In a few cases, the GNU utilities' default behavior is incompatible with the POSIX standard. To suppress these incompatibilities, define the POSIXLY_CORRECT environment variable. Unless you are checking for POSIX conformance, you probably do not need to define POSIXLY_CORRECT.
Normally options and operands can appear in any order, and programs act as if all the options appear before any operands. For example, ‘diff lao tzu -C 2’ acts like ‘diff -C 2 lao tzu’, since ‘2’ is an option-argument of -C. However, if the POSIXLY_CORRECT environment variable is set, options must appear before operands, unless otherwise specified for a particular command.
Newer versions of POSIX are occasionally incompatible with older versions. For example, older versions of POSIX allowed the command ‘diff -c -10’ to have the same meaning as ‘diff -C 10’, but POSIX 1003.1-2001 ‘diff’ no longer allows digit-string options like -10.
The GNU utilities normally conform to the version of POSIX that is standard for your system. To cause them to conform to a different version of POSIX, define the _POSIX2_VERSION environment variable to a value of the form yyyymm specifying the year and month the standard was adopted. Two values are currently supported for _POSIX2_VERSION: ‘199209’ stands for POSIX 1003.2-1992, and ‘200112’ stands for POSIX 1003.1-2001. For example, if you are running older software that assumes an older version of POSIX and uses ‘diff -c -10’, you can work around the compatibility problems by setting ‘_POSIX2_VERSION=199209’ in your environment.