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

Question

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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Originally posted by etorobot on ROS Answers with karma: 30 on 2021-03-09

Post score: 1

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Original comments

Comment by jayess on 2021-03-09:
What are you looking to extract?

Comment by etorobot on 2021-03-09:
@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]

Answer 1

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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Originally posted by stevemacenski with karma: 8272 on 2021-03-10

This answer was ACCEPTED on the original site

Post score: 1

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Original comments

Comment by etorobot on 2021-03-10:
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)

Comment by stevemacenski on 2021-03-10:
Can you mark the answer as accepted?

Answer 2

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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Originally posted by gvdhoorn with karma: 86574 on 2021-03-10

This answer was NOT ACCEPTED on the original site

Post score: 3

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Original comments

Comment by etorobot on 2021-03-10:
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]
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Comment by gvdhoorn on 2021-03-10:
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.

Comment by etorobot on 2021-03-10:
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

Comment by gvdhoorn on 2021-03-10:
This goes much further than simply printing a single field from an incoming message.

You should post a separate question.

Comment by stevemacenski on 2021-03-11:
Oh I didn't know that @gvdhoorn, that's really good to know!