Bash Copy Update 'cp -au ' Anomaly

Bogomips_ · 12-09-2015, 05:43 AM

Code:

guest@porteus:~$ echo Update Test
Update Test
guest@porteus:~$ mkdir /tmp/src
guest@porteus:~$ touch /tmp/src/{b,c,d}
guest@porteus:~$ ls -i /tmp/src/{b,c,d}
26019 /tmp/src/b  26020 /tmp/src/c  26025 /tmp/src/d

guest@porteus:~$ ln /tmp/src/b /tmp/src/a
guest@porteus:~$ ls -i /tmp/src
26019 a  26019 b  26020 c  26025 d

guest@porteus:~$ mkdir dst
guest@porteus:~$ cp -a /tmp/src/* dst

guest@porteus:~$ ls -i dst
26002 a  26002 b  26026 c  26034 d

guest@porteus:~$ ln -f dst/c dst/a
guest@porteus:~$ ls -il dst
total 0
26026 -rw-r--r-- 2 guest guest 0 Dec  7 00:00 a
26002 -rw-r--r-- 1 guest guest 0 Dec  7 00:00 b
26026 -rw-r--r-- 2 guest guest 0 Dec  7 00:00 c
26034 -rw-r--r-- 1 guest guest 0 Dec  7 00:00 d

guest@porteus:~$ ls -il /tmp/src/
total 0
26019 -rw-r--r-- 2 guest guest 0 Dec  7 00:00 a
26019 -rw-r--r-- 2 guest guest 0 Dec  7 00:00 b
26020 -rw-r--r-- 1 guest guest 0 Dec  7 00:00 c
26025 -rw-r--r-- 1 guest guest 0 Dec  7 00:00 d

guest@porteus:~$ cp -au /tmp/src/* dst
guest@porteus:~$ ls -il dst
total 0
26026 -rw-r--r-- 3 guest guest 0 Dec  7 00:00 a
26026 -rw-r--r-- 3 guest guest 0 Dec  7 00:00 b
26026 -rw-r--r-- 3 guest guest 0 Dec  7 00:00 c
26034 -rw-r--r-- 1 guest guest 0 Dec  7 00:00 d

Expect files 'b' and 'c' to always remain unchanged.

pan64 · 12-09-2015, 06:04 AM

Why do you expect that? Actually you illustrated the expected behaviour, what you think about that is not really expected.
The last cp command has no any idea about the fact: dst/a and dst/c are hard links to each other, therefore will not be able to modify it. Furthermore it will create the hard link between a and b.

Bogomips_ · 12-09-2015, 08:37 AM

^ IMHO we should not be concerning ourselves with implementation details, but rather with the correctness of the implementation.

http://manpages.ubuntu.com/manpages/...man1/cp.1.html

Code:

 -u, --update
              copy  only  when  the  SOURCE file is newer than the destination
              file or when the destination file is missing

None of the dst/* files would have been timestamped older than the src/* files, and therefore none of the dst files should have been touched.

pan64 · 12-09-2015, 08:45 AM

A hard link was missing, that was created again, therefore one of the files (a or b) must be linked to the other one. What is still unclear (for me) if a will be created again or b. That is not documented and I think you may have different result if (originally) you try: ln -f dst/c dst/b