UNIX for Dummies Questions & Answers

Hi -

I'd like to know if it's possible to change the time on a Unix box temporarily, do some stuff, and then set it back to the correct time.

I'm testing some code that reads a time (in hours) from a config file, and does something at that time. I want to be able to run regression test scripts at any time.


I envisage doing something like this:
config file:
trigger.hour=7 // do something at 07:00

Invoke regression test script at any time, eg 14:57
- Test script uses date to set the time to 06:59:59
- The code gets invoked when the system time reaches 07:00:00
- Check the right thing happened (test = pass or fail)
- Set the time back to the correct current time (14:57 + ???)

The simplest thing would be if there's a command that goes and finds out what the time is that I can use to reset the clock after my test.
An alternative would be to save the current time, set it to the fake value, run the test, calculate how long the test lasted, and then set the time to (saved_time + elapsed_time) -- but I'm not sure how I would do all that in a shell script.

Any ideas welcome!
Thanks,
Sam

The date command will reset date/time if you run as root. Not always the best idea.
A safer approach is to set the date/time the process and it's children see.

An example in the MST timezone.

Code:

export TZ=MST6.5MDT
date
export TZ=MST7MDT
date

By diddling with the n in XXXn.nXXX of a TZ setting it changes the effective time.
If your app uses any of the time.h functions, like localtime, this changes date date the application sees. The MST part can be any letters e.g., PSQ5PDQ. Just be sure to reset it after testing.

Or else :
http://www.solution-soft.com/datasheets/timemachine.pdf

Changing TZ, I only seem to be able to shift the time by whole hours (if I use a value with a decimal point the time seems to resort to UTC). Is this what you'd expect here?

e.g.:

Code:

~> date
Fri Sep 12 14:30:26 BST 2008
~> export TZ=AAA3BBB
~> date
Fri Sep 12 11:30:39 BBB 2008 // 3hours behind UTC as expected
~> export TZ=AAA3.2BBB
~> date
Fri Sep 12 13:30:45  2008 // doesn't seem to like '3.2' so it's reverted to UTC

I do need to be able to set the offset more preciesly (to the second if possible).

Is there a way around this? I can see that this way is going to be a lot nicer in terms of not messing with the actual system time, and being easier to reset afterwards.

Thanks,
Sam

What OS? TZ settings are defined to have to allow one decimal, XXX1.5YYY
Each OS also has a tztab file or whatever that you can manipulate. Don't manipulate the real one.... we'll have to dummy it so you don't screw up the whole system.

Got it - the TZ variable does work, but I was using the wrong syntax - my system seems to prefer a: b :c time to x.y.

The command shown sets my user's clock 2hr, 2min and 2sec ahead of UTC (winter), 1hr, 2min, 2sec ahead of UTC (summer), and is being run in summer.

Code:

 ~> date;export TZ=AAA-02:02:02BBB-1:2:2;date;export TZ=;date
Mon Sep 15 09:04:02 UTC 2008
Mon Sep 15 10:06:04 BBB 2008
Mon Sep 15 09:04:02 UTC 2008
 ~> uname -a
Linux 2.6.18-53.el5

As you can see above, I'm running Linux.

The format for TZ on my system is:
(3 letter identifier)(offset from UTC h:m:s)[(summer time identifier)(summer offset from UTC h:m:s)]
h:m:s can be shortened to just hours, hours:minutes or hours:min:sec, with zero-padding optional. There's more stuff available too, eg to change when summertime starts. Do

Code:

man tzset

Thanks for all the help from everyone