There is no command as such. Unlike in Windows, a program in Linux does not have to register it's existence in a list or registry. If you installed the program manually, or use a distro without a package manager, there is no way to determine what is installed. On distros that have package managers, such as Debian, Fedora, or Ubuntu, you can usually print an output of installed packages. In Debian / Ubuntu, use the command
dpkg --get-selections | less
to scroll through a list of installed packages. I do not recall offhand what you would do in Fedora or Red Hat.
whereis [name of program]
There is no utility as such. There are several different methods by which software is installed and upgraded, depending on the distro.
echo $SHELL
You can if you really want to! But, most Linux software is already compiled and ready to be downloaded and installed. Note that Linux is the kernel from which many distributions (distos) branch out - Ubuntu, Linux Mint are but two examples.
Linux is a stable operating system for using free office software such as LibreOffice. Many varieties of Linux already have LibreOffice installed by default.
Enter "java -version" into a terminal. If Java is installed, it will tell you the version number. If it is not installed, it will say "command not found."
Linux allows full access to the source code. Windows does not. The command line has always proven useful when altering software. Windows differs from Linux in command line limitations as well. Linux provides a centralized location in which software and application can be controlled.
Ubuntu is an operating system and it is an open source software. This software could be installed on many devices including laptops, smartphones and tablets.
It's a command to start the X server for running Linux in graphical mode, if you have any of the desktop environments and/or window managers installed.
Most Linux servers do not have GUIs. If it had one installed,it could be started by issuing the startx command.
In Linux the man command displays the help(man page) for a command. man fdisk is "display the man page for command fdisk"
k is not a standard command in Linux.