== <param> tag ==
The `<param>` tag defines a parameter to be set on the [[Parameter Server]]. Instead of {{{value}}}, you can specify a {{{textfile}}}, {{{binfile}}} or {{{command}}} attribute to set the value of a parameter. The `<param>` tag can be put inside of a [[roslaunch/XML/node|<node>]] tag, in which case the parameter is treated like a [[Names|private parameter]].

You can also set private parameter across a group of nodes by using the `~param` syntax (see [[Names|ROS names]]) in a `<param>` tag. The declared parameter will be set as a local parameter in the [[roslaunch/XML/node|<node>]] tags that follow that are in the same scope (i.e. group or ns tag).

=== Attributes ===
 . `name="namespace/name"`
  . Parameter name. Namespaces can be included in the parameter name, but globally specified names should be avoided.
 `value="value"`''(optional)''
  . Defines the value of the parameter. If this attribute is omitted, `binfile`, `textfile` or `command` must be specified.
 `type="str|int|double|bool|yaml"`''(optional)''
  . Specifies the type of the parameter. If you don't specify the type, roslaunch will attempt to automatically determine the type. These rules are very basic:
   * numbers with '.'s are floating point, integers otherwise;
   * "true" and "false" are boolean (not case-sensitive).
   * all other values are strings
 `textfile="$(find pkg-name)/path/file.txt"`''(optional)''
  . The contents of the file will be read and stored as a string. The file must be locally accessible, though it is strongly recommended that you use the package-relative {{{$(find)/file.txt}}} syntax to specify the location.
 `binfile="$(find pkg-name)/path/file"`''(optional)''
  . The contents of the file will be read and stored as a base64-encoded XML-RPC binary object. The file must be locally accessible, though it is strongly recommended that you use the package-relative {{{$(find)/file.txt}}} syntax to specify the location.
 `command="$(find pkg-name)/exe '$(find pkg-name)/arg.txt'"`''(optional)''
  . The output of the command will be read and stored as a string. It is strongly recommended that you use the package-relative {{{$(find)/file.txt}}} syntax to specify file arguments. You should also quote file arguments using single quotes due to XML escaping requirements.

=== Examples ===
{{{
<param name="publish_frequency" type="double" value="10.0" />
}}}

=== Parameters from YAML output ===

In order to load a YAML file, you can use:
{{{
<rosparam command="load" file="FILENAME" />
}}}

<<Version(Lunar)>>

But this doesn't work when using a command which outputs the parameters on `stdout`.
In that case the new param type `yaml` can be used:
{{{
<param name="params_a" type="yaml" command="cat '$(find roslaunch)/test/params.yaml'" />
}}}
In the above example, the parameters from the described file would be loaded under the local namespace `params_a`.