ev is a special command that enables simple operations like adding or multiplying datasets directly in a terminal. As these operations are generally simple to do in a script mode, ev is available only in the command line interface.

Command Docstrings

Reverse Polish notation

ev uses reverse Polish notation (RPN; also know as postfix notation) to input operations. Unlike in the more common infix notation, in RPN, the operators follow operands. For example 4 - 2 is written in RPN as 4 2 -; 4 - (2 / 5) is 4 2 5 / -. This can be further demonstrated on the sequence that processes this expression.

  1. 4 >>> 4 (Add 4 to the stack)
  2. 4 2 >>> 4 2 (Add 2 to the stack)
  3. 4 2 5 >>> 4 2 5 (Add 5 to the stack)
  4. 4 2 5 / >>> 4 0.4 (Divide the last two elements with each other)
  5. 4 2 5 / - >>> 3.6 (Subtract the last two elements from each other)

More information about RPN can be found, for example, on Wikipedia.

Using ev on simple datasets

The ev command has one required argument, which is the operation sequence. Datasets are specified in the sequence with f[set_index][component]. Note that Python indexing conventions are used. Specifically, when no indices are specified, everything is used, i.e., f[0][:] and f[0] are treated as identical calls. The indices need to be integers unless a global indexing mode -g is used, which ignores active and inactive sets. Then, Python slice syntax can be used as well including negative indices and strides. For example, f[2:-1:2] will select every other dataset, starting with the third one (zero-indexed), and ending with one before the last (by Python conventions, the upper bound is not included; in this example it might end with the third element from the end because of the stride).


Dataset selection in ev internally uses the same code as the activate/deactivate/pg_cmd_deactivate commands, so the following commands produce similar results (ev actually copies the datasets instead of just activating some).

pgkyl two-stream_elc_?.bp activate -i '2:-1:2'
pgkyl two-stream_elc_?.bp ev -g 'f[2:-1:2]'

However, this is probably a fringe application of ev.

The simplest example of ev is a numerical operation performed on a dataset, e.g., dividing the values by the insidious factor of 2:

pgkyl two-stream_elc_0.bp ev 'f[0] 2 /'

This can be also combined with the fact that ev can access dataset metadata as long as they are included (which is a new feature in Gkeyll introduced in January 2021). An example of this can be plotting number density from a fluid simulation (Gkeyll outputs mass density).

pgkyl 5m_fluid_elc_0.bp ev 'f[0][0] f[0].mass /' plot

Note that on top of dividing by mass, only the first component, which corresponds to density, was selected. This can be easily extended to apply on multiple datasets and create an animation using the animate command

pgkyl '5m_fluid_elc_[0-9]*.bp' ev -g 'f[:][0] f[:].mass /' animate

The capabilities are not limited to operations with float factors. As an example, ev can be used to visualize differences (--diverging mode of the plot command is well suited for this)

pgkyl two-stream_elc_0.bp two-stream_elc_80.bp interpolate ev 'f[1] f[0] -' plot --diverging

Visualizing difference between two datasets


info command, especially with the --compact -c flag can be useful to print indices for available datasets.

The same concept can be used to calculate bulk velocity from the first two moments:

pgkyl two-stream_elc_M0_0.bp two-stream_elc_M1i_0.bp interpolate ev 'f[1] f[0] /' plot

Finally, it is worth noting that this syntax cannot be used when there are datasets with more than one tag active.

Using ev on datasets with tags

The ev command is tag-aware. Tagged datasets use the following notation t.tag_name[set_index][component]. Using this, the previous example can be reproduced:

pgkyl two-stream_elc_M0_0.bp -t dens two-stream_elc_M1i_0.bp -t mom interp ev 'mom[0] dens[0] /' plot

However, unlike the previous example, this can be naturally extended for batch loading and animate:

pgkyl 'two-stream_elc_M0_[0-9].bp' -t dens 'two-stream_elc_M1i_[0-9]*.bp' -t mom interp ev -g 'mom dens /' animate

Examples of specific ev operations

In this section we provide examples of some ev operations that are less trivial or intuitive.


This operation differentiates a along a direction given by the second operand. So, for example, given the data from an ion sound wave gyrokinetic simulation we can plot the initial electrostatic potential with

pgkyl gk-ionSound-1x2v-p1_phi_0.bp interp pl -x '$x$' -y '$\phi$'

and compute the parallel electric field by differentiating the potential along \(x\) as follows:

pgkyl gk-ionSound-1x2v-p1_phi_0.bp interp ev 'f[0] 0 grad -1 *' pl -x '$x$' -y '$\phi$'

These produce the following plots:



Integrate a dataset along a direction specified by the second operand, or along multiple directions specified by a comma-separated list. If we once again take the ion sound wave gyrokinetic simulation data, we can examine the number of particles in the simulation (should be conserved) by taking the time trace of the integrated ion number density (intM0) and taking its mean:

pgkyl gk-ionSound-1x2v-p1_ion_intM0.bp ev 'f[0] mean' pr

which prints out


If we instead use ev to integrate the initial and/or the final number density GkM0, we should get roughly the same answer. We can check that this is the case by typing

pgkyl gk-ionSound-1x2v-p1_ion_GkM0_10.bp interp ev 'f[0] 0 int' pr

which produces


and we have shown that the number of particles at the end is roughly the same as the mean number of particles throughout the simulation.


Average a dataset along a direction specified by the second operand, or along multiple directions specified by a comma-separated list.