Specification of the "sequ" command

Copyright © 2013 Bart Massey
Revision 0: 1 October 2013

This specification describes the "universal sequence" command sequ. The sequ command is a backward-compatible set of extensions to the UNIX [seq](http://www.gnu.org/software/coreutils/manual/html_node/seq-invocation.html) command. There are many implementations of seq out there: this specification is built on the seq supplied with GNU Coreutils version 8.21.

The seq command emits a monotonically increasing sequence of numbers. It is most commonly used in shell scripting:

  TOTAL=0
  for i in `seq 1 10`
  do
    TOTAL=`expr $i + $TOTAL`
  done
  echo $TOTAL

prints 55 on standard output. The full sequ command does this basic counting operation, plus much more.

This specification of sequ is in several stages, known as compliance levels. Each compliance level adds required functionality to the sequ specification. Level 1 compliance is equivalent to the Coreutils seq command.

The usual specification language applies to this document: MAY, SHOULD, MUST (and their negations) are used in the standard fashion.

Wherever the specification indicates an error, a conforming sequ implementation MUST immediately issue appropriate error message specific to the problem. The implementation then MUST exit, with a status indicating failure to the invoking process or system. On UNIX systems, the error MUST be indicated by exiting with status code 1.

When a conforming sequ implementation successfully completes its output, it MUST immediately exit, with a status indicating success to the invoking process or systems. On UNIX systems, success MUST be indicated by exiting with status code 0.

Compliance Level 0

Compliance Level 0 of sequ requires absolute minimum functionality. A CL0 sequ MUST accept exactly two command-line arguments. Each argument SHOULD be a representation of an integer value. Any other supplied argument syntax is an error.

If the first integer argument is numerically greater than the second, the sequ command MUST emit no output. Otherwise, sequ MUST print on its output each of the integers between the first and second argument, inclusive. Each output integer MUST be on a line by itself, that is, a line terminated with an appropriate line terminator for the host environment.

Compliance Level 1

Compliance Level 1 of sequ adds the full functionality of GNU Coreutils seq. This includes the "--format", "--separator", "--equal-width", "--help" and "--version" arguments (as well as the one-character abbreviations of these), the increment argument, and support for floating-point numbers. The sequ initialization and increment arguments are now optional, as per the seq spec.

The sequ "--format" specifier MAY format floating-point numbers differently than seq, but it MUST follow some well-described and reasonable floating-point formatting standard.

Backslash-escapes in the "-s" argument string MUST be processed as in C printf(3).

Compliance Level 2

Compliance Level 2 of sequ adds additional convenience arguments for formatting.

The arguments that MUST be accepted are as follows:

• -W, --words: Output the sequence as a single space-separated line. Equivalent to "-s ' '".

• -p, --pad : Output the sequence with elements padded on the left to be all of equal width: the pad character is given by the single-char pad string . Backslash-escapes in MUST be processed as in C printf(3).

  Note that the "-w" command of level 2 is equivalent to "-p '0'".

• -P, --pad-spaces: Output the sequence with elements padded with spaces on the left to be all of equal width. Equivalent to "-p ' '".

Compliance Level 3

Compliance Level 3 of sequ adds the ability to have sequences of types other than floating-point numbers.

Specifically, CL3 sequ MUST accept as arguments and output as results: arbitrary-precision integers, single lowercase alphabetic (ASCII) letters, single uppercase alphabetic (ASCII) letters, and lowercase or uppercase unsigned Roman Numerals.

The sequ command MUST accept a new flag, "--format-word" or "-F", that takes a one-word argument indicating the type of the sequence. The sequ command MUST accept the format-word arguments "arabic" (for integers), "floating", "alpha" (for letters), "ALPHA", "roman" or "ROMAN"; the all-uppercase variants indicate uppercase sequences.

The sequ command MUST accept limit arguments (start, end, and increment) in the format consistent with the format-word. Arabic limit arguments MAY be "promoted" to Roman Numerals when Roman output is requested. The increment argument for alpha formats MUST be arabic. Otherwise, the limit arguments MUST be in the same format as the format-word. When no format-word is given, the format MUST be inferred from the format of the mandatory end argument.

Compliance Level 4

Compliance Level 4 of sequ adds the ability to number the lines of a textfile presented on the input.

CL4 sequ MUST accept the "--number-lines" / "-n" argument. This argument indicates that, rather than outputting the sequence on standard output, sequ will act as a filter, numbering lines of a file read from standard input to standard output. Each line "number" will be in the format specified by the "--format-word" argument, or inferred from the start or increment limit argument if the "--format-word" argument is not supplied. The end argument is irrelevant when "--number-lines" is supplied; it MUST NOT be accepted. The separator between the line number and the line may be given by the "--separator" argument, defaulting to space.

Compliance Level 5

Compliance Level 5 of sequ adds the ability to infer a sequence from a given prefix.

As an alternative to the limit arguments of previous Compliance Levels, CL5 sequ may accept a sequence specifier of the form:

value [value] [value] ... ".." value

When the ".." argument is present, the non-flag arguments MUST be parsed in inference mode.

In inference mode, sequ picks a best match for the pattern (partial sequence of values leading up to the ".."), and then continues the sequence until the end value (after the "..") is succeeded.