set $cookie_abc "$cookie_abc";

[amodpandey]

Thank you for your response.

Versions

nginx versions tried nginx/1.2.5 and nginx/1.3.9
LuaJIT 2.0.0

What I want achive?

Set the value of $cookie_abc to "a"/"b" (some logic) if the cookie value is
not coming in the request else use the value set. I am doing this in 

server level

set $cookie_abc "$cookie_abc";
set $tmp_abc "";
set_by_lua $tmp_abc '
    common.set_abc_cookie()
';

I am using set_by_lua to make sure the cookie value is set before the
rewrites are evaluated.

Why I am doing this?

I have used $cookie_abc variable in my config and I want to have "a"/"b"
value depending on a logic if the cookie is not passed.

What is not working?

inside  common.set_abc_cookie()

ngx.var.cookie_abc = "a" if the cookie is not passed. 

This is expected. That is why I am doing 
set $cookie_abc "$cookie_abc";

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,236075,236122#msg-236122