tsort - topological sort
The tsort utility shall write to standard output a totally ordered list of items consistent with a partial ordering of items contained in the input.
The application shall ensure that the input consists of pairs of items (non-empty strings) separated by <blank> characters. Pairs of different items indicate ordering. Pairs of identical items indicate presence, but not ordering.
The following operand shall be supported:
- A pathname of a text file to order. If no file operand is given, the standard input shall be used.
The standard input shall be used if no file operand is specified, and shall be used if the file operand is '-' and the implementation treats the '-' as meaning standard input. Otherwise, the standard input shall not be used. See the INPUT FILES section.
The input file shall be a text file.
The following environment variables shall affect the execution of tsort:
- Provide a default value for the internationalization variables that are unset or null. (See XBD Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
- If set to a non-empty string value, override the values of all the other internationalization variables.
- Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments and input files).
- Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
- [XSI] Determine the location of message catalogs for the processing of LC_MESSAGES.
The standard output shall be a text file consisting of the order list produced from the partially ordered input.
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
- Successful completion.
- An error occurred.
The LC_COLLATE variable need not affect the actions of tsort. The output ordering is not lexicographic, but depends on the pairs of items given as input.
The command:tsort <<EOF a b c c d e g g f g e f h h EOF
produces the output:a b c d e f g h
XBD Environment Variables
First released in Issue 2.
The normative text is reworded to avoid use of the term "must" for application requirements.
Austin Group Interpretation 1003.1-2001 #092 is applied.
The tsort utility is moved from the XSI option to the Base.
POSIX.1-2008, Technical Corrigendum 1, XCU/TC1-2008/0146  is applied.
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