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
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
2
What do you mean? The same way every other program knows (e.g.cat f | wc
orcat f | grep foo
or whatever), why would you expectsponge
to be special?
– terdon♦
Nov 22 '18 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
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 '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
orcat f | grep foo
or whatever), why would you expectsponge
to be special?
– terdon♦
Nov 22 '18 at 19:31
add a comment |
2
What do you mean? The same way every other program knows (e.g.cat f | wc
orcat f | grep foo
or whatever), why would you expectsponge
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
add a comment |
1 Answer
1
active
oldest
votes
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.
And when therename()
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 settingTMPDIR
to$(dirname target-file)
or useksh93
's>;
operator instead ofsponge
which does that automatically (and also doesn't override the target file if the redirected command failed).
– Stéphane Chazelas
Nov 22 '18 at 21:06
>;
? mind blown.
– glenn jackman
Nov 23 '18 at 16:40
add a comment |
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1 Answer
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1 Answer
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active
oldest
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strace
or similar will show the system calls used by sponge
, which is probably to write(2)
the input read(2)
from standard input out to a temporary file, and then to rename(2)
that temporary file to the desired output filename when the input ends. The input ends when a read(2)
call fails or returns 0
(which indicates end-of-file) at which point sponge
can do the rename.
And when therename()
fails with EXDEV when/tmp
is on a different file system, it ends up copying the data again into the destination file. You can avoid that by settingTMPDIR
to$(dirname target-file)
or useksh93
's>;
operator instead ofsponge
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
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.
And when therename()
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 settingTMPDIR
to$(dirname target-file)
or useksh93
's>;
operator instead ofsponge
which does that automatically (and also doesn't override the target file if the redirected command failed).
– Stéphane Chazelas
Nov 22 '18 at 21:06
>;
? mind blown.
– glenn jackman
Nov 23 '18 at 16:40
add a comment |
strace
or similar will show the system calls used by sponge
, which is probably to write(2)
the input read(2)
from standard input out to a temporary file, and then to rename(2)
that temporary file to the desired output filename when the input ends. The input ends when a read(2)
call fails or returns 0
(which indicates end-of-file) at which point sponge
can do the rename.
strace
or similar will show the system calls used by sponge
, which is probably to write(2)
the input read(2)
from standard input out to a temporary file, and then to rename(2)
that temporary file to the desired output filename when the input ends. The input ends when a read(2)
call fails or returns 0
(which indicates end-of-file) at which point sponge
can do the rename.
edited Nov 22 '18 at 20:04
answered Nov 22 '18 at 19:31
thrigthrig
24.6k23056
24.6k23056
And when therename()
fails with EXDEV when/tmp
is on a different file system, it ends up copying the data again into the destination file. You can avoid that by settingTMPDIR
to$(dirname target-file)
or useksh93
's>;
operator instead ofsponge
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
add a comment |
And when therename()
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 settingTMPDIR
to$(dirname target-file)
or useksh93
's>;
operator instead ofsponge
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
add a comment |
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2
What do you mean? The same way every other program knows (e.g.
cat f | wc
orcat f | grep foo
or whatever), why would you expectsponge
to be special?– terdon♦
Nov 22 '18 at 19:31