logfile writing utility in bash (logger does not create files)?

Question

I often write logfiles in /tmp or /var when making large or complicated bash scripts to aid in debugging or simply to track program flow. I usually do something akin to echo "$(date '+%Y-%m-%d_%H:%M:%S') something happened" > /tmp/log which is fine, but not something that can be readily ingested into programs expecting a "standard" syslog format (Apache jokes aside). I've tried getting the logger command to create a new logfile somewhere but there doesn't seem to be an option for it. I suspect it leaves that part up to syslogd and it's facility config.

I could modify my "standard" format above to include hostname, a bunk facility and PID, but there are probably other nuances and formats I might be overlooking that someone has already figured out.

Is there a utility similar to logger which can write an RFCxxx formatted logfile without going through syslog?

Answer 1

I think you could just do that by hand...

The fun part is the various formats. Here's functions for Bash to produce both RFC 5424 and RFC 3164 style messages. The %06N and %:z formats in the timestamp of the first one may require GNU date, I'm not sure.

#!/bin/bash

# print RFC 5424 syslog message, without structured data,
# somewhat like 'logger -i --rfc5424=notq', e.g.
# <13>1 2021-10-28T14:48:10.613772+03:00 myhost ilkkachu 20415 - - a problem happened
# format is
# <prio>version date host tag pid - - message
lognew() {
        local facility="${log_facility-5}"  # default 'user'
        local severity="$1"
        shift 
        local prio=$(( facility*8 + severity ))
        local version=1
        local date=$(date +"%Y-%m-%dT%H:%M:%S.%06N%:z")
        local tag="${log_tag-$USER}"        # default to username 
        local pid="$$"
        local IFS=" "
        local message="$*"
        printf "<%d>%d %s %s %s %d - - %s\n" "$prio" "$version" "$date" "$HOSTNAME" "$tag" "$pid" "$message"
}

# print RFC 3164 syslog message,
# somewhat like 'logger -i --rfc3164'
# <13>Oct 28 14:49:13 myhost ilkkachu[20418]: another problem happened
# format:
# <prio>date host tag[pid]: message
logold() {
        local facility="${log_facility-5}"  # default 'user'
        local severity="$1"
        shift
        local prio=$(( facility*8 + severity ))
        local date=$(LC_ALL=C date +"%b %_d %H:%M:%S")
        local tag="${log_tag-$USER}"        # default to username 
        local pid="$$"
        local IFS=" "
        local message="$*"
        printf "<%d>%s %s %s[%d]: %s\n" "$prio" "$date" "$HOSTNAME" "$tag" "$pid" "$message"
}

log_facility=1 # user
log_tag="test"

# 6 = info
lognew 6 testing messaging
# 4 = warning
logold 4 obsolete message format

The PID printed is that of the script, and the output is something like this:

<14>1 2021-10-28T15:06:17.413125+03:00 myhost test 20499 - - testing messaging
<12>Oct 28 15:06:17 myhost test[20499]: obsolete message format