date--date accept DATESTR from stdin?

[Trey Blancher]

I ran across something that didn't work for me, but I see no reason 
why it shouldn't.  I've tried this on a version of RHEL 7.6, as well as an 
up-to-date Arch Linux (coreutils 8.32).  I have a script/command pipeline that 
extracts the password expiration date from `chage`, and I tried to feed it into 
`date`:

chage -l ${USER} | grep 'Password expires' | awk -F':' '{print $2}' | date -d - 
+%b-%d-%Y

But this didn't work, it just printed today's date.  I guess this is because a 
bare dash/hyphen as a date specifier is ignored.  I was able to workaround 
the problem by loading the `chage` call into a subshell:

date -d "$(chage -l ${USER} | grep 'Password expires' | awk -F':' '{print 
$2}')" +%b-%d-%Y

Reading the man/info pages for `date`, it doesn't look like -d/--date accepts 
the date DATESTR from standard input.  It also doesn't look like a herestring 
will work, either.  Other coreutils programs do accept a single hyphen
('-') to mean read from standard input, so that's why I figured it would work.

As i was writing this, I saw the -f/--file option, which does allow the 
dash/hyphen to mean read from standard input.  Here's what just worked for me:

chage -l ${USER} | grep 'Password expires' | awk -F':' '{print $2}' | date -f - 
+%b-%d-%Y

Is that the answer?  Would it even make sense to read -d/--date from stdin, 
knowing that -f/--file can do it?

I could see accepting the format from standard input as well, but I don't have a 
use case for it right now.

Trey Blancher
trey@blancher.net