) call fails or returns 0 (which indicates end-of-file) at which point sponge can do the rename.
answered Nov 22 at 19:31
thrig
24.2k22956
edited Nov 22 at 20:04
answered Nov 22 at 19:31
thrig
24.2k22956
answered Nov 22 at 19:31
thrig
24.2k22956
answered Nov 22 at 19:31
thrig
24.2k22956
24.2k22956
And when therename()fails with EXDEV when/tmpis on a different file system, it ends up copying the data again into the destination file. You can avoid that by settingTMPDIRto$(dirname target-file)or useksh93's>;operator instead ofspongewhich 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
add a comment |
And when therename()fails with EXDEV when/tmpis on a different file system, it ends up copying the data again into the destination file. You can avoid that by settingTMPDIRto$(dirname target-file)or useksh93's>;operator instead ofspongewhich 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
add a comment |
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2
What do you mean? The same way every other program knows (e.g.
cat f | wcorcat f | grep fooor whatever), why would you expectspongeto be special?– terdon♦
Nov 22 at 19:31