How to output variable in nginx log for debugging

Question

I am testing nginx and want to output variables to the log files. How can I do that and which log file will it go (access or error).

yuvilio · Accepted Answer · 2017-03-08T16:34:08.247

218

You can send nginx variable values via headers. Handy for development.

add_header X-uri "$uri";

and you'll see in your browser's response headers:

X-uri:/index.php

I sometimes do this during local development.

It's also handy for telling you if a subsection is getting executed or not. Just sprinkle it inside your clauses to see if they're getting used.

location ~* ^.+.(jpg|jpeg|gif|css|png|js|ico|html|xml|txt)$ {
    add_header X-debug-message "A static file was served" always;
    ...
}

location ~ \.php$ {
    add_header X-debug-message "A php file was used" always;
    ...
}

So visiting a url like http://www.example.com/index.php will trigger the latter header while visiting http://www.example.com/img/my-ducky.png will trigger the former header.

edited Mar 08 '17 at 16:34

answered Mar 08 '14 at 19:58

yuvilio

2,312
2
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36

Note that `add_header` will work [on successful requests only](http://wiki.nginx.org/NginxHttpHeadersModule#add_header). Documentation states that it can only be applied to responses with codes 200, 204, 301, 302 or 304. Therefore, it can't be used to debug HTTP errors. – John WH Smith Aug 25 '14 at 14:43
42

@JohnWHSmith : As marat noted in [this answer](http://serverfault.com/questions/418709/nginx-add-header-for-a-50-page/647552#647552). As of [version 1.7.5](http://nginx.org/en/CHANGES), nginx added an ["always" parameter](http://nginx.org/en/docs/http/ngx_http_headers_module.html) to ```add_header``` which will return the header, no matter what the response code. So for example, ```add_header X-debug-message "A php file was used" always;```, should work even for 500 error code. – yuvilio Sep 22 '15 at 20:24
16

This doesn't answer the question at all. The guy wants to log to **log file** not to the client. – Avamander Apr 17 '18 at 11:31
omg.. so handy. thanks – Shaun Apr 08 '20 at 03:23
Doesn't work. I am doing proxy_pass. The header is NOT present in the response. – Arrow_Raider Mar 29 '21 at 16:07

score 64 · Answer 2 · edited May 25 '22 at 05:54

64

You can return a simple string as HTTP response:

location / {
    add_header Content-Type text/plain;
    return 200 $document_root;
}

Or use interpolation for watching multiple variables at the same time:

location / {
    add_header Content-Type text/plain;
    return 200 "document_root: $document_root, request_uri: $request_uri";
}

edited May 25 '22 at 05:54

Nick Roz

103
4

answered Feb 21 '15 at 17:39

Thomas Decaux

1,239
11
13

2

What if you want to return two variables values? – BringBackCommodore64 Mar 16 '18 at 16:41
Going to the location immediately opens the Save As dialog of my browser (tested on Opera, Chromium). No response whatsoever. – BringBackCommodore64 Mar 16 '18 at 17:27
@BringBackCommodore64 Adding a text/html Content-Type header may help. – CuriousGeorge May 09 '19 at 20:26
Using [Postman](https://getpostman.com), this works without any additional headers. Thanks! – mehov Oct 07 '19 at 08:10
2

I want to return multiple variables and solved it like this; `return 200 "xforwardedfor:$proxy_add_x_forwarded_for--remote_addr:$remote_addr--scheme:$scheme--host:$host"; ` – kursat sonmez Nov 27 '20 at 13:15

score 23 · Answer 3 · answered Jul 04 '12 at 05:35

23

You can set a custom access log format using the log_format directive which logs the variables you're interested in.

