Create timestamp variable in bash script

If you want to get unix timestamp, then you need to use:

timestamp=$(date +%s)

%T will give you just the time; same as %H:%M:%S (via http://www.cyberciti.biz/faq/linux-unix-formatting-dates-for-display/)

In order to get the current timestamp and not the time of when a fixed variable is defined, the trick is to use a function and not a variable:

#!/bin/bash

# Define a timestamp function
timestamp() {
  date +"%T" # current time
}

# do something...
timestamp # print timestamp
# do something else...
timestamp # print another timestamp
# continue...

If you don't like the format given by the %T specifier you can combine the other time conversion specifiers accepted by date. For GNU date, you can find the complete list of these specifiers in the official documentation here: https://www.gnu.org/software/coreutils/manual/html_node/Time-conversion-specifiers.html#Time-conversion-specifiers

DATE=`date "+%Y%m%d"`

DATE_WITH_TIME=`date "+%Y%m%d-%H%M%S"` #add %3N as we want millisecond too