Can the date command take as argument a date string in a file given by another date command?

Question

I am reading through the GNU Core Utils manual and I am experimenting with the date command. Here is what I found in the text:

1. date command has a -f datefile option:

   • Parse each line in datefile as with -d and display the resulting date and time
   • that means date is looking at each line in datefile and treating it as a date string for use to display that particular date.

2. In chapter 28 (Date input formats) it is stated:

The output of the date command is not always acceptable as a date string, not only because of the language problem, but also because there is no standard meaning for time zone items like ‘IST’. When using date to generate a date string intended to be parsed later, specify a date format that is independent of language and that does not use time zone items other than ‘UTC’ and ‘Z’. Here are some ways to do this:

$ LC_ALL=C TZ=UTC0 date

Mon Mar 1 00:21:42 UTC 2004

• this statement (especially the bold part) implies to me that I should be able to put in the datefile a date command such as the one above and the date command I issue in the terminal should be able to parse that date command in the datefile and output the corresponding date.

For some reason it is not working for me. Either there is something wrong with my date command in the datefile (wrong quotation marks, some special syntax which I am not aware of....), or the -f datefile option is not capable of handling this situation.

What I am asking in summary is:

• can the date -f command be fed a file where the date string on one line is another date command? e.g. can I put in a file a string "date" on the first line (which as a command would return the system time), and then feed that file in a date -f command and get the current system time (generated by the date command in the file)?

Many thanks

Andrei

Answer 1

The correct format for a date-time that will be correctly parsed by GNU date is:

$ LC_ALL=C TZ=UTC0 date +"%Y-%m-%dT%H:%M:%S%z"
2016-09-28T04:08:55+0000

And that avoids spaces also, which may generate other problems.

If your file contains one line per date like:

$ cat dates.txt
1992-03-28T14:26:01+0000
2004-02-29T21:02:02+0000
2005-02-28T21:02:03+0000
2015-07-02T11:52:04+0000
2016-03-31T01:36:05+0000

Then this will work:

$ TZ=America/Los_Angeles date -f infile
Sat Mar 28 06:26:01 PST 1992
Sun Feb 29 13:02:02 PST 2004
Mon Feb 28 13:02:03 PST 2005
Thu Jul  2 04:52:04 PDT 2015
Wed Mar 30 18:36:05 PDT 2016

---

---

If what you want to do is create a file with "date commands" as in:

$ cat dates.txt
date -d "1992-03-28T14:26:01+0000"
date -d "2004-02-29T21:02:02+0000"
LC_ALL=C TZ=UTC0 date -d "2005-02-28T21:02:03+0000" +"%Y-%m-%d"
TZ=America/Los_Angeles date -d "2005-02-28T21:02:03+0000"
date -d "2015-07-02T11:52:04+0000"
date -d "2016-03-31T01:36:05+0000"

Then you need to execute each line with the shell.
One fast solution is to source the file and all commands will be excuted.

$ . ./dates.txt
Sat Mar 28 14:26:01 GMT 1992
Sun Feb 29 21:02:02 GMT 2004
2005-02-28
Mon Feb 28 13:02:03 PST 2005
Thu Jul  2 11:52:04 GMT 2015
Thu Mar 31 01:36:05 GMT 2016

Does this answer your question?

Answer 2

No, you can not read LC_ALL=C TZ=UTC0 from a file with date -f.

cat tz
LC_ALL=C TZ=UTC0

date -f tz
date: invalid date ”LC_ALL=C TZ=UTC0”

You can, however, put the entire command in a file, like this:

cat date
#!/bin/bash

LC_ALL=C TZ=UTC0 date

Then run it from the command line, which gives you:

./date 
Tue Sep 27 06:56:29 UTC 2016

As opposed to the timezone on my server which is:

date
tis 27 sep 2016 08:56:52 CEST

Answer 3

With date from coreutils 8.21, TZ=UTC and LC_ALL=C I get

$ date > x; sleep 1; date >> x; sleep 1; date >> x
$ date -f x
Wed Sep 28 03:32:39 UTC 2016
Wed Sep 28 03:32:40 UTC 2016
Wed Sep 28 03:32:41 UTC 2016
$

as expected. I checked the source code, too: The magic happens in the routine batch_convert where essentially there's a loop over parse_datetime()/show_date() just as in the command line case (using -d).