answered Jul 04 '12 at 05:35

mgorven

30,036
7
76
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2

thanks, and I guess there is no easier way to output one variable by itself? – lulalala Jul 04 '12 at 05:39
@lulalala Not that I know of. – mgorven Jul 04 '12 at 05:42
It is possible set the log level in the directive `error_log` to `debug` so you can see the value of the variables and that block that are execute. Example `error_log file.log debug` – Victor Aguilar Jan 10 '17 at 00:55
2

notice thar empty variables are shown as `-` in the log, but are really empty in the nginx code, you should not check for `-` at any time. This sometimes confuse users. – higuita May 05 '17 at 12:56

score 8 · Answer 4 · answered Nov 21 '13 at 22:30

8

Another option is to include the echo module when you build nginx, or install OpenResty which is nginx bundled with a bunch of extensions (like echo.)

Then you can simply sprinkle your configuration with statements like:

echo "args: $args"

answered Nov 21 '13 at 22:30

Mark Evans

189
1
2

2

When I try this it echos out to a plain text file on the server interrupting the output of the actual page. – JaredMcAteer May 07 '15 at 17:36
1

There is an `echo_log` directive in development. – Gajus Jul 14 '17 at 16:16
1

@Gajus Three years later now, and I googled for this, but can't find anything about `echo_log`. Do you know if it ever got out of development? – Randall Oct 30 '20 at 18:14

score 4 · Answer 5 · answered Mar 06 '21 at 15:32

none of these answer the question as asked (log)

However, @Victor Aguilar - has a comment on this answer, that should be an answer! It says how to log variables, and it works. Thanks!

https://serverfault.com/a/404628/400075

i.e. in /etc/nginx/nginx.conf

error_log /var/log/nginx/error.log debug;

results in the following type of logging:

2021/03/06 15:14:48 [debug] 2550#2550: *1 fastcgi param: "SCRIPT_FILENAME: /usr/lib/cgit/cgit.cgi/something.git/cgit.cgi"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script copy: "QUERY_STRING"
2021/03/06 15:14:48 [debug] 2550#2550: *1 fastcgi param: "QUERY_STRING: "
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script copy: "REQUEST_METHOD"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script var: "GET"
2021/03/06 15:14:48 [debug] 2550#2550: *1 fastcgi param: "REQUEST_METHOD: GET"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script copy: "CONTENT_TYPE"
2021/03/06 15:14:48 [debug] 2550#2550: *1 fastcgi param: "CONTENT_TYPE: "
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script copy: "CONTENT_LENGTH"
2021/03/06 15:14:48 [debug] 2550#2550: *1 fastcgi param: "CONTENT_LENGTH: "
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script copy: "SCRIPT_NAME"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script var: "/cgit/cgit.cgi/something.git/cgit.cgi"
2021/03/06 15:14:48 [debug] 2550#2550: *1 fastcgi param: "SCRIPT_NAME: /cgit/cgit.cgi/something.git/cgit.
cgi"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script copy: "REQUEST_URI"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script var: "/cgit/cgit.cgi/something.git/"
2021/03/06 15:14:48 [debug] 2550#2550: *1 fastcgi param: "REQUEST_URI: /cgit/cgit.cgi/something.git/"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script copy: "DOCUMENT_URI"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script var: "/cgit/cgit.cgi/something.git/"
2021/03/06 15:14:48 [debug] 2550#2550: *1 fastcgi param: "DOCUMENT_URI: /cgit/cgit.cgi/something.git/"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script copy: "DOCUMENT_ROOT"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script var: "/usr/lib"
2021/03/06 15:14:48 [debug] 2550#2550: *1 fastcgi param: "DOCUMENT_ROOT: /usr/lib"
2021/03/06 15:14:48 [debug] 2550#2550: *1 http script copy: "SERVER_PROTOCOL"
2021/03/06 15:14:48 [debug] 2550#2550: *....

not all nginx packages ar buit with --with-debug flag so debug directive is useless in error_log if nginx built with --with-debug (as I see most of nginxes are built without) — Ivan Borshchov, May 27 '21 at 11:02

How to output variable in nginx log for debugging

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