Azizi,

Sorry for so late a response, but I've been away from the site for a while.

"You're in the army now.
You're not behind a plow.
You'll never get rich
by digging a ditch
You're in the army now."

As has been stated by Joe_F, the fourth line is more often quoted as "You son-of-a-bitch" (presumably you heard the bowdlerised vesion).

While, as PoppaGator says, the song may have appeared in a Hollywood film of the WWII period, a version also appears amongst the 'dialog panels' of King Vidor's silent masterpiece "The Big Parade" (in the version I saw, the musical accompanyment actually made reference to the familiar tune to the above - whether this was a 'back reference' or not, I can't say).

Walrus.