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In the very beginning of a script stands:
if [ -z $DISPLAY ]
then
DIALOG=dialog
else
DIALOG=Xdialog
fi
I understand, that
$DISPLAY holds the value of which pseudo-terminal one is attached to[ -z $DISPLAY ] tests whether $DISPLAY has a value or is an empty stringWhat is the purpose of $DISPLAY keeping a value, what is different when it has an empty string?
Why would it be crucial for the further execution of the script to know whether [ -z $DISPLAY ] is true or false ?
Whatever this script is, it's using the $DISPLAY value to determine if it's running in an X11 environment or not (if the script is running in e.g. a desktop environment, $DISPLAY will be set). It doesn't care about the actual value at all.
If it is running in X11, it sets DIALOG to Xdialog, which presumably is a X11 program for displaying dialog boxes.
If it's not running in X11 (for example, it's on a headless machine or in an SSH-session), it sets DIALOG to dialog, which presumably is a curses (or at least text-based) program for displaying dialog boxes.
Later, the script may use $DIALOG to display a dialog box appropriate for the user.
$DISPLAYexpands to at most one word of course)DISPLAY. I don't think it usually contains more than one word, but of course someone could set it to anything. Also, it's about X, not pseudoterminals.