tsort—topological sort

/usr/ccs/bin/tsort [filename]

The tsort command produces, on the standard output, an ordered list of items consistent with a partial ordering of items mentioned in the input filename. If no filename is specified, the standard input is understood. The input consists of pairs of items (nonempty strings) separated by blanks. Pairs of different items indicate ordering. Pairs of identical items indicate presence, but not ordering.