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lulalala
Asked: 2012-07-04 21:29:37 +0800 CST

How to output variable in nginx log for debugging

  • 772

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).

nginx

5 Answers

  1. Best Answer

    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.

    • 218

  2. 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";
}
    • 64

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

    • 23

  4. 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"
    • 8

  5. 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: *....
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