I've gathered some information about French military ranks and commissions from Google searches. I've not edited or commented, just consolidated what seems most relevant. The following link is the best information I found ( "Proust the Private Soldier"); some of you have probably already found this site.

http://www.readingproust.com/orleans.htm

Military Ranks

Source: http://en.wikipedia.org/wiki/La_Grande_Arm%C3%A9e#History

This Wikipedia article includes lots of detailed description of Napolean I and Napolean III’s armies. I’m not sure, but it seems to be implied that the French Army of 1900 (St Loup’s army) was similar to that of Napolean III. (jm)

Ranks of the Grande Armée: Unlike the armies of the Ancien Régime and other monarchies, advancement in the Grande Armée was based on proven ability rather than social class or wealth. Napoleon wanted his army to be a meritocracy, where every soldier, no matter how humble of birth, could rise rapidly to the highest levels of command, much as he had done (provided, of course, they did not rise too high or too fast). This was equally applied to the French and foreign officers, and no less than 140 foreigners attained the rank of Général.[48] By and large this goal was achieved. Given the right opportunities to prove themselves, capable men could rise to the top within a few years, whereas in other armies it usually required decades if at all. It was said that even the lowliest private carried a marshal's baton in his knapsack.

Grande Armée rank Modern U.S. equivalent

Commissioned officers

Maréchal Lieutenant General

Général de division Major General

Général de brigade Brigadier General

Colonel Colonel

Colonel en second Lieutenant Colonel

Chef de bataillon or Chef d'escadron Major

Capitaine Captain

Lieutenant First Lieutenant

Sous-lieutenant Second Lieutenant

Non-commissioned officers

Adjudant-Chef Warrant Officer

Adjudant Sergeant-Major

Sergent-Major or Maréchal des logis Chef] First sergeant

Sergent or Maréchal des Logis Sergeant

Caporal-Fourrier or Brigadier-Fourrier Company clerk/supply Sergeant

Caporal or Brigadier Corporal

Soldat or Cavalier(Cavalry) Private

Canonnier(Artillery) Private

********************************************

Source: http://napoleonistyka.atspace.com/FRENCH_ARMY.htm#_under_Napoleon_III

Army of Emperor Napoleon III: 1850-1900

Promotion in the army was determined by a law that had been passed in 1832. Approx. 66 % of the officers were promoted on the basis of seniority, up to the rank of commandant.

***************************************************

Commissioned v Non-commissioned Officers

Source: Wikipedia, the free encyclopedia

A non-commissioned officer, called a sub-officer in some countries, is a military officer who does not have a high rank and who has not been given a commission. Non-commissioned officers (usually) obtain their position of authority by promotion from the lower ranks.

Commissioned officers generally receive training as leadership and management generalists, in addition to training relating to their specific military occupational specialty or function in the military. Most developed nations have set the goal of having their officer corps university-educated, although exceptions exist in some nations to accommodate officers who have risen from the non-commissioned ranks (e.g. the battlefield commission). Many advanced militaries require university degrees as a prerequisite for commissioning, even from the enlisted ranks.

A commission is a physical document issued to certify the appointment of a commissioned officer by a sovereign power. The more specific terms commissioning parchment or commissioning scroll are often used to avoid ambiguity, due to "commission" being a homonym which directs the individual in carrying out their duty regardless of what authority or responsibility they may have at any time. However the document is not usually in the form of a scroll and is more often printed on paper instead of parchment. Commissions are typically signed by the Head of State or other Commander-in-Chief.

The sale of commissions was a common practice in most European armies where wealthy and noble officers purchased their rank. Only the Imperial Russian Army and the Prussian Army never used such a system. While initially shunned in the French Revolutionary Army, it was eventually revived in the Grande Armée of Napoleon I (mainly in the French allied and satellite states). The British Army, which used this practice through most of its history, was last to abolish it.

http://www.heraldica.org/topics/france/noblesse.htm (Also has info on how titles are conferred)

....military commissions: in the Middle ages, the owner of a noble fief could be ennobled if he wasn't so, but after 1275 a condition that three consecutive generations hold the fief ("tierce foi") was added, and the privilege was abolished in 1579. The Edict of November 1750, when some military commissions were opened to non-nobles, it was decided that officers reaching the rank of general would automatically receive hereditary nobility. Officers of lesser rank who received the Order of Saint-Louis and fulfilled certain requirements were exempt from the taille (a tax on non-nobles); the third generation meeting the requirements received hereditary nobility.

Officers’ Servants - (couldn't find anything very relevant to Donciers)

A batman (or batwoman) is a soldier or airman assigned to a commissioned officer as a personal servant. The term is derived from the obsolete bat, "packsaddle" (from French bât, from Old French bast, from Late Latin bastum) + man. A batman's duties often include:

* acting as a "runner" to convey orders from the officer to subordinates

* maintaining the officer's uniform and personal equipment as a valet

* driving the officer's vehicle, sometimes under combat conditions

* acting as the officer's bodyguard in combat

* other miscellaneous tasks the officer does not have time or inclination to do

The action of serving as a batman was referred to as "batting". In armies where officers typically came from the upper class, it was not unusual for a former batman to follow the officer into later civilian life as a domestic servant. In the French Army the term for batman was ordonnance. Batmen were abolished after World War II.

[JRR Tolkien took the relationship of his characters Samwise Gamgee and Frodo Baggins from his observations during his military service during World War I of the relationship between a batman and his officer. [The Letters of J.R.R. Tolkien, ed. Humphrey Carpenter]