Format date with linux

Write at 2017 Apr 23 in notes linux date day bash linux-daily

Time is part of DevOps life. We use timestampt everywhere, in backup, in cronjob, compare different between two times. However, working with date/time in bash maybe a bit cumbersome.

On Bash, lots of time, we have to rely on an external utility and use the output to feed into another variable. That’s Linux way.

The command we are looking at is date. It’s extremely powerful.

The most basic use is just call it:

$ date
Sun Apr 23 08:27:40 UTC 2017

This is useless most of time. We can format it by passing the format string as firt argument.

Format is to epoch

$ date +%s

Format it as we always do with our language:

$ date +%Y-%m-%d
$ date +%D
$ date "+%Y-%m-%D %H:%M:%S"
2017-04-04/23/17 08:30:08

What happen if we want to get yesterday time, tomorrow, 5 hours ago, as we always do in Rails


Or when we want to format a specificed date, not current day time. We can pass an --date parameter:

$ date "+%Y-%m-%D %H:%M:%S" --date=now
2017-04-04/23/17 08:31:51

Let’s do time travel:

$ date "+%Y-%m-%D %H:%M:%S" --date=tomorrow
2017-04-04/24/17 08:32:02

$ date "+%Y-%m-%D %H:%M:%S" --date=yesterday
2017-04-04/22/17 08:32:06

Back to our Rails code, we love ago, from now etc. We can do that with date as well. By default, it advance time into future, unless we specifiy ago. Such as

$ date "+%Y-%m-%D %H:%M:%S" --date='5 minutes ago'
2017-04-04/23/17 08:29:48
$ date "+%Y-%m-%D %H:%M:%S" --date='1 hours ago'
2017-04-04/23/17 07:35:14
$ date "+%Y-%m-%D %H:%M:%S" --date='1 hours'
2017-04-04/23/17 09:35:25

It’s suprisingly very powerful and can parse almost anything

$ date "+%Y-%m-%D %H:%M:%S" --date='next monday'
2017-04-04/24/17 00:00:00
$ date "+%Y-%m-%D %H:%M:%S" --date='next monday 5:00'
2017-04-04/24/17 05:00:00
$ date "+%Y-%m-%D %H:%M:%S" --date='next monday 5:00pm'
2017-04-04/24/17 17:00:00

And finally, don’t forgot to

man date

to learn more about what it can do.