NAME    [Toc]    [Back]

     tsort - topological sort of a directed graph

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     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  NP-complete  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.

EXAMPLES    [Toc]    [Back]

     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.

SEE ALSO    [Toc]    [Back]

     ar(1), lorder(1), make(1)

     Donald E. Knuth, The Art of Computer Programming, Vol. 1, pp
258-268,
     1973.

HISTORY    [Toc]    [Back]

     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