Setting rules in config.h

What does 'rules' do?

The rules array allows treating of certain applications (clients) uniquely. A rule has a matching and an action part. When a new client appears (sends a maprequest), it is matched against the rules based on its class, instance (WM_CLASS) and title (WM_NAME) properties and then the given tag and floating mode setting actions are performed. The default tag mask is 0, which means the currently viewed tags and the default mode is tiled so isfloating is False or 0.

Example from the default config:

static Rule rules[] = {
	/* class      instance    title       tags mask     isfloating   monitor */
	{ "Gimp",     NULL,       NULL,       0,            1,           -1 },
	{ "Firefox",  NULL,       NULL,       1 << 8,       1,           -1 },
	{ "deadbeef", NULL,       NULL,       1 << 7,       0             0 }

These rules make every Gimp and Firefox window floating and makes Firefox windows appear on tag 9 instead of the currently viewed tags. deadbeef similarly displays its window on tag 8 for a secondary display monitor.

How does the matching work?

A client is matched if its properties contain the given strings as substrings (case-sensitively) or NULL is given (which means anything is matched there).

More than one rule can be applied to a client, the rules are matched in order.

How to check these properties of a client?

The xprop utility can be used to get this information: WM_CLASS is (instance, class) WM_NAME (or _NET_WM_NAME) is the title.

For example this shell script prints the relevant properties of the selected client (if the properties does not contain '=' or ','):

xprop | awk '
	/^WM_CLASS/{sub(/.* =/, "instance:"); sub(/,/, "\nclass:"); print}
	/^WM_NAME/{sub(/.* =/, "title:"); print}'

How to add exception to a tagging rule?

It cannot be simply done. For example it is difficult to achieve that each Firefox window goes to tag 9 except one specific dialog, which goes to tag 8, because the tag masks of different matched rules are 'or'ed (and not overwritten).