Is sed line deletion atomic?

Question

Lets say I have a file 'queue'. From what I understand, appending to it using '>>' is atomic as defined by POSIX. But if I have multiple processes appending, is it safe to do the following without data loss?

sed -e '1d' -i queue

I have several different script services on Linux that may want to pass information between them. Each has a 'queue' where new data is pushed on the bottom with the next item to be popped from the top.

Is my use of sed guaranteed to not miss an append operation from another process between the time it reads the file to a buffer and writes it to back to disk?

Thanks.

Answer 1

I'll defer to anyone else who makes a stronger claim, but I think it is not safe to use that way. -i works by making a temporary copy of the file and then moving that back in place, which would replace any additional changes in the middle.

Source: http://www.gnu.org/software/sed/manual/sed.html

-i[SUFFIX] --in-place[=SUFFIX] This option specifies that files are to be edited in-place. GNU sed does this by creating a temporary file and sending output to this file rather than to the standard output.1. This option implies -s.

When the end of the file is reached, the temporary file is renamed to the output file's original name. The extension, if supplied, is used to modify the name of the old file before renaming the temporary file, thereby making a backup copy2).

This rule is followed: if the extension doesn't contain a *, then it is appended to the end of the current filename as a suffix; if the extension does contain one or more * characters, then each asterisk is replaced with the current filename. This allows you to add a prefix to the backup file, instead of (or in addition to) a suffix, or even to place backup copies of the original files into another directory (provided the directory already exists).

If no extension is supplied, the original file is overwritten without making a backup.