Is there a way to extract specific info or lines from a ROS topic?

asked 2021-03-09 15:13:47 -0500

etorobot
30 ●5 ●6 ●10

updated 2021-03-09 21:33:40 -0500

echo /joint_states has the following output (shortened).

I made a node (using the rospy tutorial) to subscribe to /joint_states and I want the.py script to print() just the position list for use outside ROS, not the whole topic.

How can I extract only that one line from the topic?

Any help appreciated!

---
header: 
  seq: 1734
  stamp: 
    secs: 26
    nsecs:  47000000
  frame_id: ''
name: 
  - L_CROTCH_Y
  - L_CROTCH_R
  - L_CROTCH_P
  - L_KNEE_P
  - L_ANKLE_R
  - L_ANKLE_P
  - CHEST_Y
  - LC-AI-linear-joint
  - LC-AO-linear-joint
  - RC-W-linear-joint
  - LC-W-linear-joint
  - RC-E-linear-joint
  - LC-E-linear-joint
position: [0.00015912794687559303, -0.00013126947362762615, -0.5109341293645083, 1.1760409850349758, 0.000248513719677129, -0.621929533407773, -0.000435712016344022, -0.25723620888891585, 0.0001570613057798813, -0.00010196824444583892, -0.5109459927618113, 1.176033708862523, 0.00026405600006946736, -0.6219324270315635, 2.4978990728770677e-06, -0.0017088975996239417, -2.073140234539537, 0.1084099023783044, -0.058023574829095245, -1.0747267840396235, 0.07317868560597149, 0.0032100904000482025, 1.6205784723865542, 0.5235246181475844, -2.073396637452086, -0.10915019469469348, 0.05786516725301176, -1.0742435456501649, -0.07401658373418843, 0.003328953066405395, -1.6208511322518637, 0.5236036706122303, 0.2857, 0.2857, 0.2229, 0.294777, 0.2977, 0.2857, 0.2857, 0.2229, 0.2977, 0.2977, 0.2641, 0.2641, 0.1878, 0.1878]
velocity: [-1.3228298182769372e-05, 0.003776438677399348, -0.0004475842306390639, 0.0005277640174040183, 0.00017784552316915054, -1.3651984069540118e-05, 0.0005193152271762515, -1.614980274114962e-05, -2.00906204147508e-05, 0.003781257562926932, 0.0003987885595227354, -0.0001448261644036203, 0.0001767970472550341, -6.29682030487069e-06, 1.0756488468918085e-05, 8.071757825288712e-06, 0.006530372894968822, -0.0022088213690768304, -0.0016988138511383664, -0.008485019320501625, 0.0011095488166591757, -0.0024041403239194944, 0.0010985414608061655, -0.004589661263847082, -0.006443145635355184, -0.0024326191176086316, -0.0015454480678420079, 0.007643253393764543, 0.0002669792459987253, 0.002038055197724077, 0.0002605698826494082, 0.003338718115727315, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
effort: [-0.003956578081039798, -0.103970838003065, -10.348208900103906, 12.521092591095684, 0.031017161031442676, -0.12736535358104373, 0.024752505226941078, 22.123612918291016, 0.0034862249945595775, -0.2969554758624179, -10.330423298916536, 12.466804055037972, -0.021633696134477715, -0.1281664874752675, -0.00023514434759137692, 1.1822407190049664, 28.710767380445425, 4.002011405040487, 1.9539374192954484, -55.435105205047684, -0.08386314941827289, -17.16595565303094, -0.08274324761003982, -0.0006322724365340204, 29.273691838522797, -3.495521534240986, -1.4834178286291244, -55.63014000109101, -0.09213406138346147, -16.939107019010283, -0.09077836527859873, -0.00042287233021992554, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

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Comments

What are you looking to extract?

jayess ( 2021-03-09 15:18:18 -0500 )edit

@jayess just this part:

position: [0.00015912794687559303, -0.00013126947362762615, -0.5109341293645083, 1.1760409850349758, 0.000248513719677129, -0.621929533407773, -0.000435712016344022, -0.25723620888891585, 0.0001570613057798813, -0.00010196824444583892, -0.5109459927618113, 1.176033708862523, 0.00026405600006946736, -0.6219324270315635, 2.4978990728770677e-06, -0.0017088975996239417, -2.073140234539537, 0.1084099023783044, -0.058023574829095245, -1.0747267840396235, 0.07317868560597149, 0.0032100904000482025, 1.6205784723865542, 0.5235246181475844, -2.073396637452086, -0.10915019469469348, 0.05786516725301176, -1.0742435456501649, -0.07401658373418843, 0.003328953066405395, -1.6208511322518637, 0.5236036706122303, 0.2857, 0.2857, 0.2229, 0.294777, 0.2977, 0.2857, 0.2857, 0.2229, 0.2977, 0.2977, 0.2641, 0.2641, 0.1878, 0.1878]

etorobot ( 2021-03-09 15:52:04 -0500 )edit

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2 Answers

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answered 2021-03-09 23:00:09 -0500

stevemacenski

8272 ●34 ●503 ●129 https://www.linkedin.c...

