2.5 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.

Newer versions of POSIX are occasionally incompatible with older versions. For example, older versions of POSIX required the command `sort +1' to sort based on the second and succeeding fields in each input line, but starting with POSIX 1003.1-2001 the same command is required to sort the file named `+1', and you must instead use the command `sort -k 2' to get the field-based sort.

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 `sort +1', you can work around the compatibility problems by setting `_POSIX2_VERSION=199209' in your environment.