Depended a lot on the army you fought in but the short answer would be no it was not a death sentence.

The Roman army would be well shielded and armored and losses were generally rather low. You would advance walking toward the enemy in tight formation. If the enemy was a barbarian they would probably advance and then charge when close enough in loose formation as their tactics generally relied less on formation and more on shock and mobility.

You would meet them with your shield and stab with your pugio. After a few minutes the guy behind would take your place and you would get in the rear for a while to rest a bit, then if the battle was still going you would get back to the front line eventually etc... This tactic was excellent for morale as tired soldiers would always know the would get to rest for a while if they held just a few more minutes. The enemy troops would therefore always face fresh and vigilant soldiers while themselves were getting wearier with every minute.

The front line sure had more casualties because they had to take the enemy charge sure, but it was far from a death sentence and as a roman legionnary you would have had good chances of getting back without dying thanks to your training, excellent equipment and iron discipline. As long as the Legion stayed in formation your chances were good and the Legion generally remained in formation because they were very highly disciplined soldiers.

If you somehow could not assume formation (generally because of the terrain : trees / rocks / whatever) the chances of survival would drop a lot whatever your position because the Legion kind of sucked when not in open ground, but the Legion avoided this type of terrain generally as they totally knew they were highly vulnerable in these.

The Greek style phalanxes was basically the same deal, the armor was lighter and less inclusive (armpits for example were often highly vulnerable, the shield was smaller as well) but once again the formation protected you as long as you stayed disciplined and the Greeks had an awesome system to force soldiers to maintain formation : for protection you had to count on the left part of the shield of the soldier at your right, meaning any soldier in difficulty would not tend to flee or break formation but in the contrary would tend to get closer from the mate at his right, the soldier on his left would adapt to stay within the protection of his shield etc...

Because of that Greek phalanxes tended to swerve toward the right so the wise commander would make sure they cannot go too much on the right, whether by making sure there is another unit there or even better a natural obstacle.

In the end the casualties were not as grievous as one could think seeing war movies.

The loose formation armies such as barbarians or persians (except Greek merc who would fight in phalanxes like their Greek counterparts of course) would use their infantry to pin the enemy infantry but would not count on it to win the battle and it was kind of expected that it would rout at some time, they just had to hold long enough for highly mobile units of cavalry to flank the enemy. Losses in the infantry was of course higher with this strategy but their infantry was not trained a lot and was lightly equipped and therefore much more expendable.

In conclusion the huge part of the casualties in this type of battle was if your side broke formation and / or routed. The enemy would launch his cavalry after you and you would be easy preys then. If the morale broke the first line were generally the last to rout and therefore the first to be cut. But it rarely happened in highly disciplined armies such as Rome of Greece. But if you maintained formation and retreat in a disciplined way, even if the battle was a defeat you had okey chances to see your family back.

So if you are given the choice between getting sent back in time as a Roman front line legionnaire or as a French soldier from WW1, do not hesitate one second and pick the Roman. WW1 soldiers had much more chances of not coming back than Roman ones.

First you would have to define what the 'front line' is and then break it down by each of the groups you listed. All four had very different ways of committing to battle.

As a general answer, no, it was not a death sentence. However the answer varies depending on who exactly you are talking about and much more importantly, what battle you're referring to. Let's use Romans as the first example. A manipular army was traditionally made up of four lines. At the front line there would be velites, Roman skirmishers. The manipular formation was slotted to allow the velites to safely withdraw after expending their ammunition. The lanes between maniples were very important. It meant the velites could walk between units without disrupting their cohesion which would not be possible if it was a solid phalanx.

I think this establishes the survivability of front line soldiers for Roman armies until the Marian reforms. Their model of warfare was based around allowing front line troops to move through the army as described which would be pointless if skirmishers were regularly annihilated. Skirmishers were often the youngest and the poorest under the pre-Marian system. I wouldn't say it was a death sentence but neither was it remotely safe.

Questions like this always seem to ask for and get responses pertaining to European antiquity; can anyone shed light on tactics of other places, like Asia, Africa, Americas, etc.?