Perform a topological sort of a directed graph (POSIX)
tsort [-l] [-q] [file]
The tsort utility takes a list of pairs of node names representing directed arcs in a graph and prints the nodes in topological order on standard output. Input is taken from the named file, or from standard input if no file is given.
Node names in the input are separated by white space, and there must be an even number of node names.
The presence of a node in a graph can be represented by an arc from the node to itself. This is useful when a node isn't connected to any other nodes.
If the graph contains a cycle (and therefore cannot be properly sorted), one of the arcs in the cycle is ignored, and the sort continues. Cycles are reported on standard error.