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is 'sed' thread safe

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2

If I have a shell/python script that uses sed to modify a file in place based on user inputs, and then two users run the same script at the same time or approx. same time, is 'sed' thread safe ? Or perhaps it is not an issue because the file_descripor that was opened by the first thread will be used to lock the file anyway ? thx

linux sed python

share|improve this question

edited 5 hours ago

Jeff Schaller

42.9k1159137

asked 5 hours ago

terreysterreys

1613

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Slightly more info wanted. How do you use sed from Python (and why, can't Python do things like that fairly effortlessly?).
  
  – Kusalananda
  5 hours ago
  

  
  
  
  

add a comment | 

2

If I have a shell/python script that uses sed to modify a file in place based on user inputs, and then two users run the same script at the same time or approx. same time, is 'sed' thread safe ? Or perhaps it is not an issue because the file_descripor that was opened by the first thread will be used to lock the file anyway ? thx

linux sed python

share|improve this question

edited 5 hours ago

Jeff Schaller

42.9k1159137

asked 5 hours ago

terreysterreys

1613

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Slightly more info wanted. How do you use sed from Python (and why, can't Python do things like that fairly effortlessly?).
  
  – Kusalananda
  5 hours ago
  

  
  
  
  

add a comment | 

If I have a shell/python script that uses sed to modify a file in place based on user inputs, and then two users run the same script at the same time or approx. same time, is 'sed' thread safe ? Or perhaps it is not an issue because the file_descripor that was opened by the first thread will be used to lock the file anyway ? thx

linux sed python

share|improve this question

edited 5 hours ago

Jeff Schaller

42.9k1159137

asked 5 hours ago

terreysterreys

1613

share|improve this question

edited 5 hours ago

Jeff Schaller

42.9k1159137

asked 5 hours ago

terreysterreys

1613

share|improve this question

edited 5 hours ago

Jeff Schaller

42.9k1159137

asked 5 hours ago

terreysterreys

1613

edited 5 hours ago

Jeff Schaller

42.9k1159137

edited 5 hours ago

Jeff Schaller

42.9k1159137

asked 5 hours ago

terreysterreys

1613

asked 5 hours ago

terreysterreys

1613

1 Answer
1

active

oldest

votes

5

I'm not going to nitpick on the awful terminology, but yes, GNU sed with its -i ("in-place") flag could be safely used by more than one process at the same time without any extra locking, because sed is not actually modifying the file in-place, but it's redirecting the output to a temporary file, and if everything goes well, it will rename(2) (move) the temporary file to the original file, and the rename(2) is guaranteed to be atomic:

$ strace sed -i s/o/e/g foo.txt

open("foo.txt", O_RDONLY)               = 3

...

open("./sedDe80VL", O_RDWR|O_CREAT|O_EXCL, 0600) = 4

...

read(3, "foon", 4096)                  = 4

...

write(4, "feen", 4)                    = 4

read(3, "", 4096)                       = 0

...

close(3)                                = 0

close(4)                                = 0

rename("./sedDe80VL", "foo.txt")        = 0

At any point, foo.txt will refer either to the complete original file or to the complete processed file, never to something in between the two.

share|improve this answer

edited 5 hours ago

answered 5 hours ago

mosvymosvy

7,7921530

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  But what can happen is that two copies of sed start reading the file, make different changes, storing them to their respective temporary files, which are then renamed into place one after other, without regard for the fact that there was another sed working on the file at the same time. The changes made by one of the sed processes would be lost.
  
  – ilkkachu
  5 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @ilkkachu that will still be completely consistent -- the file will be either modified by both processes in turn (in any order) or by just one of them. At no point will the file contain garbage resulting from both processes modifying it at the same time.
  
  – mosvy
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  it won't be total garbage, but having changes lost can still be an issue. The question refers to the file "being locked", and locking would usually refer to the second sed process waiting until the first completed. That might just be awful terminology on their part; it's hard to say what they really care about, but that particular issue is still possible.
  
  – ilkkachu
  4 hours ago
  

  
  
  
  

add a comment | 

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5

I'm not going to nitpick on the awful terminology, but yes, GNU sed with its -i ("in-place") flag could be safely used by more than one process at the same time without any extra locking, because sed is not actually modifying the file in-place, but it's redirecting the output to a temporary file, and if everything goes well, it will rename(2) (move) the temporary file to the original file, and the rename(2) is guaranteed to be atomic:

$ strace sed -i s/o/e/g foo.txt

open("foo.txt", O_RDONLY)               = 3

...

open("./sedDe80VL", O_RDWR|O_CREAT|O_EXCL, 0600) = 4

...

read(3, "foon", 4096)                  = 4

...

write(4, "feen", 4)                    = 4

read(3, "", 4096)                       = 0

...

close(3)                                = 0

close(4)                                = 0

rename("./sedDe80VL", "foo.txt")        = 0

At any point, foo.txt will refer either to the complete original file or to the complete processed file, never to something in between the two.

share|improve this answer

edited 5 hours ago

answered 5 hours ago

mosvymosvy

7,7921530

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  But what can happen is that two copies of sed start reading the file, make different changes, storing them to their respective temporary files, which are then renamed into place one after other, without regard for the fact that there was another sed working on the file at the same time. The changes made by one of the sed processes would be lost.
  
