I fear you may be operating under some inclarity here: while line formations were certainly in use during the Napoleonic and Franco-Prussian Wars, there is some difference in precisely what constitutes a 'line' during such, as these posts will expand on:

• The now-deleted u/elos_ has this excellent overview on the Austro-Prussian and Franco-Prussian Wars;
• u/Superplaner provides, with pictures, a rundown of Prussian tactics during said war.

On the matter of how WW1-era battles were fought before the digging-in period, we can look to the appropriate section of the FAQ (1914, if that doesn't automatically take you there); specifically, this particular post also by u/elos_, on this matter precisely.