Also only some small hints (and also still in TODO):

yum list postgresqlShow available packages (all)RedHat, CentOS
yum search plproxysearch for available packagesRedHat, CentOS
rpm -qa|grep postgresqlShows which postgresql packages are installed.RedHat, CentOS
yum list installed|grep postgresSimillar result using yumRedHat, CentOS
apt-cache search show available packagesUbuntu
dpkg --get-selections | grep postgresqlInstalled packages of PostgreSQL on UbuntuUbuntu
chkconfig --list|grep postgres|cut -d' ' -f1shows name of postgresql service RedHat, CentOS 5
chkconfig|grep postgres|cut -d' ' -f1shows name of postgresql service RedHat, CentOS - 6/7
On ubuntu PostgreSQL usually runs as service "postgresql" without number of the version.Ubuntu
sudo apt-get --purge remove postgresqlClean your machine from postgresql Ubuntu
sudo apt-get --purge remove snap snapdClean postgresql if it was installed using snapUbuntu
service $( chkconfig|grep postgres|cut -d' ' -f1) statusfind the name of postgresql service and shows its status - do not be surprised if you see message about "postmaster" with port numberRedHat, CentOS
service postgresql-9.3 restartAs root - restarts postgres server running as service - if you know directly the name of the serviceall
service $( chkconfig|grep postgres|cut -d' ' -f1) statusAs root - finds the name of postgresql service an restarts it. This work directly only on RedHat, CentOSRedHat, CentOS
vmstat 1 10shows 10 lines immediate statistics of cpu, mem, swap, buffer etcall
cat /proc/swapsshows infos about swaps on machineall
cat /proc/meminfoshows full memory infoall
free -mshows immediate memory and swap statisticsall
cat /etc/*releaseshows you Distro and Version and sometimes more info about Linux you are working onall
uname -aShows Linux kernel infoall
ulimit -ashows per-user process limits in linuxall
lshwInfo about all hardware - sometimes very long list so you would want to limit it.all
lshw -class storageShows info about all disks / disk arraysall
iostatShows immediate statistics of reads / writes for all disks. May not be installed but is available for all distros.all
htopVery useful shows in dynamic screen usage of CPUs, memory, swap and top processes. May not be installed but it is available.all