How to set use_sim_time

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If you need to set use_sim_time from the commandline, then rosparamis your friend. Please read https://wiki.ros.org/rosparam

If you need to set it from a launch file, you can use either the <rosparam>tag to read a yaml file of parameters (https://wiki.ros.org/roslaunch/XML/ro...) or you can use the <param> tag to set a particular parameter, like use_sim_time(https://wiki.ros.org/roslaunch/XML/param). For example, add <param name="/use_sim_time" value="true" />

Reading the documentation helps alot.

Also helpful is the documentation regarding the ros clock: https://wiki.ros.org/roslaunch/XML/ro...

You only need to set the /use_sim_time (note that it is in the root namespace) parameter once, and all your nodes will start using simulation time.