Logging to /dev/stdout and to file at the same time.

[markmadlangbayan]

Hello,

Is it possible to log to /dev/stdout and the same time to a file in /var/log directory? I tried to redirect all longs to /dev/stdout, and I can actually see the logs as output in docker, but not in a file. Then I read the documentation, and used the logutil-service by creating a file called log/run. Now it redirected all outputs from screen to the file in /var/log/*/current file.

I'd like to set it up where I see the outputs on screen, and also in file.

Thanks,
Mark

[jprjr]

I don't think there's an easy way to do it using logutil-service (it only reads in one argument), but if you use s6-log directly you can do it.

Example Dockerfile

FROM ubuntu:16.04
ADD https://github.com/just-containers/s6-overlay/releases/download/v1.21.8.0/s6-overlay-amd64.tar.gz /tmp/
RUN tar xzf /tmp/s6-overlay-amd64.tar.gz -C /

COPY root /

ENTRYPOINT ["/init"]

Contents of root folder

jprjr test $ tree root
root
└── etc
    ├── cont-init.d
    │   └── setup-someservice-logdirs
    └── services.d
        └── some-service
            ├── log
            │   └── run
            └── run

5 directories, 3 files

Contents of root/etc/cont-init.d/setup-someservice-logdirs

#!/usr/bin/env bash
set -e

mkdir -p /var/log/some-service
chown nobody:nogroup /var/log/some-service

Contents of root/etc/services.d/some-service/run

#!/usr/bin/env bash

while true ; do
  sleep 1
  echo "Some text"
done

Contents of root/etc/services.d/some-service/log/run

#!/usr/bin/execlineb

s6-envuidgid nobody
s6-applyuidgid -U
s6-log -bp T 1 n20 s1000000 T /var/log/some-service

The log will be written to both /var/log/some-service as well as standard output.

The key is that s6-log reads in a whole "logging script" on the command line, and you can have multiple actions - here's the link to the manual.

Here's the breakdown of what I'm doing in my s6-log command:

• s6-log -bp - call s6-log in blocking, protected mode (same as logutil-service does)
• T 1 -- this is my first action directive
  • take all lines of input (because I don't have any line selection directives)
  • prepend with an ISO 8601 timestamp (done with T)
  • forward to stdout (done with the 1)
• n20 s1000000 T /var/log/some-service -- this is second first action directive, and it's the default mode for logutil-service
  • take all lines of input (again, no selection directives)
  • keep up to 20 files (n20)
  • each file is max 1000000 bytes (s1000000)
  • prepend with an ISO 8601 timestamp (T)
  • save to logdir /var/log/some-service

[markmadlangbayan]

Thank you for the detailed response, it worked!

[felipecrs]

Any plans to add an option, or environment variable, to allow logutil-service to abstract this configuration?

[skarnet]

S6_LOGGING_SCRIPT?

[felipecrs]

Got it. Now I understand, to retain the behavior described above, I suppose I would have to set:

S6_LOGGING_SCRIPT="T 1 n20 s1000000 T"

Is this correct?

[skarnet]

Something like that, yes. :-)

[felipecrs]

Thanks a lot!