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Can this be done without starting a nodeos instance? Perhaps with a js or cleos command?

I am asking since I want to be able to identify the chain_id for private blockchains without starting a nodeos just for this.

2

The chain id is the is the SHA256 serialization of the genesis_state.

genesis_state.hpp :

/**
    * Get the chain_id corresponding to this genesis state.
    *
    * This is the SHA256 serialization of the genesis_state.
    */
   chain_id_type compute_chain_id() const;

genesis_state.cpp :

/**
 *  @file
 *  @copyright defined in eos/LICENSE.txt
 */

#include <eosio/chain/genesis_state.hpp>

// these are required to serialize a genesis_state
#include <fc/smart_ref_impl.hpp>   // required for gcc in release mode

namespace eosio { namespace chain {

genesis_state::genesis_state() {
   initial_timestamp = fc::time_point::from_iso_string( "2018-06-01T12:00:00" );
   initial_key = fc::variant(eosio_root_key).as<public_key_type>();
}

chain::chain_id_type genesis_state::compute_chain_id() const {
   digest_type::encoder enc;
   fc::raw::pack( enc, *this );
   return chain_id_type{enc.result()};
}

} } // namespace eosio::chain

chain_id_type.hpp :

namespace chain {

   struct chain_id_type : public fc::sha256 {
      using fc::sha256::sha256;

      template<typename T>
      inline friend T& operator<<( T& ds, const chain_id_type& cid ) {
        ds.write( cid.data(), cid.data_size() );
        return ds;
      }

      template<typename T>
      inline friend T& operator>>( T& ds, chain_id_type& cid ) {
        ds.read( cid.data(), cid.data_size() );
        return ds;
      }

      void reflector_verify()const;

      private:
         chain_id_type() = default;

         // Some exceptions are unfortunately necessary:
         template<typename T>
         friend T fc::variant::as()const;

         friend class eosio::chain_apis::read_only;

         friend class eosio::net_plugin_impl;
         friend struct eosio::handshake_message;

         friend struct ::hello; // TODO: Rushed hack to support bnet_plugin. Need a better solution.
   };

} }  // namespace eosio::chain

chain_id_type.cpp :

/**
 *  @file
 *  @copyright defined in eos/LICENSE.txt
 */

#include <eosio/chain/chain_id_type.hpp>
#include <eosio/chain/exceptions.hpp>

namespace eosio { namespace chain {

   void chain_id_type::reflector_verify()const {
      EOS_ASSERT( *reinterpret_cast<const fc::sha256*>(this) != fc::sha256(), chain_id_type_exception, "chain_id_type cannot be zero" );
   }

} }  // namespace eosio::chain

namespace fc {

   void to_variant(const eosio::chain::chain_id_type& cid, fc::variant& v) {
      to_variant( static_cast<const fc::sha256&>(cid), v);
   }

   void from_variant(const fc::variant& v, eosio::chain::chain_id_type& cid) {
      from_variant( v, static_cast<fc::sha256&>(cid) );
   }

} // fc 

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