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## Standards conformance

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.