To run bitcoin core testnet on Ubuntu 16.04

After having installed Bitcoin core on your machine, run the following command to start synchronising the testnet blockchain

bitcoind -testnet -daemon

The synchronisation starts as a daemon and runs in the background, you can issue the following the command to see the progress

tailf ~/.bitcoin/testnet3/debug.log

To access the command line apis , just add the -testnet parameter

bitcoin-cli -testnet getinfo

Update: The RPC getinfo is deprecated, use getblockchaininfo for it to work with the latest version

bitcoin-cli -testnet getblockchaininfo

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