use one date variable but format it different ways

Question

In a shell script, there are the following variables:
datestamp=$(date '+%Y-%m-%d_T%H-%M-%S')
datestamp_pretty=$(date '+%m/%d/%Y at %I:%M:%S %p')

The first one is used as part of the output filename while the second one is used to display a nicely readable date and time in the contents of the file. Since these are created separately, the seconds can be off slightly. Is there a way to create a single date variable and then format it two different ways? If so, how is this done?

https://unix.stackexchange.com/questions/107290/extract-date-from-a-variable-in-a-different-format — Kate, Dec 03 '19 at 22:04
That's not a duplicate. Here, a better approach would be something like eval "$(date +"datestamp='%Y-%m-%d_T%H-%M-%S' datestamp_pretty='%m/%d/%Y at %I:%M:%S %p'")" and avoid relying on that -d GNU extension. — Stéphane Chazelas, Dec 10 '19 at 09:37
Note that that datestamp_pretty format of yours is US-specific and would generally not work properly outside a US or C/POSIX locale. Using %c is probably better for a human-readable date adapted to the user's locale. — Stéphane Chazelas, Dec 10 '19 at 09:43
@StéphaneChazelas How would %c be used in datestamp_pretty to replace the code that's there but yield the same output? — knot22, Dec 11 '19 at 02:42

score 2 · Accepted Answer · edited Dec 10 '19 at 09:30

2

This is pretty easy if you are using GNU date.

First, get the most generic time format from date. I chose to get the epoch time:

epoch=$(date +%s)

Then, convert the epoch into whatever format you want however many times you want:

datestamp=$(date -d @$epoch '+%Y-%m-%d_T%H-%M-%S')
datestamp_pretty=$(date -d @$epoch '+%m/%d/%Y at %I:%M:%S %p')

It should always represent the time when you first captured $epoch.

If you do not have GNU date, you would need some other method of converting the epoch to a human date.

edited Dec 10 '19 at 09:30

Toby Speight

8,678

answered Dec 03 '19 at 22:28

GracefulRestart

4,901

In @$epoch, what does the @ do? – knot22 Dec 03 '19 at 23:06
According to this coreutils example page, it is shorthand to represent date -d "1970-01-01 UTC $epoch seconds" – GracefulRestart Dec 03 '19 at 23:48
1

The "@" should make you read man date. – waltinator Dec 04 '19 at 13:43

score 1 · Answer 2 · answered Dec 17 '20 at 22:32

Here’s a way to do it that doesn't require GNU date and doesn't require you to run date more than once.

Pick a string that will never appear in any of the date/time strings. It can be a single character — for example, ~, ^ and | would all work. But I’ll demonstrate with /foo/.

Simply concatenate your date string formats with your chosen string between them. (Remove the + from the second format.) Capture the output from date into a variable, and then split it at your delimiter string:

combined=$(date '+%Y-%m-%d_T%H-%M-%S/foo/%m/%d/%Y at %I:%M:%S %p')
datestamp="${combined%/foo/*}"
datestamp_pretty="${combined#*/foo/}"

use one date variable but format it different ways

2 Answers2