请考虑：
$ id # Me
uid=100(user1) gid=200(group1) groups=200(group1)
$ ls -l tnsnames.ora # So user1 has only read permission on below file.
-rw-rw-r-- 1 oracle dba 411024 Jul 28 2010 tnsnames.ora
$ cp tnsnames.ora tnsnames.ora_bak
$ cat tnsnames.ora_bak > tnsnames.ora # No write access
-bash: tnsnames.ora: Permission denied
$ cp -p tnsnames.ora tnsnames.ora_copy # Copy file and preserve mode, ownership, timestamps. Requires sudo to succeed completely.
$ ls -l tnsnames.ora_copy # Mode and timestamps preserved; ownership not preserved.
-rw-rw-r-- 1 …