November 26, 2014

How to check MySQL Config files

Probably most of MySQL users sometime had a situation when they would do changes in MySQL config file without restarting server (may be matching SET GLOBAL command to do it in run time) but either because option is misspelled or because given version does not support such option the server when would refuse to start when it restarted, either on operating system restart or recovering from MySQL Server crash. In any case it is quite nasty.

Unlike Apache MySQL service control script does not have “configtest” option so there is no straight way to check MySQL config file for errors. However you can do it by running mysqld –help option:

This does not do complete check, ie it is possible to allocate 80GB to buffer pool instead of 8G you intended but at least it checks options for names which is good sanity check.

I would encourage you to run such check after you made changes to your config file for sanity purposes.

About Peter Zaitsev

Peter managed the High Performance Group within MySQL until 2006, when he founded Percona. Peter has a Master's Degree in Computer Science and is an expert in database kernels, computer hardware, and application scaling.


  1. walrus says:

    I believe the proper way is to submit a feature request on and ask them to add special options, say, –check_config to mysqld

  2. peter says:

    It would,

    Though I honestly do not expect little things as these to be given a lot of priority :)

  3. peterwang says:

    hi, what’s your mysql version?
    it doesn’t work for me.
    /usr/libexec/mysqld Ver 4.1.18 for pc-linux-gnu on i686 (Source distribution)

  4. peter says:

    Hm. I think it worked for MySQL 4.1
    I tested it with MySQL 5.0

  5. sv says:

    Yup. I think the problem might be with your configuration. I use Mysql version 5.0.
    and I get following message, I am not use of the warning message. Can you help

    090922 2:30:57 [Warning] option ‘max_join_size': unsigned value 18446744073709551615 adjusted to 4294967295
    090922 2:30:57 [Warning] option ‘max_join_size': unsigned value 18446744073709551615 adjusted to 4294967295

    /usr/libexec/mysqld Ver 5.0.67 for redhat-linux-gnu on i386 (Source distribution)
    Copyright (C) 2000 MySQL AB, by Monty and others
    This software comes with ABSOLUTELY NO WARRANTY. This is free software,
    and you are welcome to modify and redistribute it under the GPL license

    Starts the MySQL database server

    Usage: /usr/libexec/mysqld [OPTIONS]

    For more help options (several pages), use mysqld –verbose –help

  6. rajeev says:

    Copy the following code into “mysql_test.php” and click save. (Make sure to replace the MYSQL_PASS constant with the MySQL Password you specified during the MySQL installation).


    # Define MySQL Settings
    define("MYSQL_HOST", "localhost");
    define("MYSQL_USER", "root");
    define("MYSQL_PASS", "password");
    define("MYSQL_DB", "test");

    $conn = mysql_connect("".MYSQL_HOST."", "".MYSQL_USER."", "".MYSQL_PASS."") or die(mysql_error());
    mysql_select_db("".MYSQL_DB."",$conn) or die(mysql_error());

    $sql = "SELECT * FROM test";
    $res = mysql_query($sql);

    while ($field = mysql_fetch_array($res))
    $id = $field['id'];
    $name = $field['name'];

    echo 'ID: ' . $field['id'] . '';
    echo ‘Name: ‘ . $field[‘name’] . ”;

    Open up Internet Explorer and type in “http://localhost/mysql_test.php”. If the “mysql_test.php” page returns something similiar to:

    ID: 1
    Name: John

    after run on broser:

    Fatal error: Call to undefined function mysql_connect() in C:\Program Files\Apache Software Foundation\Apache2.2\htdocs\mysql_test.php on line 9

    why given error in line no-9
    plz help

  7. gorenje says:

    Once thing that happened to me was that the file was deleted even though nothing happened to the server. This was an annoyance since it caused nagios to go critical because the pid was missing.

    I assume that /etc/init.d/mysql would have had a fit had I tried to use it (i.e. server running but not pid file for the server).

    Version I was using: Ver 5.1.56 for pc-linux-gnu on x86_64

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