Episode: 2469
Title: HPR2469: A flight itinerary in Bash
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr2469/hpr2469.mp3
Transcribed: 2025-10-19 03:45:14

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This is HPR episode 2004 and 169 entitled, A Flight Itinerary in Bash and in part of the series
Bash Cripting.
It is hosted by Dave Morris and in about 18 minutes long and can in the next visit flag.
The summer is working out mates and time in a Bash Cript.
This episode of HPR is brought to you by An Honesthost.com.
Get 15% discount on all shared hosting with the offer code HPR15, that's HPR15.
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Hello everyone, this is Dave Morris for Hacker Public Radio.
I'm going to do another Bash related show today.
It's fairly brief.
It's probably a bit silly, but I thought since I've done the thing I'm going to talk
about, somebody might find it useful and interesting, so bear with me.
It's first of January today and my daughter flew out to New Zealand before Christmas Eve
she went.
She's gone there with her mum, my ex-wife, to hang out with her brother.
She's been there since around November with his girlfriend, they're on a bit of a travel
binge moment.
Anyway, I was interested in her flights and times and stuff and I saw her itinerary that
came in from the airline, but the times on those things are all local times, which is
what you want when you're flying places, you want to know what time your next plane's
going on whatever as you arrive in a place.
But I was interested in tracking where she was at on her journey using local time.
So I took this itinerary and turned it into a little bash script, which allowed me to
calculate times in my local time zone, which is GMT or UTC.
Both of my kids are fairly frequent travellers, I've got the wanderlust both of them I think.
And I like just to have an idea of where they are, not at the point of view of monitoring
them or anything, but just how they're progressing through their journeys.
Because I tend to worry otherwise, I don't know where they are and stuff.
So it's just a parental thing, maybe this parent, I don't know.
This was a fairly simple journey, two flights, one from Edinburgh to Doha in Qatar and then
another one out to Auckland, not a long way in between in Doha.
Of course the overall journey is incredibly long, it's hard to believe that people are
doing these such things on a regular basis, but that's just me showing my age probably.
It's simple in comparison to many into this flight, when she flew out to Indonesia in 2015
and I did an HBR show chatting to her when she came back by what she'd been up to
and stuff.
She was learning to dive and being an assistant with people who were doing research in remote
Ireland in the Indonesian archipelago.
Anyway, she took four flights and a boat trip and it took her over 38 hours.
There was a break along the way to get out there and I made a spreadsheet for that.
She's an okay way of doing this, but it was good to do, I found it gave me relief from
worrying.
This time I chose the different means of distracting my brain.
So let's talk about the script.
The basic algorithm was that I had the start and arrival times of each flight as well
as each flight duration and I also had the connection time, the time in the linking airport
waiting for the next plane.
I decided to use the date command to perform date and time calculations and I've known
that you can do some reasonably sophisticated things of date for a number of years but
I've not really, I always forget how to do it so I was keen to get back into remembering
learning how to do this properly and finding all the ins and outs of it and that's why
I'm doing this show of course because I'm sharing what I found with you.
The date command can take a date specification with an offset.
So when you just type date you get back the current date and time and time zone.
If you give it an offset, if you give it a base date which you do by giving it a hyphen
D option followed by date specification.
People who wrote this have actually gone to a lot of trouble to provide a piezo which
can handle all sorts of wonderful date formats.
So my example here is if you typed date space hyphen D space then in quotes doesn't matter
which quotes.
Today plus one week with spaces where you'd expect them to be.
Then you get back and I did this on the 30th of December 11th 13, I got back the answer
and I couldn't bother about the format, I just let it default it.
Sadly the 6th of January 11th 13 and 12 second as GMT 2018, that's what I got back.
So that was just added a week.
It has problems if you give it things like a month because what is the length of a month
because they're all well a lot of them are different.
Dates and times are so horrible to fiddle with.
But anyway if you give it a start date and time in the form that you might expect so
I had to start date of my cat in the background.
The start date of in ISO format 2017 hyphen 12 hyphen 24 space 144 coal on 55.
So that was the date that the first flight started and then I put plus 415 minutes which
actually the flight duration.
And I got back Sunday the 24th of December at 1041.
So the answer was before the time I was actually trying to calculate from adding 415 minutes
which is 6 hours 55 minutes to 1455 should result in 2150.
So something's broken here.
The problem I believe is caused by the date and time being separated by a space.
So the parser has only read the date not the time.
So they started midnight on the 24th and added the appropriate time to that.
Does that work?
They should have worked as a.
I don't know what I know if it was doing.
That's strange actually.
I never thought about that until just now.
I don't know where I got that date from.
But so my next attempt was to see what is the universal date format that you can use to
manipulate dates, add stuff to dates or indeed subtract stuff from dates.
And I tried another approach where the example I don't want to read this out because it's
complicated.
But basically there's after the hyphen D option there is a quoted string in it and in it
there is another command substitution with another date.
What this other in the inner date does is it takes the nicely formatted date the one
I was using before.
And it converts it to ISO 8601 format.
You have to put in there hyphen capital I followed immediately by the word minutes.
There are different formats that you can get and there's some that's got fractional
seconds and stuff which this one just gives you an hour and minutes format thing as well
as the date.
So I've given an example here of feeding in this two date and the answer I get back is
an ISO 8601 date which is the 2017-1224 and then a capital T 14 colon 55 plus 00 colon
00.
In other words it's the UTC time zone which is where I am at and it splits up the date
and the time and so forth.
I mean I could have put the T in between the two fields originally but I wanted to be
able to potentially at least learn how to convert from one date format to another and find
a consistent format that I could do stuff to.
So going back to the command within a command thing having computed that and adding 415 minutes
to it the answer I get back is the correct one Sunday the 24th December at 2150.
