I have large files that I am inputing the beginning of my texts that contain
all my newcommands. They are quite a few. Most of them will not be used in a given document. But which ones are used ?

Is it possible to write something to an auxiliary file saying if the newcommand that was defined was used or not in the text.

Although not very useful, I am including a MWE

documentclass[12pt]article
newcommandtitiHemixos castanonotus
newcommandeagHaliaeetus leucocephalus
begindocument
I my travels, I have seen many birds of type titi.
enddocument

An auxiliary file should then list the commands that were defined

Defined commands

titi
eag

Used commands

titi

Unused commands

eag

I am aware that packages define many newcommands. But I would be interested by user-defined newcommands only.

documentclass[12pt]article

usepackageetoolbox

makeatletter
 % This macro will contain all the tracked commands:
 def@mycommands
 % These macros enable and disable tracking the commands:
 defstarttrackingnewcommands%
 letold@@newcommand@newcommand
 def@newcommand##1%
 expandafterdefexpandafter@mycommandsexpandafter@mycommandsoneof@mycommands##1%
 old@@newcommand##1%
 %
 
 defstoptrackingnewcommands%
 let@newcommandold@@newcommand
 
 % These macros are used to write to the log file:
 defmycommand@used#1typeoutMy command `string #1' was used.
 defmycommand@unused#1%
 GenericWarning(mycommands)LaTeX Warning:
 My command `string #1' was not used!%
 %
 
 % These macros mark a command as used or unused:
 defmycommand@markunused#1%
 expandaftergdefcsname mycommand@status@expandafter@gobblestring #1endcsnamemycommand@unused #1%
 pretocmd #1mycommand@markused #1GenericWarning(mycommands)Could not patch `string #1' as unused!%
 aftergroupmycommand@markunusedaftergroup #1%
 
 defmycommand@markused#1%
 expandaftergdefcsname mycommand@status@expandafter@gobblestring #1endcsnamemycommand@used #1%
 patchcmd #1mycommand@markused #1GenericWarning(mycommands)Could not patch `string #1' as used!%
 aftergroupmycommand@markusedaftergroup #1%
 
 % This macro calls the appropriate logging macro for a command:
 defmycommand@evaluateuse#1%
 csname mycommand@status@expandafter@gobblestring#1endcsname
 
 % Mark all commands as unused at begindocument:
 AtBeginDocument%
 letoneof@mycommandsmycommand@markunused
 @mycommands
 
 % Evaluate the use of the commands at enddocument:
 AtEndDocument%
 letoneof@mycommandsmycommand@evaluateuse
 letmycommand@unused@gobble% first, only the used commands
 @mycommands
 letmycommand@used@gobble% then, only the unused commands
 @mycommands
 %
 
makeatother

starttrackingnewcommands
 newcommandfoofoo
 newcommandbazbar
 newcommandtitiHemixos castanonotus
 newcommandeagHaliaeetus leucocephalus
stoptrackingnewcommands

begindocument

I my travels, I have seen many birds of type titi.

enddocument

Towards the end of the log file you will find

My command `titi' was used.

LaTeX Warning: My command `foo' was not used! on input line 68.


LaTeX Warning: My command `baz' was not used! on input line 68.


LaTeX Warning: My command `eag' was not used! on input line 68.

Some Notes

This will break something if the first usage of one of your commands is used in a context like this:

defsomethingwithargument#1expansion
expandaftersomethingwithargumentoneofyourcommands

That is, if some command (somethingwithargument) consumes the start of the expansion of one of your commands, the fact that you inserted something at the start of that command to keep track of its usage will break this code. I do not know of any safer way to do this, though.

I chose to format the message for unused commands as a warning and the message for a used command as just that. You can of course choose to do this differently (or even write to an entirely different file if you prefer this).

Since this is a rather invasive process (modifying all your commands temporarily), there may well be side effects i cannot think of right now. If you spot any, feel free to add them here.

In any case, you should not use this as a permanent addition to your preamble, only as a technique to get a quick overview over which commands you use.

Here is a way to display used commands, for unused commands I did not find a solution. Search and replace your commands adding a display
i.e replace titi with displaytiti

 documentclass[12pt]article
newcountcommandnum
commandnum=0
newwritedisplayed
immediateopenoutdisplayed=jobname.dis
defdisplayedcommands#1vfill section#1 vskipbaselineskip immediatecloseoutdisplayed inputjobname.dis
defdisplay#1advancecommandnum by 1 #1textsuperscript(thecommandnum) immediatewritedisplayednoindent #1 P.thepage called by (thecommandnum) noexpandstringnoexpand#1'' vskip0.2baselineskip
newcommandtiti``Hemixos castanonotus''
newcommandeag``Haliaeetus leucocephalus''
begindocument
sectionTest
In my travels, I have seen many birds of type displaytiti
 In my travels, I have seen many birds of type displayeag

In my travels, I have seen many birds of type displayeag
displayedcommandsUsed Commands
enddocument

Another Option that occurs to me off the top of my head, depending on the diagnostic purposes of your question... it may be of interest to know not just which commands were used or not, but rather how often they were used. To this end, consider following:

documentclass[12pt]article
usepackageetoolbox
newcommanddisplayTracked
makeatletter

newcommandtrackedCommand[2]% This will define a tracked command.
 % Define the command as it was original intended.
 expandafternewcommandcsname#1endcsname#2 refstepcounter#1Count

 % Define a counter to track that specific command.
 newcounter#1Count
 setcounter#1Count0% Initialize it to zero as the command hasn't been used yet.

 % Update the display command to include the new command counter.
 csapptodisplayTrackedThe command #1 has been used arabic#1Count times \
 

makeatother
trackedCommandtitiHemixos castanonotus
trackedCommandeagHaliaeetus leucocephalus
trackedCommandanothercommandThis command won't be used at all, but still tracked.
begindocument
I my travels, I have seen many birds of type titi. What about titi or eag or just titi ?

displayTracked
enddocument

Someone better at LaTeX than I might be able to get the command trackedCommand to take in a csname rather than the text name (ie allow you to define a command identically to the newcommand syntax using titi rather than titi), but I couldn't get a nice solution working in the time frame I was willing to work on this. It's worth a note that the trackedCommand command will take in nonstandard command names (eg those with spaces, or arabic numerals); moreover you can decide if you want to track each command individually, rather than always tracking every command (in case there are commands you already know are heavily used or rarely used, and don't need to display the stats).

Moreover the displayTracked is what gives you the output, so simply commenting out (or not including) that command will allow you to only get tracking info when desired. You could also use the command and pipe it out to an aux file or latex warning or whatever else you might want it for.