sort has three modes of operation: sort (the default), merge, and check for sortedness. The following options change the operation mode:
A pair of lines is compared as follows: if any key fields have been specified, sort compares each pair of fields, in the order specified on the command line, according to the associated ordering options, until a difference is found or no fields are left.
If any of the global options Mbdfinr are given but no key fields are specified, sort compares the entire lines according to the global options.
Finally, as a last resort when all keys compare equal (or if no ordering options were specified at all), sort compares the lines byte by byte in machine collating sequence. The last resort comparison honors the -r global option. The -s (stable) option disables this last-resort comparison so that lines in which all fields compare equal are left in their original relative order. If no fields or global options are specified, -s has no effect.
GNU sort has no limits on input line length or restrictions on bytes allowed within lines. In addition, if the final byte of an input file is not a newline, GNU sort silently supplies one.
If the environment variable TMPDIR is set, sort uses it as the directory in which to put temporary files instead of the default, /tmp. The "-T tempdir" option is another way to select the directory for temporary files; it overrides the environment variable.
The following options affect the ordering of output lines. They may be specified globally or as part of a specific key field. If no key fields are specified, global options apply to comparison of entire lines; otherwise the global options are inherited by key fields that do not specify any special options of their own.
Other options are:
A position has the form f. c, where f is the number of the field to use and c is the number of the first character from the beginning of the field (for +pos) or from the end of the previous field (for -pos). The . c part of a position may be omitted in which case it is taken to be the first character in the field. If the -b option has been given, the . c part of a field specification is counted from the first nonblank character of the field (for +pos) or from the first nonblank character following the previous field (for -pos).
A +pos or -pos argument may also have any of the option letters Mbdfinr appended to it, in which case the global ordering options are not used for that particular field. The -b option may be independently attached to either or both of the +pos and -pos parts of a field specification, and if it is inherited from the global options it will be attached to both. If a -n or -M option is used, thus implying a -b option, the -b option is taken to apply to both the +pos and the -pos parts of a key specification. Keys may span multiple fields.
In addition, when GNU sort is invoked with exactly one argument, the following options are recognized:
Historical (BSD and System V) implementations of sort have differed in their interpretation of some options, particularly -b , -f , and -n . GNU sort follows the POSIX behavior, which is usually (but not always!) like the System V behavior. According to POSIX -n no longer implies -b . For consistency, -M has been changed in the same way. This may affect the meaning of character positions in field specifications in obscure cases. If this bites you the fix is to add an explicit -b .