<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def cmd():
 """Implementation of field subcommand. Other Parameters: conf.field conf.core """

    sdat = StagyyData(conf.core.path)
    sovs = set_of_vars(conf.field.plot)
    minmax = {}
    if conf.plot.cminmax:
        conf.plot.vmin = None
        conf.plot.vmax = None
        for step in sdat.walk.filter(snap=True):
            for var, _ in sovs:
                if var in step.fields:
                    if var in phyvars.FIELD:
                        dim = phyvars.FIELD[var].dim
                    else:
                        dim = phyvars.FIELD_EXTRA[var].dim
                    field, _ = sdat.scale(step.fields[var], dim)
                    if var in minmax:
                        minmax[var] = (min(minmax[var][0], np.nanmin(field)),
                                       max(minmax[var][1], np.nanmax(field)))
                    else:
                        minmax[var] = np.nanmin(field), np.nanmax(field)
    for step in sdat.walk.filter(snap=True):
        for var in sovs:
            if var[0] not in step.fields:
                print("'{}' field on snap {} not found".format(var[0],
                                                               step.isnap))
                continue
            opts = {}
            if var[0] in minmax:
                opts = dict(vmin=minmax[var[0]][0], vmax=minmax[var[0]][1])
            fig, axis, _, _ = plot_scalar(step, var[0], **opts)
            if valid_field_var(var[1]):
                plot_iso(axis, step, var[1])
            elif var[1]:
                plot_vec(axis, step, var[1])
            oname = '{}_{}'.format(*var) if var[1] else var[0]
            misc.saveplot(fig, oname, step.isnap)