TSORT(1) BSD Reference Manual TSORT(1)

**tsort** - topological sort of a directed graph

**tsort** [**-flqrvw**] [**-h** *file*] [*file*]

**tsort** takes a list of pairs of node names representing directed arcs in a
graph and prints the nodes in topological order on standard output. That
is, the input describes a partial ordering relation, from which **tsort**
computes a total order compatible with this partial ordering.
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 pairs.
Presence of a node in a graph can be represented by an arc from the node
to itself. This is useful when a node is not 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.
The options are as follows:
**-f** Resolve ambiguities by selecting nodes based on the order of ap-
pearance of the first component of the pairs.
**-h** *file*
Use *file*, which holds an ordered list of nodes, to resolve ambi-
guities. In case of duplicates, the first entry is chosen.
**-l** Search for and display the longest cycle. Can take a very long
time, as it may need to solve an NP-complete problem.
**-q** Do not display informational messages about cycles. This is pri-
marily intended for building libraries, where optimal ordering is
not critical, and cycles occur often.
**-r** Reverse the ordering relation.
**-v** Inform on the exact number of edges broken while breaking cycles.
If a hints file was used, inform on seen nodes absent from that
file.
**-w** Exit with exit code the number of cycles **tsort** had to break.

Faced with the input:
a b
b c
b d
d f
c e
**tsort** outputs:
a
b
c
e
d
f
which is one total ordering compatible with the individual relations.
There is no unicity, another compatible total ordering would be:
a
b
c
d
e
f
**tsort** is commonly used to analyze dependencies and find a correct build
order in a static way, whereas make(1) accomplishes the same task in a
dynamic way.

ar(1), lorder(1), make(1)
Donald E. Knuth, *The Art of Computer Programming*, Vol. 1, pp 258-268,
1973.

A **tsort** command appeared in Version 7 AT&T UNIX. This **tsort** command was
completely rewritten by Marc Espie for OpenBSD, to finally use the well-
known optimal algorithms for topological sorting.
MirBSD #10-current November 1, 1999 1