06-06-2008
${!} an alternate syntax for $! which is the PID of the last background command (presumably here, your graph application). The script simply waits for the application to finish.
Generally, ${anything} is just another way to write $anything -- in some situations, the braces are required for disambiguation (${anything}too means something else than $anythingtoo which means ${anythingtoo}) but some people like to put them everywhere just out of habit, or for clarity.
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LEARN ABOUT CENTOS
tkwait
tkwait(n) Tk Built-In Commands tkwait(n)
__________________________________________________________________________________________________________________________________________________
NAME
tkwait - Wait for variable to change or window to be destroyed
SYNOPSIS
tkwait variable name
tkwait visibility name
tkwait window name
_________________________________________________________________
DESCRIPTION
The tkwait command waits for one of several things to happen, then it returns without taking any other actions. The return value is always
an empty string. If the first argument is variable (or any abbreviation of it) then the second argument is the name of a global variable
and the command waits for that variable to be modified. If the first argument is visibility (or any abbreviation of it) then the second
argument is the name of a window and the tkwait command waits for a change in its visibility state (as indicated by the arrival of a Visi-
bilityNotify event). This form is typically used to wait for a newly-created window to appear on the screen before taking some action. If
the first argument is window (or any abbreviation of it) then the second argument is the name of a window and the tkwait command waits for
that window to be destroyed. This form is typically used to wait for a user to finish interacting with a dialog box before using the
result of that interaction.
While the tkwait command is waiting it processes events in the normal fashion, so the application will continue to respond to user interac-
tions. If an event handler invokes tkwait again, the nested call to tkwait must complete before the outer call can complete.
KEYWORDS
variable, visibility, wait, window
Tk tkwait(n)