## NAME

`tsort`

—
topological sort of a directed
graph

## SYNOPSIS

`tsort` |
[`-flqrvw` ] [`-h`
file] [file] |

## DESCRIPTION

`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 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.

## EXIT STATUS

The `tsort`

utility exits 0 on
success, and >0 if an error occurs.

## EXAMPLES

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

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

## STANDARDS

The `tsort`

utility is compliant with the
IEEE Std 1003.1-2008 (“POSIX.1”)
specification.

The flags [`-fhlqrvw`

] are extensions to
that specification.

## HISTORY

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.