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<!--- What behavior did you expect? -->
Up to 0.16 it was sufficient to only specify the -testnet command line option causing bitcoind to write data to the ~/.bitcoind/testnet3 directory by default and not messing with the mainnet directory ~/.bitcoin. However, with 0.17 and -testnet seems to use the old mainnet directory ~/.bitcoin.
Not sure if this intended or a bug. Do you have to explicitely specify a different datadir for TESTNET now, if you want to run TESTNET and MAINNET on the same machine?
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$ /usr/local/bin/bitcoind -conf=/etc/bitcoin/test_bitcoin.conf -testnet
Bitcoin Core version v0.17.0 (release build)
InitParameterInteraction: parameter interaction: -proxy set -> setting -discover=0
InitParameterInteraction: parameter interaction: -whitelistforcerelay=1 -> setting -whitelistrelay=1
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EXCEPTION: N5boost10filesystem16filesystem_errorE
boost::filesystem::create_directory: Permission denied: "/mnt/data/bitcoind/.bitcoin/blocks"
bitcoin in AppInit()
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EXCEPTION: N5boost10filesystem16filesystem_errorE
boost::filesystem::create_directory: Permission denied: "/mnt/data/bitcoind/.bitcoin/blocks"
bitcoin in AppInit()
Shutdown: In progress...
Shutdown: done
<!-- What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)? -->
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ARM-32 (Armbian)
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