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 names.
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 appearance of
the first component of the pairs.


h
file
 Use file, which holds an ordered list of
nodes, to resolve ambiguities. 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 NPcomplete problem.


q
 Do not display informational messages about cycles. This is primarily
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.
The
tsort
utility exits 0 on success,
and >0 if an error occurs.
Faced with the input:
tsort
outputs:
which is one total ordering compatible with the individual relations. There is
no unicity, another compatible total ordering would be:
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 258268,
1973.
The
tsort
utility is compliant with the
IEEE Std 1003.12008
(“POSIX.1”) specification.
The flags [
fhlqrvw
]
are extensions to that specification.
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 wellknown
optimal algorithms for topological sorting.