Linux common commands
1. Check folder size
$ du -hs /path/to/directory
2. Zip a folder
$ zip -r filename.zip /path/to/folder
3. Mount a portable harddrive / usb drive
$/sbin/fdisk -l
4. Check memory usage
$ free -m
by look into the free column of -/+ buffers/cache line, it’s the free memory that can be used by applications.
5. Check if a package is installed (RedHat Enterprise/Fedora/Suse/CentOS)
$rpm -qa | grep PACKAGE-NAME