CROSSBOW(5) | File Formats Manual (urm) | CROSSBOW(5) |
crossbow-outfmt
—
Output format reference
crossbow outfmt |
-f format
-o output_mode |
crossbow set |
-f format
-o output_mode
[...] |
Feeds that are registered for monitoring with crossbow-add(1) are individually associated to an output mode that determines how new items (stories, articles, etc) are handled by crossbow-fetch(1).
The default output mode is called print
and consists in printing to stdout(3)
all the properties of the item. The
pretty
mode also prints on
stdout(3), but allows the user to
define a custom textual representation in place of the default one. The last
two modes, subproc
and pipe
,
correspond to two flavours of parametric subprocess execution.
All output modes, except for print
,
require a format string as an additional parameter. A format string can
contain a number of placeholders, that are expanded with the properties of
each handled item. The syntax of placeholders resembles
the printf(3) function, with some
differences and simplifications:
subproc
and pipe
mode, while a non-escaped whitespace is be interpreted as a separator.
An escaped whitespace has no effect in pretty
mode, where spaces are maintained verbatim.The interpretation of the format string depends on the output mode.
If the output mode is pretty
, the format
string is treated as a template for the textual representation of each
item. The placeholders are expanded with the string
representation of the corresponding properties (listed below), and the
resulting string is printed to
stdout(3).
If the output mode is subproc
or
pipe
, the format string is split on white-space into
an array of tokens, and the placeholders are expanded for each of them. The
obtained array is interpreted as a command line, with the first token being
the command to execute, and the subsequent being the arguments. For security
reasons the command line is not parsed by a shell interpeter.
Quotation is currently not supported, thus literal white-spaces must be individually escaped by backslash characters ("\").
Please mind security when using the
subproc
and pipe
modes. The
invocation of a shell interpreter as subcommand is especially discouraged,
as a specially crafted feed item might result in remote command
execution.
Follows a list of the supported placeholders, and the corresponding fields of a struct mrss_item_t data type (see library “mrss” Ns ). Each field correspond to one property of a feed item:
The following extra placeholders are also available:
This is an important security feature: the value of this number is not controlled by the feed, thus it can be used safely as filename. See crossbow-cookbook(7).
The value is padded with zero to make lexicographical order trivial. Increment happens even when the execution of a subprocess fails, so that the same value is never used twice.
The language recognized by the output format parser allows the placeholders to be composed by multiple characters. While this feature makes it easier to have mnemonic placeholders (such as "%a" for "Author" and "%am" for "Author eMail"), it introduces some additional edge cases.
The zero-width break sequence ("\:") has been introduced to cover a case of ambiguity which can be easily explained by means of an example.
Let's consider the case in which both "%x" and "%xn" are valid placeholders. In such case it would get impossible to express the expansion of "%x" followed by a literal "n", as the sequence "%x\n" would be rendered as the expansion of "%x" followed by a new-line, while "%xn" would be rendered as the expansion of "%xn".
The behaviour is summarized by the following table, where expand(x) expresses the expansion of the "%x" placeholder into the string representation of the corresponding field, and the "." operator expresses string concatenation.
By supplying
crossbow-fetch(1) with the
-d
flag, it is possible to test the configuration to
work as intended without marking new items as seen. Even though
-d
stands for
dry-run,
it is worth noting that the subproc
and
pipe
output mode will still execute the supplied
command.
The recommended way to verify the invocation to be correct is to prefix the actual command with a printf(1), so that the actual invocation of the command is prevented, and printf(1) will represent clearly how the command line is actually composed:
$ crossbow set -i some-feed -o subcmd -f 'printf [\%s]\\n parse -t %t'
When verifying the correctness of a feed configuration, it might
be useful to restrict the fetch operation to the such feed by means of the
-i
flag:
$ crossbow fetch -i some-feed
crossbow(1), crossbow-del(1), crossbow-fetch(1), crossbow-query(1), crossbow-set(1)
June 15, 2020 |