Hi, I am looking for advice how to implement best this kind of usecase:
User rd creates a file on /tmp: rd@h370:~/tmp.nobackup$ touch /tmp/123 rd@h370:~/tmp.nobackup$ chgrp users /tmp/123 rd@h370:~/tmp.nobackup$ chmod g+w /tmp/123 rd@h370:~/tmp.nobackup$ ls -l /tmp/123 -rw-rw-r-- 1 rd users 0 30. Aug 20:42 /tmp/123 rd@h370:~/tmp.nobackup$ User ka overwrites it with the content of another file (atomically): ka@h370:~$ echo test > 123 ka@h370:~$ mv 123 /tmp/123 mv: cannot move '123' to '/tmp/123': Operation not permitted ka@h370:~$ id uid=1401(ka) gid=1401(ka) groups=1401(ka),20(dialout),21(fax),24(cdrom), 30(dip),44(video),46(plugdev),100(users),1000(sispmctl) ka@h370:~$ Although ka has write permissions as group member, this does not work. Maybe moving removes a node in /tmp, so I am trying to append to the file as a test: rd@h370:~/tmp.nobackup$ touch /tmp/123 rd@h370:~/tmp.nobackup$ chgrp users /tmp/123 rd@h370:~/tmp.nobackup$ chmod g+w /tmp/123 rd@h370:~/tmp.nobackup$ ls -l /tmp/123 -rw-rw-r-- 1 rd users 0 30. Aug 20:35 /tmp/123 rd@h370:~/tmp.nobackup$ ka@h370:~$ id uid=1401(ka) gid=1401(ka) Gruppen=1401(ka),20(dialout),21(fax),24(cdrom), 30(dip),44(video),46(plugdev),100(users),1000(sispmctl) ka@h370:~$ ls -l /tmp/123 -rw-rw-r-- 1 rd users 0 30. Aug 20:35 /tmp/123 ka@h370:~$ echo test >> /tmp/123 -bash: /tmp/123: Permission denied ka@h370:~$ Even that does not work. Why not? Is there something special with /tmp? kan@h370:~$ ls -ld /tmp drwxrwxrwt 26 root root 32768 Aug 30 20:51 /tmp ka@h370:~$ Now attempting to do the same in a regular home directory: rd@h370:~/tmp.nobackup$ touch 123 rd@h370:~/tmp.nobackup$ chgrp users 123 rd@h370:~/tmp.nobackup$ chmod g+w 123 rd@h370:~/tmp.nobackup$ ka@h370:~$ echo test >> /home/rd/tmp.nobackup/123 ka@h370:~$ Appending works! But replacing the file with a mv command does not work in the /home directory: rd@h370:~/tmp.nobackup$ touch 123 rd@h370:~/tmp.nobackup$ ls -l 123 -rw-rw-r-- 1 rd users 5 30. Aug 20:39 123 rd@h370:~/tmp.nobackup$ ka@h370:~$ mv 123 /home/rd/tmp.nobackup/123 mv: cannot move '123' to '/home/rd/tmp.nobackup/123': Permission denied ka@h370:~$ If I redirect the output and overwrite the file instead of using mv, the system allows me to do that: rd@h370:~/tmp.nobackup$ touch 123 rd@h370:~/tmp.nobackup$ chgrp users 123 rd@h370:~/tmp.nobackup$ chmod g+w 123 rd@h370:~/tmp.nobackup$ ls -l 123 -rw-rw-r-- 1 rd users 0 30. Aug 20:51 123 rd@h370:~/tmp.nobackup$ ka@h370:~$ echo test > 123 ka@h370:~$ cat 123 > /home/rd/tmp.nobackup/123 ka@h370:~$ cat /home/rd/tmp.nobackup/123 test ka@h370:~$ It is weird that mv is forbidden, but redirecting the output is allowed. The end result on the file system would be the same! Downside of redirecting is that /home/rd/tmp.nobackup/123 is not updated atomically (assuming another process is reading it asynchronically). So essentially what I want to achieve: - Updating a file atomically - Preferably in /tmp Any advice or hint is welcome. Thanks Rainer -- Rainer Dorsch http://bokomoko.de/
