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 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.
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
258268,
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 wellknown
optimal algorithms for topological sorting.
OpenBSD 3.6 November 1, 1999
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