if

caution

Nginx if implement is weird, see

• Internal Implementation
• If is Evil

if ($args = "alice") {
    # ...
}

• The left side must be variable $...

Operator	Description
	'' same as ""
	"" == false
	"0" == false
=	equal
!=	not equal
~	match RegExp (case-sensitive)
~*	match RegExp (case-insensitive)
!~	not match RegExp (case-sensitive)
!~*	not match RegExp (case-insensitive)

Flow

• Only ngx_http_rewrite_module directives (break, if, return, rewrite, ... ) execute as flow
• If if matched, Nginx change the context to the if block
• All support if context but not ngx_http_rewrite_module directives (add_header, proxy_pass, ...), Nginx does not execute that, Nginx will only set context block as that

Example

Only X-Second 2 header will be present in response

location / {
    # 1: if this location is matched, Nginx set the config block to `location /`

    set $true 1;

    if ($true) {
        # 2: if matched, Nginx set the config block to this `if` block

        # because add_header is not ngx_http_rewrite_module directive, Nginx does not execute add_header

        add_header X-First 1;
    }

    if ($true) {
        # 3: if matched, Nginx set the config block to this `if` block

        # Finally, Nginx use this `if` block as final config block, so the response header only has `X-Second 2`

        add_header X-Second 2;
    }

    return 204;
}

Operator and

caution

• Nginx change the context block when if matched (If is evil)
• To avoid if side effect, use map if possible

Method 1

Recommended

# if ($arg_a == "alice" && $arg_b == "bob")

http {
    map $arg_a $match_a {
        default 0;
        alice 1;
    }
    map $arg_b $match_b {
        default 0;
        bob 1;
    }
    map "$match_a$match_b" $match {
        default 0;
        11 1;
    }

    server {
        if ($match) {
            # ...
        }
        # ...
    }
}

Method 2

set $and 1;

if (<not condition>) {
    set $and 0;
}

if (<not condition>) {
    set $and 0;
}

if ($and) {
    # ...
}

Method 3

set $and "";

if (<condition>) {
    set $and "0";
}

if (<condition>) {
    set $and "${and}1";
}

if ($and = "01") {
    # ...
}