  – ilkkachu
  5 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @ilkkachu that will still be completely consistent -- the file will be either modified by both processes in turn (in any order) or by just one of them. At no point will the file contain garbage resulting from both processes modifying it at the same time.
  
  – mosvy
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  it won't be total garbage, but having changes lost can still be an issue. The question refers to the file "being locked", and locking would usually refer to the second sed process waiting until the first completed. That might just be awful terminology on their part; it's hard to say what they really care about, but that particular issue is still possible.
  
  – ilkkachu
  4 hours ago
  

  
  
  
  

add a comment | 

5

I'm not going to nitpick on the awful terminology, but yes, GNU sed with its -i ("in-place") flag could be safely used by more than one process at the same time without any extra locking, because sed is not actually modifying the file in-place, but it's redirecting the output to a temporary file, and if everything goes well, it will rename(2) (move) the temporary file to the original file, and the rename(2) is guaranteed to be atomic:

$ strace sed -i s/o/e/g foo.txt

open("foo.txt", O_RDONLY)               = 3

...

open("./sedDe80VL", O_RDWR|O_CREAT|O_EXCL, 0600) = 4

...

read(3, "foon", 4096)                  = 4

...

write(4, "feen", 4)                    = 4

read(3, "", 4096)                       = 0

...

close(3)                                = 0

close(4)                                = 0

rename("./sedDe80VL", "foo.txt")        = 0

At any point, foo.txt will refer either to the complete original file or to the complete processed file, never to something in between the two.

share|improve this answer

edited 5 hours ago

answered 5 hours ago

mosvymosvy

7,7921530

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  But what can happen is that two copies of sed start reading the file, make different changes, storing them to their respective temporary files, which are then renamed into place one after other, without regard for the fact that there was another sed working on the file at the same time. The changes made by one of the sed processes would be lost.
  
  – ilkkachu
  5 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @ilkkachu that will still be completely consistent -- the file will be either modified by both processes in turn (in any order) or by just one of them. At no point will the file contain garbage resulting from both processes modifying it at the same time.
  
  – mosvy
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  it won't be total garbage, but having changes lost can still be an issue. The question refers to the file "being locked", and locking would usually refer to the second sed process waiting until the first completed. That might just be awful terminology on their part; it's hard to say what they really care about, but that particular issue is still possible.
  
  – ilkkachu
  4 hours ago
  

  
  
  
  

add a comment | 

I'm not going to nitpick on the awful terminology, but yes, GNU sed with its -i ("in-place") flag could be safely used by more than one process at the same time without any extra locking, because sed is not actually modifying the file in-place, but it's redirecting the output to a temporary file, and if everything goes well, it will rename(2) (move) the temporary file to the original file, and the rename(2) is guaranteed to be atomic:

$ strace sed -i s/o/e/g foo.txt

open("foo.txt", O_RDONLY)               = 3

...

open("./sedDe80VL", O_RDWR|O_CREAT|O_EXCL, 0600) = 4

...

read(3, "foon", 4096)                  = 4

...

write(4, "feen", 4)                    = 4

read(3, "", 4096)                       = 0

...

close(3)                                = 0

close(4)                                = 0

rename("./sedDe80VL", "foo.txt")        = 0

At any point, foo.txt will refer either to the complete original file or to the complete processed file, never to something in between the two.

share|improve this answer

edited 5 hours ago

answered 5 hours ago

mosvymosvy

7,7921530

$ strace sed -i s/o/e/g foo.txt

open("foo.txt", O_RDONLY)               = 3

...

open("./sedDe80VL", O_RDWR|O_CREAT|O_EXCL, 0600) = 4

...

read(3, "foon", 4096)                  = 4

...

write(4, "feen", 4)                    = 4

read(3, "", 4096)                       = 0

...

close(3)                                = 0

close(4)                                = 0

rename("./sedDe80VL", "foo.txt")        = 0

share|improve this answer

edited 5 hours ago

answered 5 hours ago

mosvymosvy

7,7921530

answered 5 hours ago

mosvymosvy

7,7921530

answered 5 hours ago

mosvymosvy

7,7921530