Minecraft wiki entry for bossbar, associated text: bossbar add <id> <name>
Create a new boss bar.
bossbar get <id> (max|players|value|visible)
Return the requested setting as a result of the command.
bossbar list
Display a list of existing boss bars.
bossbar remove <id>
Remove an existing bossbar.
bossbar set <id> (color|max|name|players|style|value|visible)
... color (blue|green|pink|purple|red|white|yellow)
Set the text color (if no color was specified as part of a text component) and bar color. Defaults to white upon creation.
... max <max>
Set the boss bar's maximum value. Defaults to 100 upon creation.
... name <name>
Set the boss bar's name.
... players [<targets>]
Change the set of players to whom the bar is visible. Defaults to none upon creation.
... style (notched_6|notched_10|notched_12|notched_20|progress)
Set the boss bar's visual amount of segments: continuous, 6 segments, 10 segments, 12 segments, or 20 segments. Defaults to progress upon creation.
... value <value>
Set the boss bar's current value. Defaults to 0 upon creation.
... visible <visible>
Set the boss bar's visibility. Defaults to true upon creation.