ATTENTION: use at your own risk, no warranty of any kind This seems to work in High Sierra, but may break in future versions
cat > /tmp/lockscreen.c <<EOF
// lockscreen.c
extern void SACLockScreenImmediate();
int main()
{
SACLockScreenImmediate();
return 0;
}
EOF
clang -F /System/Library/PrivateFrameworks -framework login -o /tmp/lockscreen /tmp/lockscreen.c
cp /tmp/lockscreen /usr/local/bin/