So that's fine that works that's how to do it.
I did actually I didn't put this in the notes discovered that you can say you can produce
epoch times out of date and you can use that as your sort of base and epoch times the number
of seconds since is it first of January 1970 I've forgotten what epoch date is but you
get a big integer number anyway and you can then feed that into date and produce a more
readable date.
I also discovered that instead of giving it plus 415 minutes which means you've got to
have converted the hours and minutes into minutes you could give it a plus 6 hours plus
55 minutes so you can do that you couldn't put in that date string plus 6 colon 5.5 that's
not accepted which is a shame there you go.
So that was some of the stuff that led up to what I was doing.
What I've done is I've included the script itself it's 52 lines including a big comment
so it's not particularly big it's got a function as well just for the convenience.
So talking about the function first of all there's a function called minutes whose purpose
was to take a formatted time in the format hours colon minutes and convert it into a number
of minutes.
I'm not going to talk much about it I've covered up most of these points before in other
shows just whizz through it briefly the key element of it is that it takes that time
in and it also has a second argument, a mandatory argument which is the name of variable
to put the minutes into it calls it ref but it uses a method called a name ref.
I've got another show pending which I'm in the process of writing I write my notes
first and then talk to the notes as you are probably well aware and it's talking more
about name ref and I plan to go into even more detail on this yeah the further show
I talk about functions but this is a piece of bash magic if you like which allows you
inner function to hand an answer back to the caller but instead of handing it back as
you would in more normal languages as Mike Ray would probably put it it can hand it back
as a variable.
So basically what the minutes thing does it splits the time up into two pieces there
are no checks approved to check that it really is this format so it's just a rough and
ready type of function then it takes the values it gets and multiply the hours by 60 and
adds in the minutes in a what's it called an arithmetic substitution expression that's
what it does so the main script works on the principle that it generates times and saves
them into variables and these times are then printed out in a nice format so I get a
little report of the different flight times and stuff so it's not not a very complex thing
but basically I start with a variable which I call DEP underscore eddy I've just used
the terminology that you find on flight itineraries DEP mean to part a double R meaning arrive
and then a three letter airport key EDI's Edinburgh DOH which will come to the minute is
doer Auckland is a KL and stuff like just seemed logical you might not agree.
Anyway DEP eddy sad silly when I say it is converted into an ISO 8601 I then compute
the flight time in minutes which is 6 hours 55 then I compute the arrival time in Doha as
the departure time plus the flight time again all in ISO 8601 format then I put a couple of
print state print F statements straight after that just to print them out rather than save them
all up to the end. Print F's I've not talked much about print F in bash thought Clackier was
going to do a show on it but he's a busy guy but anyway just in very very brief print F takes
format string and then it takes a list of arguments and the format string has got specifiers same
as we have talked a little bit about this in the org theory it takes things that says I want you
to treat each of the arguments in the following way the first one is percent hyphen 20s what that
means is take the first argument print it as a string inner field of 20 spaces and make sure
it's left justified because the default is to right justify it so the first print F the text
is leave Edinburgh at so it does that then it does another percent S field which is the date
on time the departure occurs and that argument consists of another command substitution which is
date hyphen D and then in quotes dollar depth edd so that's that ISO-860 one time then the format
specification for date is plus percent capital F space percent capital R space percent Z and
that what that does is generate a date to shorthand wave of specifying a sort of natural format date
ISO sort of format date which is to me very logical the y-y-y-y-m-m-dd with hyphen's in between
24 hour time and then followed by the time zone and of course it's the local time zone because
I'm interested in knowing what the times are in my time zone as a whole point so the rest of the
script is sort of the similar sort of thing I then compute the time in doha in minutes which
one hour 40 and then that's used in the next calculation compute the duration of the flight from
doha to Auckland which is 16 hours and 10 minutes whoa and then compute the arrival time in Auckland
print those out then I thought oh I'll see if I can print it out as a New Zealand time
which gets a little convoluted there are several ways of doing it but the one I found
was successful probably because I messed up the other one was to create a time command substitution
which starts with capital T capital Z equals and then the name of the time zone region
now various ways you can specify this I believe I think you can put in there plus 13 000 and possibly
others which specifies the the Pacific Auckland time zone but I use the actual name of it as
you'll see from that from the notes and then I computed that arrival date in New Zealand time
I also threw in a duration calculation because I knew the durations of each flight and the waiting
in between to work out how long the overall journey was so you can the these are arithmetic
substitutions and just add together the three things and then we print that print out a little
calculation of that duration divided by 60 that's the number of hours because it doesn't
integer division and then it modulo 60 which gives you the minutes so that's it really so the
output and it's always produced the same output forever and ever because it's just a one-off I think
I said in the notes it's just a throw away script more for my amusement and to keep me busy
should have done some mining I don't think but now anyway this this exercise my brain a bit more
it prints stuff out in a reasonably pleasant way and you can see the New Zealand time has got a
plus 13 time zone spec it was 440 in the morning there they arrived on the 26 they started
24 but there you go that's time zones for you and it took 24 hours and 45 minutes so I don't
I don't envy I don't envy them at all I'm not very good at long flights anymore anyway so in
conclusion writing this was was interesting and it get me distracted and I also watched the flight
progress said flight numbers on one of the the flight monitoring sites I use flight radar 24
I'm sure you will have done this I'm sure everybody's far more sophisticated in this area than I am
learned stuff about the date command and it was pretty good my kids come back in mid-January
and of course I've written the script for the return flight and the script is there for download if
you want it's obviously listed in the in the notes I called it EDI underscore AKL and I thought I
better explain why because that's the name of the two airports so make sense eh okay well that's
it I hope you found it interesting bye now
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