This could be a shirt or a frock, depending on the weight of the material used. Frocks were generally made using a heavier material than was used for making an undershirt. This one appears to be buttoned at the neck, so it is more than likely a shirt and not a frock.
This claims to be a mid 18th century French undershirt, but it also claims to be made of a heavy flax linen. This is therefore in my opinion in fact a French froc/frock. Also note the linen ties at the neck.
An 18th century bricklayer wearing a long sleeved weskit over a shirt.
A French peasant working in the fields. Note his undershirt is worn inside his breeches.
Again, no waistcoat for this peasant field worker, but his shirt is worn inside his breeches.
These two soap makers are wearing frocks.
This is a Dutch postman wearing a frock.
An Italian butcher wearing a frock.
A field worker in Brussels wearing a frock.
I think we can safely assume that these living historians interpreting French milice at Bedford Village are in fact either wearing breeches and frock, or they are wearing breechclout and shirt or frock.
Bedford Villiage- La Milice de la Belle Riviere 2009