how to change date-format in a log file using bash, avoiding while loop

Question

This is not a new question here and here, but the details make it differ.

My input log file looks like:

TEMP MON -=- Sat Aug 15 02:20:24 EEST 2020 -=-          


        

Answer 1

With awk using only string functions, you can avoid calling the GNU awk datetime functions or the external command date, as we want to modify only the month and re-order the data.

> cat tst.awk
BEGIN { OFS=FS="-=-" }
{
    split($2, arr, " ")
    m=(index("JanFebMarAprMayJunJulAugSepOctNovDec", arr[2])+2)/3
    $2=sprintf(" %04d-%02d-%02d_%s %s ", arr[6], m, arr[3], arr[4], arr[5])
    print
}

Usage:

> awk -f tst.awk file
TEMP MON -=- 2020-08-15_02:20:24 EEST -=- 48.6
TEMP MON -=- 2020-08-15_02:20:50 EEST -=- 49.1
TEMP MON -=- 2020-08-15_02:21:13 EEST -=- 49.1
TEMP MON -=- 2020-08-15_02:21:44 EEST -=- 49.1
TEMP MON -=- 2020-08-15_02:21:45 EEST -=- 48.6
TEMP MON -=- 2020-08-15_02:21:52 EEST -=- 49.1
TEMP MON -=- 2020-08-15_02:21:53 EEST -=- 48.6
TEMP MON -=- 2020-08-15_02:21:54 EEST -=- 49.6
TEMP MON -=- 2020-08-15_02:21:56 EEST -=- 49.1
TEMP MON -=- 2020-08-15_02:21:57 EEST -=- 49.1