Enable slow query / general log to output to stdout

[tsposato]

Currently, the system logs output in a way where you can use 'docker logs' to view them.
This is beneficial for a variety of reasons as you don't need to specifically access the containers filesystem to view them.
Is there a way to also enable the slow query or general query log to output in the same way?

[tianon]

I haven't tried, but I bet you could do something with /dev/stdout + https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_slow_query_log_file 👀

(ie, --slow-query-log-file=/dev/stdout or --slow-query-log-file=/dev/stderr)

[tsposato]

I haven't tried, but I bet you could do something with /dev/stdout + https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_slow_query_log_file 👀

(ie, --slow-query-log-file=/dev/stdout or --slow-query-log-file=/dev/stderr)

I'll try this and see how I go. Will report back my findings.

[dotneet]

--slow-query-log-file=/dev/stdout doesn't work on my environment.

2022-08-10T06:39:23.313012Z 0 [ERROR] Could not use /dev/stdout for logging (error 2 - No such file or directory). Turning logging off for the server process. To turn it on again: fix the cause, then either restart the query logging by using "SET GLOBAL SLOW_QUERY_LOG=ON" or restart the MySQL server.

[tianon]

That's weird - it works OK here: 😬

$ docker run -it --rm --pull=always -e MYSQL_RANDOM_ROOT_PASSWORD=1 mysql:8 --slow-query-log-file=/dev/stdout
8: Pulling from library/mysql
32c1bf40aba1: Pull complete 
3ac22f3a638d: Pull complete 
b1e7273ed05e: Pull complete 
20be45a0c6ab: Pull complete 
410a229693ff: Pull complete 
1ce71e3a9b88: Pull complete 
c93c823af05b: Pull complete 
c6752c4d09c7: Pull complete 
d7f2cfe3efcb: Pull complete 
916f32cb0394: Pull complete 
0d62a5f9a14f: Pull complete 
Digest: sha256:ce2ae3bd3e9f001435c4671cf073d1d5ae55d138b16927268474fc54ba09ed79
Status: Downloaded newer image for mysql:8
2022-08-11 00:05:27+00:00 [Note] [Entrypoint]: Entrypoint script for MySQL Server 8.0.30-1.el8 started.
2022-08-11 00:05:27+00:00 [Note] [Entrypoint]: Switching to dedicated user 'mysql'
2022-08-11 00:05:27+00:00 [Note] [Entrypoint]: Entrypoint script for MySQL Server 8.0.30-1.el8 started.
2022-08-11 00:05:27+00:00 [Note] [Entrypoint]: Initializing database files
2022-08-11T00:05:27.410861Z 0 [Warning] [MY-011068] [Server] The syntax '--skip-host-cache' is deprecated and will be removed in a future release. Please use SET GLOBAL host_cache_size=0 instead.
2022-08-11T00:05:27.410932Z 0 [System] [MY-013169] [Server] /usr/sbin/mysqld (mysqld 8.0.30) initializing of server in progress as process 42
2022-08-11T00:05:27.416907Z 1 [System] [MY-013576] [InnoDB] InnoDB initialization has started.
2022-08-11T00:05:27.951388Z 1 [System] [MY-013577] [InnoDB] InnoDB initialization has ended.
2022-08-11T00:05:29.450069Z 6 [Warning] [MY-010453] [Server] root@localhost is created with an empty password ! Please consider switching off the --initialize-insecure option.
2022-08-11 00:05:32+00:00 [Note] [Entrypoint]: Database files initialized
2022-08-11 00:05:32+00:00 [Note] [Entrypoint]: Starting temporary server
mysqld will log errors to /var/lib/mysql/c82b49dded53.err
mysqld is running as pid 95
2022-08-11 00:05:33+00:00 [Note] [Entrypoint]: Temporary server started.
'/var/lib/mysql/mysql.sock' -> '/var/run/mysqld/mysqld.sock'
Warning: Unable to load '/usr/share/zoneinfo/iso3166.tab' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/leapseconds' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/tzdata.zi' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/zone.tab' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/zone1970.tab' as time zone. Skipping it.
2022-08-11 00:05:34+00:00 [Note] [Entrypoint]: GENERATED ROOT PASSWORD: 7yU3dQrzXNgwZjWEvBkew2HsC72fd9nA

2022-08-11 00:05:34+00:00 [Note] [Entrypoint]: Stopping temporary server
2022-08-11 00:05:36+00:00 [Note] [Entrypoint]: Temporary server stopped

2022-08-11 00:05:36+00:00 [Note] [Entrypoint]: MySQL init process done. Ready for start up.

2022-08-11T00:05:36.661573Z 0 [Warning] [MY-011068] [Server] The syntax '--skip-host-cache' is deprecated and will be removed in a future release. Please use SET GLOBAL host_cache_size=0 instead.
2022-08-11T00:05:36.662487Z 0 [System] [MY-010116] [Server] /usr/sbin/mysqld (mysqld 8.0.30) starting as process 1
2022-08-11T00:05:36.667203Z 1 [System] [MY-013576] [InnoDB] InnoDB initialization has started.
2022-08-11T00:05:36.757377Z 1 [System] [MY-013577] [InnoDB] InnoDB initialization has ended.
2022-08-11T00:05:36.937099Z 0 [Warning] [MY-010068] [Server] CA certificate ca.pem is self signed.
2022-08-11T00:05:36.937125Z 0 [System] [MY-013602] [Server] Channel mysql_main configured to support TLS. Encrypted connections are now supported for this channel.
2022-08-11T00:05:36.938629Z 0 [Warning] [MY-011810] [Server] Insecure configuration for --pid-file: Location '/var/run/mysqld' in the path is accessible to all OS users. Consider choosing a different directory.
2022-08-11T00:05:36.953614Z 0 [System] [MY-011323] [Server] X Plugin ready for connections. Bind-address: '::' port: 33060, socket: /var/run/mysqld/mysqlx.sock
2022-08-11T00:05:36.953628Z 0 [System] [MY-010931] [Server] /usr/sbin/mysqld: ready for connections. Version: '8.0.30'  socket: '/var/run/mysqld/mysqld.sock'  port: 3306  MySQL Community Server - GPL.

[tianon]

For further help, I'd suggest a dedicated support forum, such as the Docker Community Forums, the Docker Community Slack, or Stack Overflow.

[fnwbr]

@tianon You forgot to pass --general=log=1; please re-open this issue.
Verified this issue both on 8.0 but also latest 8.4.4:

docker run -it --rm --pull=always -e MYSQL_RANDOM_ROOT_PASSWORD=1 mysql:8 --general-log=1 --general-log-file=/dev/stdout

2025-03-03T17:41:36.688030Z 0 [ERROR] [MY-011263] [Server] Could not use /dev/stdout for logging (error 2 - No such file or directory). Turning logging off for the server process. To turn it on again: fix the cause, then either restart the query logging by using "SET GLOBAL GENERAL_LOG=ON" or restart the MySQL server.

EDIT: To clarify, this issue exists for both, general_log but also slow_query_log.

[yosifkit]

Look like stdout or stderr are not supported (https://stackoverflow.com/a/32939348); only files, tables, or none are supported (https://dev.mysql.com/doc/refman/8.4/en/log-destinations.html). So, there is nothing we can document or add to the image to change this. If you want this functionality, submit a feature request with upstream MySQL (https://bugs.mysql.com/, https://github.com/mysql/mysql-server).