I don't understand completely but I think you are looking for saveStrings()

thank you for responding,

I send a message from a web page, the message is displayed on the  processing console ; I want to retrieve the message and put it automaticly on a txt file.

Then saveStrings() should work. It is worth noting that if your Processing sketch is on your website (as opposed to just receiving data from it) that you will have to sign your sketch for saveStrings() to work.

sorry but it does not work because to use saveStrings()i should have my string , but in my case the string is in the console or in the web page, i could not save it or put it as an argument in the saveStrings() function
My probleme is: how can recoveri this message in a variable or put it in a txt file!!! Please please please help me