X3270 is an application program available for desktop/laptop POSIX systems (which includes Linux) which emulates an IBM 3270 family terminal. X3270 is the reference 3270 emulator in the ZNETBOOT package. It is not the only 3270 emulator available.
X3270 uses TELNET protocol to connect to your z/VM host and simulates the operation of an IBM 3270 terminal. X3270 can use traditional unprotected cleartext TELNET or can use SSL-enabled or SSL-always modes. SSL is recommended, unless you use other security schemes such as proxy through the Tor network.
If you don't already have X3270 installed on your laptop/desktop Linux system, find it using your package manager and install it. X3270 is available with most Linux distributions, not a special order. You can then connect to z/VM with a command like ...
x3270 myvmhost.mydomain
If z/VM TELNET is listening on an alternate port, you can specify that ...
x3270 myvmhost.mydomain 1234
... where myvmhost.mydomain is the z/VM system
and 1234 is the port to use. SSL negotiation should be automatic.
There are real 3270 terminals in use, but they are far and away less common than emulated 3270 terminals. When using a 3270 terminal (emulated or real) remember that nothing is sent to the server until you press an action key such as a PF key or the <Enter> key.
Be aware that click to focus may also affect the 3270 cursor.
The usual way of directing focus to an X application is by clicking anywhere in the application's window. Some applications are sensitive and may use the click for their own pointing. This is true of X3270, so when you click to focus you will also move the text cursor.
In case 3270 input has been incorrectly re-focused, use the <Tab> key to re-focus on the correct input field.
There is no reliable way to show the locations of the fields on a 3270 screen, but <Tab> will move naturally from one input field to the next.
Using a 3270 interface is a bit like using a web form. It's easy and becomes natural after some time, but is not byte-at-a-time like other terminals.
Once logged on, z/VM divides your 3270 screen into three areas: output from your guest operating system (most of the screen), input to your guest operating system (the bottom two lines), and a status area (far right of the bottom line).
z/VM uses its 3270 interface to present a virtual 3215 line-mode terminal. Think typewriter terminal. Think IBM Selectric. This model persists for reasons which are beyond the scope of this document, but it is worth noting that there are bypass modes where your virtual machine can drive the 3270 in full-screen mode. One of these is the CMS text file editor XEDIT.
Linux can drive 3270 terminals directly. When it does, it too simulates a line-mode terminal when presenting to most applications. Linux too has ways of bypassing line-mode operation and going full-screen.
z/VM uses your 3270 session according to its own rules. One of those rules is that the lower right corner displays status "RUNNING", "VM READ", "CP READ", "MORE...", or "HOLDING".
The default linend character is pound sign (#). It would introduces a comment in a Unix or Linux shell, but on z/VM it is the equivalent of the semi-colon in the shell.
While "#" by itself in the middle of a command in a shell means "all that follows is a comment", on z/VM it means "all that follows is the next line".
The linend character can be changed.
#cp is mentioned elsewhere as a way to enter CP commands
from your z/VM virtual console (from your 3270 session).
The intercept is actually <linend>\CP.
If you are accustomed to Ctrl-A, Ctrl-E and other hotkey short-cuts for command-line editing, you will not find these with the 3270. Whether physical or emulated, a 3270 is a block-mode device. Some control characters do have meaning to your 3270 emulator, but they have no equivalent on a real 3270 terminal and they are not passed to the remote system. Any line editing effects are isolated to your local screen. The resulting command is sent to z/VM when an action key is pressed. (Usually <Enter>.)
You can usually jump to the start of your command-line with the <Tab> key. A hotkey to jump to the end of the command-line would vary from one 3270 emulator to another. Many 3270 emulators have an "erase to end of field" hotkey.