How to check if X server is running?

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I often need to run an X command on a server that is running many X servers, so the ps based answers do not work. Naturally, $DISPLAY has to be set appropriately. To check that that is valid, use xset q in some fragment like:

if ! xset q &>/dev/null; then
    echo "No X server at \$DISPLAY [$DISPLAY]" >&2
    exit 1
fi

EDIT

Some people find that xset can pause for a annoying amount of time before deciding that $DISPLAY is not pointing at a valid X server (often when tcp/ip is the transport). The fix of course is to use timeout to keep the pause amenable, 1 second say.

if ! timeout 1s xset q &>/dev/null; then
    ⋮

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