The date command is pretty hard to use...

So, I don’t know about you, but, when I use the date command it’s usually to match some arcane format. This means I am then scrolling to the bottom of the date man page to find the name of the man page where all the format options are listed. Every time is just like the 1st time, but, not in a good way! (It’s strftime by the way… good luck remembering it.)

It gets worse. Let’s say you need multiple date formats in a script, for example, because you don’t know exactly what date formats the sources you are matching against will use. The obvious thing to do would be to call the date command a few times in separate variables:

date_dd_mon_yyyy="$(/bin/date "+%d %b %Y")"
date_d_mon_yyyy="$(/bin/date "+%e %b %Y" | /usr/bin/sed 's/^[[:space:]]//')"
date_d_month_yyyy="$(/bin/date "+%e %B %Y" | /usr/bin/sed 's/^[[:space:]]//')"

Which yield the following (example) date strings:

01 Jun 2020
1 Jun 2020
1 June 2020

But, there is a problem crystallized in this nugget of wisdom from Bash Pitfalls.

So, what to do, what to do?

I think, perhaps, this:

eval "$(/bin/date +'century="%C" year_00_Mon="%g" year_00_Sun="%y" year_Mon="%G" year_Sun="%Y" month_01="%m" month_abrv="%b" month_full="%B" week_00_Mon="%W" week_00_Sun="%U" week_01_Mon="%V" weekday_0_Sun="%w" weekday_1_Mon="%u" day_001="%j" day_01="%d" day_1="%e" day_abrv="%a" day_full="%A" hour_00="%H" hour_01="%I" hour_0="%k" hour_1="%l" minute_00="%M" second_00="%S" second_epoch="%s" ampm="%p" timezone_alpha="%Z" timezone_utc="%z"')"

Using the Bash Pitfalls idiom for ensuring the date command is only called once, my version creates human-readable bash variables for all of the separate components that can make up a date string using the formats available in strftime. If I were being fancy I would maybe call it a “library” of possible options that you might want to include in a date string in your script. It’s modular! I have intentionally excluded the strftime options that create combined strings like %F, %D, %X.

You might be thinking we’ve traded one opaque solution for another, but, my goal here is to NEVER HAVE TO LOOK AT OR REMEMBER THE NAME OF THE strftime MAN PAGE AGAIN. Before judging too quickly, pull up the definitions, take a look at my variable names, then, see if they make sense to you…

Using the ampm variable will print out the current “meridiem” for the time.

The day_01 variable will print a 2-digit version of today’s date while day_1 will print a 1-digit version (one that doesn’t use a leading 0.)

The hour_00 variable prints the 24-hour 2-digit version of the hour, hour_01 prints the 12-hour 2-digit version, hour_0 prints a 24-hour 1-digit version, hour_1 the 12-hour 1-digit version.

The second_00 variable gives you a 2-digit version of the current time’s seconds while second_epoch gives you current time in seconds since the epoch (unix time).

And so on… if I wanted to print the ISO 8601 time stamp string in a log, for example, I could do this:

echo "${year_Sun}-${month_01}-${day_01}T${hour_00}:${minute_00}:${second_00}"

which yields:

2020-09-02T18:47:25

Is the eval command super easy to read? Hmm, no. Is it easier to read than the strftime man page? Yes! Is it copy / paste-able so you can just put it somewhere in every script you write that calls date? Yes! Can you change the variable names I created to make them more “human-readable” to you? Yes! We all win.

As always, I hope some of you find this useful.

Source: https://community.jamf.com/t5/jamf-pro/the-date-command-is-pretty-hard-to-use/m-p/234571

Comments Link to heading

Bartlomiej Sojka

“Great and very tidy solution indeed! Although usually ’eval is evil’, this one seems pretty much unexploitable. I’ve always avoided running plain date multiple times by using epoch as a source format for later conversion, i.e.: EPOCH=$(date +%s); date -jf "%s" ${EPOCH} "+%T" — This however doesn’t solve the strftime ‘pain’ problem, so kudos.”