Re: date--date accept DATESTR from stdin?

[Bernhard Voelker]

On 6/2/21 5:22 AM, Trey Blancher wrote:
> 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?

Yes, the -f, --file option is the way to tell date(1) to read from standard 
input:

https://www.gnu.org/software/coreutils/manual/html_node/Options-for-date.html#Options-for-date

  ‘-f datefile’
  ‘--file=datefile’
    Parse each line in datefile as with -d and display the resulting date and 
time.
    If datefile is ‘-’, use standard input.  [...]

While the feature also aims at processing many dates at a time, it is perfectly 
legit
to use it for one date string only to avoid a sub-shell (by possibly adding a 
pipe).

BTW: 'chage -l $USER' doesn't seem to reliably print a valid date string as 
value
for "Password expires".  At least, also "never" seems to be common.

  $ chage -l $USER | grep 'Password expires' | awk -F':' '{print $2}' | date -f 
- +%b-%d-%Y
  date: invalid date ' never'

Therefore, your script should cater for that as well.

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

I don't think so: the use of -d is fine if the date string is in a variable, and
the -f option is fine when that string is coming from standard input.
FWIW: the "herestring" you mentioned above is also regular data read from 
standard
input, as far as the invoked command is concerned.

Have a nice day,
Berny