in reply to Writing to a file atomically without renaming

I think this does was you want, but... I could be wrong... 
copy file_to_modify temp_file
# modify file
cat temp_file > file_to_modify

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I ran a test on my system and it overwrites the contents without affecting the owner or permissions... although I don't know if that redirection is guaranteed to be atomicm but you could make a third copy of the original, just in case, and remove it when everything is successfully completed.

HTH!

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Re^2: Writing to a file atomically without renaming
by waswas-fng (Curate) on Jun 30, 2005 at 20:30 UTC
    If you run out f space n the filesystem you have just clobbered the file. I don't think he is just looking for Atomic write -- but also rollback on err.


    -Waswas
[reply]
      Which is why I suggested making the third copy, which could be "rolled back" in case of a problem. It's similar to DB transactions with redo logs. They don't have to worry about the file permissions problems, though. 

       eval {
 copy file to temp_file
 copy file to backup_file
 modify temp_file
 cat temp_file > file
 verify file is OK
 remove temp_file and backup_file
 };
 if ( $@ ) {
   remove temp_file
   cat backup_file > file
   remove backup_file
 }

      [download]
      Worst case scenario the cat of the backup_file to temp_file fails and you're left with a corrupt "file" and the extra "backup_file" which would have to be moved back manually.
[reply]
[d/l]
        I think it can be done with three files: the target, a "log" file and a temporary:
        • when starting the program, if the log file exist => cat it to the target and then remove it
        • to modify the file, write the data to a temporary file, them rename the temporary file to be log file, cat the log over the target and finally remove the log file.
[reply]
        no. worse case is when you are "rolling back" that another user on your multi user system steals the disk space needed for the cat backupfile > file between the time the > zeros file out and the time cat is done -- leaving you with a broken file. That is why the unlink,rename method is popular -- at no time do you destroy the rollback until the new file is in place.


        -Waswas
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