Docker – How to add a prefix / label for services to supervisor stdout output

dockerloggingsupervisord

I'm using supervisor in docker images when I have to run multiple services in one image e.g. postfix and other mail services.
When I redirect the stdout/stderr from all services to supervisor and supervisor does also log to stdout/stderr I would prefer to have a prefix/label infront of the actual log output on the console to know which log is coming from which service. I can't find any config setting for this but maybe you know a way.

Here is an example how it looks like with Foreman:
Foreman log output

Best Answer

@flocki Thanks for your great answer! Below a slightly more complete example:

[program:nginx]
command=/usr/local/bin/prefix-log /usr/sbin/nginx -g "daemon off; error_log /dev/stderr info;"
autostart=true
autorestart=true
priority=10
stdout_events_enabled=true
stderr_events_enabled=true
stdout_logfile=/dev/stdout
stdout_logfile_maxbytes=0
stderr_logfile=/dev/stderr
stderr_logfile_maxbytes=0
stopsignal=QUIT

prefix-log:

#!/usr/bin/env bash
# setup fd-3 to point to the original stdout
exec 3>&1
# setup fd-4 to point to the original stderr
exec 4>&2

# get the prefix from SUPERVISOR_PROCESS_NAME environement variable
printf -v PREFIX "%-10.10s" ${SUPERVISOR_PROCESS_NAME}

# reassign stdout and stderr to a preprocessed and redirected to the original stdout/stderr (3 and 4) we have create eralier
exec 1> >( perl -ne '$| = 1; print "'"${PREFIX}"' | $_"' >&3)
exec 2> >( perl -ne '$| = 1; print "'"${PREFIX}"' | $_"' >&4)

# from here on everthing that outputs to stdout/stderr will be go through the perl script

exec "$@"
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