unix yyyymmddhhmmss format conversion to specific date format - date

Question

There is a bash script running that outputs folder names appended with time logs_debug_20190213043348. I need to be able to extract the date into a readable format yyyy.mm.dd.hh.mm.ss and also may be convert to GMT timezone. I'm using the below method to extract.
echo "${folder##*_}" | awk '{ print substr($0,1,4)"."substr($0,5,2)"."substr($0,7,2)"."substr($0,9,6)}'
Is there a better way to print the output without writing complex shell scripts?

Answer 1

The internal string conversion functions are too limited, so we use sed and tr when needed.
## The "readable" format yyyy.mm.dd.hh.mm.ss isn’t understood by date.
## yyyy-mm-dd hh:mm:ss is. So we first produce the latter.
# Note how to extract the last 14 characters of ${folder} and that, since
# we know (or should have checked somewhere else) that they are all digits,
# we match them with a simple dot instead of the more precise but less
# readable [0-9] or [[:digit:]]
# -E selects regexp dialect where grouping is done with simple () with no
# backslashes.
d="$(sed -Ee's/(....)(..)(..)(..)(..)(..)/\1-\2-\3 \4:\5:\6/'<<<"${folder:(-14)}")"
# Print the UTF date (for Linux and other systems with GNU date)
date -u -d "$d"
# Convert to your preferred "readable" format
# echo "${d//[: -]/.}" would have the same effect, avoiding tr
tr ': -' '.'<<<"$d"
For systems with BSD date (notably MacOS), use
date -juf'%Y-%m-%d %H:%M:%S' "$d"
instead of the date command given above. Of course, in this case the simplest way would be:
# Convert to readable
d="$(sed -Ee's/(....)(..)(..)(..)(..)(..)/\1.\2.\3.\4.\5.\6/'<<<"${folder:(-14)}")"
# Convert to UTF
date -juf'%Y.%m.%d.%H.%M.%S' "$d"

Answer 2

Here's a pipeline that does what you want. It certainly isn't simple looking, but taking each component it can be understood:
echo "20190213043348" | \
sed -e 's/\([[:digit:]]\{4\}\)\([[:digit:]]\{2\}\)\([[:digit:]]\{2\}\)\([[:digit:]]\{2\}\)\([[:digit:]]\{2\}\)\([[:digit:]]\{2\}\)/\1-\2-\3 \4:\5:\6/' | \
xargs -d '\n' date -d | \
xargs -d '\n' date -u -d
The first line is simply printing the date string so sed can format it (so that it can easily be modified to fit the way you are passing in the string).
The second line with sed is converting the string from the format you give, to something like this, which can be parsed by date: 2019-02-13 04:33:48
Then, we pass the date to date using xargs, and it formats it with the timezone of the device running the script (CST in my case): Wed Feb 13 04:33:48 CST 2019
The final line converts the date string given by the first invocation of date to UTC time rather than being stuck in the local time: Wed Feb 13 10:33:48 UTC 2019
If you want it in a different format, you can modify the final invocation of date using the +FORMAT argument.