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

Question

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:

Try using supervisor-stdout in your supervisor.conf: https://github.com/coderanger/supervisor-stdout — Gustavo Kawamoto, Jan 26 '17 at 01:06
@GustavoKawamoto http://meta.stackexchange.com/questions/8231/are-answers-that-just-contain-links-elsewhere-really-good-answers — chicks, Feb 02 '17 at 14:05

score 7 · Answer 1 · answered Dec 21 '18 at 13:33

@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 "$@"

Why setup additional FDs? wouldn't you get exactly the same results with just `exec 1> >( perl -ne '$| = 1; print "'"${PREFIX}"' | $_"' >&1)` ? — istepaniuk, Jan 17 '23 at 19:51

score 4 · Answer 2 · answered Feb 22 '18 at 09:47

I was having the same problem recently, and could not figure out supervisor-stdout. Here is what I did:

In bash you can use I/O redirection and Process Substitution to pipe stdout, stderr through another process.

    #!/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
    echo "I will be prefixed"
    # don't forget to use exec

Now you can setup supervisor to use stdout and stderr for logging:

    stdout_logfile=/dev/stdout
    stdout_logfile_maxbytes=0
    stderr_logfile=/dev/stderr
    stderr_logfile_maxbytes=0

and as command use a bash script that setups the I/O redirection.

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

2 Answers2