Simple script to check if the org you are using is a Dev Hub. Run by passing a username or alias to the script.

NotDevHub.sh

#!/bin/bash

# specify your username or alias
usernameOrAlias=$1

hub=$(sfdx force:data:soql:query -q "SELECT DurableId, SettingValue FROM OrganizationSettingsDetail WHERE SettingName = 'ScratchOrgManagementPref'" -t -u ${usernameOrAlias} --json)

# parse response
size="$(echo ${hub} | jq -r .result.size)"

if [[ $size -eq 0 ]];
then
  echo "${usernameOrAlias} is not a dev hub"
  exit 0
fi

records="$(echo ${hub} | jq -r .result.records)"
value="$(echo ${records} | jq -r .[].SettingValue)"

# evaluate the setting value
if $value === 'true'
then
  echo "${usernameOrAlias} is a dev hub"
else
  echo "${usernameOrAlias} is not a dev hub"
fi

exit 0