Not getting any logged output on clients

I'm trying to use `net-hydra` to run `iperf` on a pool of clients all at the same time - my goal is to measure the impact of increasing the number of wifi clients on average request latency in a LAN.

My directives are all of the form `clientId = $loadtest`, and the `loadtest` directive is defined as:

```
$loadtest = "/usr/bin/iperf --trip-times -l 128 -b 51.2k,5k -t 30 -e -c 10.3.0.125 >> /tmp/symmetric-30-$when.log"
```

The actual load test runs fine, and the log file ~/symmetric-30-NNNNNNN.log gets created, but there's never any output captured in the actual log file. When I run the exact same command line locally on the individual test nodes, they _do_ generate log files with the expected content. I'm not sure why it's not working when the command is run by net-hydra. Is it possibly related to daemonisation by `whenits`?