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

240

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

2,532

36

Note that add_header will work on successful requests only. 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
43

@JohnWHSmith : As marat noted in this answer. As of version 1.7.5, nginx added an "always" parameter 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
Doesn't work. I am doing proxy_pass. The header is NOT present in the response. – Arrow_Raider Mar 29 '21 at 16:07
answering @Avamander you are right! OP asked a simple question how to render custom logging... can be done using Nginx directive: set $append_log 'custom_field=$custom_data'; – bellasys Sep 19 '23 at 15:12
also one trick we do is to nest $append_log statements, using a default set of custom logging in the Server{} block level, example above, then we add any granular detail in the Location{} block level as follows: set $append_log '$append_log v_loc=$my_loc_name'; – bellasys Sep 19 '23 at 15:14

score 70 · Answer 2 · edited May 25 '22 at 05:54

70

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

answered Feb 21 '15 at 17:39

Thomas Decaux

1,327

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, this works without any additional headers. Thanks! – ᴍᴇʜᴏᴠ 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 24 · Answer 3 · answered Jul 04 '12 at 05:35

24

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

31,041

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

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 5 · 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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