Greeting Your Neighbor

[Croat]

I read somewhere that it is a sin against 5th commandment not to greet your neighbor. Today I was shopping with my mother and one of my friends saw me and i did not greet him because I was ashamed(I do not hate him).

Is this matter grave enough?
Did I commit a mortal sin in these or any other similar situations?

[cappie]

First, we should not harm anyone, either by hand or deed. Next, we should not use our tongue to advocate or advise harming anyone. Furthermore, we should neither use nor sanction any means or methods whereby anyone may be mistreated. Finally, our heart should harbor no hostility or malice against anyone in a spirit of anger and hatred. Thus you should be blameless in body and soul toward all people, but especially toward anyone who wishes or does you evil. For to do evil to someone who desires good for you and does you good is not human but evil.

In the second place, this commandment is violated not only when we do evil, but also when we have the opportunity to do good to our neighbors and to prevent, protect, and save them from suffering bodily harm or injury, but fail to do so.

I don't think you sinned mortally as failing to greet someone would if anything be a lesser offense against the commandment.