Via commandline

rostopic echo /joint_states | grep position

Via code make a subscriber and in the callback, take the message object and print(msg.position)

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Comments

thank you! they both work!

print(joint_states.position) in callback gives:

 >>= python simplesub.py          
(0.00015512971358007838, -0.00015967116068381712, -0.5109057242141067, 1.1760468735261653, 0.00024737518151100293, -0.6219273124713371, -0.00043378914150490606, -0.25728330147008904, 0.00015295854040879757, -0.00013035336442333807, -0.5109116400316044, 1.176043473406131, 0.00026297911536165907, -0.6219294475672827, 2.3686681173184236e-06, -0.0017019379157545052, -2.0731745346858754, 0.10844604105347311, -0.057998265002420835, -1.0747005312985296, 0.07318421198008158, 0.0032205623245452246, 1.6205839810166822, 0.5235180193010072, -2.0735647804400736, -0.10926420886570778, 0.0577272133804709, -1.0744295533357717, -0.07392402403748027, 0.003115224999999684, -1.6207593400342353, 0.5235428314880762, 0.2857, 0.2857, 0.2229, 0.294777, 0.2977, 0.2857, 0.2857, 0.2229, 0.2977, 0.2977, 0.2641, 0.2641, 0.1878, 0.1878)

etorobot ( 2021-03-09 23:14:02 -0500 )edit

Can you mark the answer as accepted?

stevemacenski ( 2021-03-10 01:01:17 -0500 )edit

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answered 2021-03-10 03:08:16 -0500

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

updated 2021-03-10 03:15:04 -0500

I made a node (using the rospy tutorial) to subscribe to /joint_states and I want the.py script to print() just the position list for use outside ROS, not the whole topic.

rostopic echo can actually print individual fields. The grep should not be needed, nor a custom node.

See the rostopic echo documentation:

echo <topic-name/field>

Display specific fields in a message.
$ rostopic echo /my_topic/field_name

And see #q54895 for a previous question about this.

For JointState:

rostopic echo /joint_states/position

should work

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rostopic echo /joint_states/position works in console

>>= rostopic echo /joint_states/position
[0.00015846172468917373, -0.00013917082967925174, -0.5109520651984194, 1.176033061817138, 0.0002480363156142424, -0.6219319452419783, -0.00043551693502062657, -0.25721493268895024, 0.00015645224814834745, -0.00010988429784202527, -0.5109659467739442, 1.1760253582176348, 0.0002635422334038843, -0.621934983228341, 2.8759981429691814e-06, -0.001725037542044635, -2.0730975836732286, 0.10848981362952725, -0.057972814677291484, -1.0746624245112641, 0.07318469499950343, 0.00323110174674267, 1.6205845054169492, 0.5236412224584802, -2.073346746902119, -0.1092237021374939, 0.057807670438532716, -1.074215977780912, -0.07399643209973757, 0.0033379011578772684, -1.620831187381248, 0.5237314724596493, 0.2857, 0.2857, 0.2229, 0.294777, 0.2977, 0.2857, 0.2857, 0.2229, 0.2977, 0.2977, 0.2641, 0.2641, 0.1878, 0.1878]
---

etorobot ( 2021-03-10 06:22:13 -0500 )edit

Yes, that's what you asked about, isn't it?

Creating a separate node just to print the value of a single field in messages on a topic shouldn't be necessary.

gvdhoorn ( 2021-03-10 06:23:34 -0500 )edit

Is there an equivalent of this that can work in code (.py)? This is what I'm trying to achieve:

Node subscribes to /joint states to get positions
Script outputs (outside ROS) the list of positions from /joint_state/position
External program gives new list of positions
Node publishes this as JointTrajectoryPoint

etorobot ( 2021-03-10 06:35:58 -0500 )edit

This goes much further than simply printing a single field from an incoming message.

You should post a separate question.

gvdhoorn ( 2021-03-10 06:45:59 -0500 )edit

Oh I didn't know that @gvdhoorn, that's really good to know!

stevemacenski ( 2021-03-11 14:17:09 -0500 )edit

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Is there a way to extract specific info or lines from a ROS topic?

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