UNIX Shells by Example (4th Edition)

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.