how to convert from one date format to another format in linux ?

 how to convert from one date format to another format in linux ?

echo 04-11-2021 | { IFS=- read d m y && echo "$y$m$d"; }

20211104

echo 04-11-2021 | { IFS=- read d m y && echo "$y/$m/$d"; }

2021/11/04

 echo 04-11-2021 | { IFS=- read d m y && echo "$d$m$y"; }

04112021

 echo 04-11-2021 | awk -F- '{print $3$2$1}'

20211104

echo 04-11-2021 |gawk -F, '{split($1, a, "-"); print a[3] a[2] a[1]}' 

20211104

echo 04/11/2021 | { IFS=/ read d m y && echo "$y$m$d"; }

20211104

date -d "20211104" "+%Y-%m-%d"

2021-11-04

date -d 04/11/2021 +%F

2021-04-11

 %F   full date; same as %Y-%m-%d

How to Convert Epoch Seconds To the Current Time in linux?

Linux / UNIX: Convert Epoch Seconds To the Current Time

how you can obtain the UNIX epoch time (number of seconds since 1970-01-01

00:00:00 UTC) using the Linux bash "date" command. It also shows how you can

convert a UNIX epoch time to a human readable time.

Obtain UNIX epoch time using bash

Obtaining the UNIX epoch time using bash is easy. Use the build-in date command and

instruct it to output the number of seconds since 1970-01-01 00:00:00 UTC. You can do this

by passing a format string as parameter to the date command. The format string for UNIX

epoch time is '%s'.

Print Current UNIX Time

Type the following command to display the seconds since the epoch:

Syntax: date +%s

date +%s

1569569962

Convert Epoch To Current Time

Type the command:

date -d @Epoch

date -d @1569569962

date -d "1970-01-01 1569569962 sec GMT"

Sample outputs:

Fri Sep 27 13:09:22 IST 2019

To convert number of seconds back to a more readable form, use a command like this:

date -d @1569569962 +"%d-%m-%Y %T %z"

Using AWK

awk 'BEGIN { print strftime("%Y-%m-%d %H:%M:%S", 1569569962); }'

Output:

2019-09-27 13:09:22

Using Perl

perl -e 'print scalar(localtime(1569569962)), "\n"'

Output:

Fri Sep 27 13:09:22 2019

How to count days since a specific date until today using Bash shell ?

The easiest way to perform days count since a specifics day is to first get a number of seconds

since epoch time ( 1970-01-01 ) for both dates. As an example let’s count number of days

since 28.12.1999 until today 8.1.2018. Consider a following example:

$ echo $((($(date +%s)-$(date +%s --date "1999-12-28"))/(3600*24))) days
6586 days

Let's add little bit of readability to the above command by using variables. First, we get

seconds since epoch time ( 1970-01-01 ) until now:

$ now=$(date +%s)

$ echo $now
1515370378

Next we do the same for the 28.12.1999 date:

past=$(date +%s --date "1999-12-28")
$ echo $past
946299600

Next, calculate the difference:

$ difference=$(($now-$past))
$ echo $difference
569070778

Lastly, convert the difference in seconds to days:
$ echo $(($difference/(3600*24)))
6586

All done. The same principle can be used to calculate days between any specific days.

For example let's count days between 1.1.2016 and 31.12.2016 dates:

$ echo $((($(date +%s --date "2016-12-31")-$(date +%s --date "2016-1-1"))/(3600*24))) days
365 days