zeek/doc/script-reference/directives.rst
2015-05-30 01:35:55 -05:00

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Directives
==========
The Bro scripting language supports a number of directives that can
affect which scripts will be loaded or which lines in a script will be
executed. Directives are evaluated before script execution begins.
.. bro:keyword:: @DEBUG
TODO
.. bro:keyword:: @DIR
Expands to the directory pathname where the current script is located.
Example::
print "Directory:", @DIR;
.. bro:keyword:: @FILENAME
Expands to the filename of the current script.
Example::
print "File:", @FILENAME;
.. bro:keyword:: @load
Loads the specified Bro script, specified as the relative pathname
of the file (relative to one of the directories in Bro's file search path).
If the Bro script filename ends with ".bro", then you don't need to
specify the file extension. The filename cannot contain any whitespace.
In this example, Bro will try to load a script
"policy/misc/capture-loss.bro" by looking in each directory in the file
search path (the file search path can be changed by setting the BROPATH
environment variable)::
@load policy/misc/capture-loss
If you specify the name of a directory instead of a filename, then
Bro will try to load a file in that directory called "__load__.bro"
(presumably that file will contain additional "@load" directives).
In this example, Bro will try to load a file "tuning/defaults/__load__.bro"
by looking in each directory in the file search path::
@load tuning/defaults
The purpose of this directive is to ensure that all script dependencies
are satisfied, and to avoid having to list every needed Bro script
on the command-line. Bro keeps track of which scripts have been
loaded, so it is not an error to load a script more than once (once
a script has been loaded, any subsequent "@load" directives
for that script are ignored).
.. bro:keyword:: @load-plugin
Activate a dynamic plugin with the specified plugin name. The specified
plugin must be located in Bro's plugin search path. Example::
@load-plugin Demo::Rot13
By default, Bro will automatically activate all dynamic plugins found
in the plugin search path (the search path can be changed by setting
the environment variable BRO_PLUGIN_PATH to a colon-separated list of
directories). However, in bare mode ("bro -b"), dynamic plugins can be
activated only by using "@load-plugin", or by specifying the full
plugin name on the Bro command-line (e.g., "bro Demo::Rot13"), or by
setting the environment variable BRO_PLUGIN_ACTIVATE to a
comma-separated list of plugin names.
.. bro:keyword:: @load-sigs
This works similarly to "@load", except that in this case the filename
represents a signature file (not a Bro script). If the signature filename
ends with ".sig", then you don't need to specify the file extension
in the "@load-sigs" directive. The filename cannot contain any
whitespace.
In this example, Bro will try to load a signature file
"base/protocols/ssl/dpd.sig"::
@load-sigs base/protocols/ssl/dpd
The format for a signature file is explained in the documentation for the
`Signature Framework <../frameworks/signatures.html>`_.
.. bro:keyword:: @unload
This specifies a Bro script that we don't want to load (so a subsequent
attempt to load the specified script will be skipped). However,
if the specified script has already been loaded, then this directive
has no affect.
In the following example, if the "policy/misc/capture-loss.bro" script
has not been loaded yet, then Bro will not load it::
@unload policy/misc/capture-loss
.. bro:keyword:: @prefixes
Specifies a filename prefix to use when looking for script files
to load automatically. The prefix cannot contain any whitespace.
In the following example, the prefix "cluster" is used and all prefixes
that were previously specified are not used::
@prefixes = cluster
In the following example, the prefix "cluster-manager" is used in
addition to any previously-specified prefixes::
@prefixes += cluster-manager
The way this works is that after Bro parses all script files, then for each
loaded script Bro will take the absolute path of the script and then
it removes the portion of the directory path that is in Bro's file
search path. Then it replaces each "/" character with a period "."
and then prepends the prefix (specified in the "@prefixes" directive)
followed by a period. The resulting filename is searched for in each
directory in Bro's file search path. If a matching file is found, then
the file is automatically loaded.
For example, if a script called "local.bro" has been loaded, and a prefix
of "test" was specified, then Bro will look for a file named
"test.local.bro" in each directory of Bro's file search path.
An alternative way to specify prefixes is to use the "-p" Bro
command-line option.
.. bro:keyword:: @if
The specified expression must evaluate to type :bro:type:`bool`. If the
value is true, then the following script lines (up to the next "@else"
or "@endif") are available to be executed.
Example::
@if ( ver == 2 )
print "version 2 detected";
@endif
.. bro:keyword:: @ifdef
This works like "@if", except that the result is true if the specified
identifier is defined.
Example::
@ifdef ( pi )
print "pi is defined";
@endif
.. bro:keyword:: @ifndef
This works exactly like "@ifdef", except that the result is true if the
specified identifier is not defined.
Example::
@ifndef ( pi )
print "pi is not defined";
@endif
.. bro:keyword:: @else
This directive is optional after an "@if", "@ifdef", or
"@ifndef". If present, it provides an else clause.
Example::
@ifdef ( pi )
print "pi is defined";
@else
print "pi is not defined";
@endif
.. bro:keyword:: @endif
This directive is required to terminate each "@if", "@ifdef", or
"@ifndef".