Fork athena and compile

setup.sh

# Fork main athena repository: https://gitlab.cern.ch/atlas/athena/forks/new
git clone https://:@gitlab.cern.ch:8443/[YOUR_USER_NAME]/athena.git
cd athena
git remote add upstream https://:@gitlab.cern.ch:8443/atlas/athena.git
git fetch upstream  # Ensure sync with main repository
# Use useful name for branch "-b your useful name" 
git checkout -b 21.0-fix-for-something upstream/21.0 --no-track

# Use the latest date in r2017-XX-XX or the day vefore
cd ..
asetup 21.0,r2017-07-05,Athena
cp athena/Projects/WorkDir/package_filters_example.txt pf.txt
$EDITOR pf.txt # leave only "+ Trigger/TrigT1/TrigT1CaloMonitoring" or any other packages that you requie
mkdir build && cd build

# ...
# Provide your changes
# ...
cmake -DATLAS_PACKAGE_FILTER_FILE=../pf.txt ../athena/Projects/WorkDir
make -j
source x86_64-slc6-gcc62-opt/setup.sh
# Run your tests
# athena ...