Various systems have differing df output. POSIX added a -P option (`portability') to force a portable output that could be used by scripts. However, GNU df has a non-portable -P option, totally useless, and only produces the output required by POSIX when also the environment variable POSIXLY_CORRECT is set. The -P option should imply POSIXLY_CORRECT. Andries [observed on df from fileutils 4.0]
