hpr-knowledge-base/hpr_transcripts/hpr2709.txt

Episode: 2709
Title: HPR2709: Bash Tips - 16
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr2709/hpr2709.mp3
Transcribed: 2025-10-19 07:56:14

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This is HPR episode 2007-109 entitled Bashtip, 16, and in part of the series Bash Crypting.
It is hosted by Dave Morris and is about 24 minutes long and carries an explicit flag.
The summary is Arain in Bash Part 1.
This episode of HPR is brought to you by an honesthost.com.
Get 15% discount on all shared hosting with the offer code HPR-15, that's HPR-15.
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Hello everybody. Welcome to Hack Public Radio. This is Dave Morris and I'm talking about
Bash again, Bashtip's number 16.
I've dropped using the titles with interesting words from the Thessauras.
As I mentioned on the community news, so yeah 16, so what I'm doing now is talking about
a raise in Bash. I'm going to do a bunch of shows on the subject to try not to make each
one too long. So we've encountered Bash arrays at various points throughout this sub-series
I've referred to them and given examples using them but never really explained them.
So now's the time to get into some explanations.
First thing is that there are two types of arrays and they're called indexed and associative.
They're both one-dimensional so other languages have multi-dimensional arrays but not in Bash.
Indexed arrays are indexed by positive integers starting at zero but the indices don't have
to be sequential so you can have gaps. That's called being sparse.
Active arrays are indexed by strings and if you following this series on Ork then we've
talked about the equivalent in that language. Both rate types have no limit on the length
and the actual values you store in there are strings. It might be numbers.
Well actually it's not quite as simple as that but I'm not getting to that particular point
for a while, not in this episode anyway. So there are several ways of creating and
raising Bash and the methods of doing it very slightly between the two types.
So let's first look at indexed arrays. First thing is that you can simply create an index
array and put a value in it by using the format name where the name is the name of the array.
Then in square brackets the subscript that would be a number indexed array.
Then after the closing square bracket equals and then a value which can just be an
unquoted word if you wish but usually wiser to quote things because Bash can contribute
up if you don't put quotes around strings. So I've got an example which shows an array
called fruits and element zero in square brackets is being set equal to the string apple.
It's one it's being set to pair. So fruits is the array name and there are two subscripts
which are zero and one subscripts or indices whichever you like to say and so it's an assignment
but if you've used any languages that you use to raise this will not be a big surprise
this is the way it's usually done. The subscript must be a number or an expression but it's
an expression which we evaluate so a number string would not do. So I've got an example
where a variable i is set to zero then fruits dollar i as the subscript set to apple then
i is incremented in an arithmetic assignment and an arithmetic expansion I should say inside
the square brackets so plus plus i is in two round brackets two parentheses with a dollar
on the front. We looked at this stuff in show 1951 which is a link to in the show notes.
So that is being set to the value pair then we do the same again fruits open square brackets
dollar two open parentheses plus plus i two close parentheses close square brackets equals
then in quotes grape so we're just incrementing that variable it started to zero each time
we add a value to it. It's not the best way of doing it it's just to prove that that's
a way of doing it. There's also a thing called a compound assignment where you use the name
of your array so I've got the sort of generic form name equals then open parenthesis followed
by a list of things and close parenthesis. The list is square brackets containing a subscript
equals value so that would be for the first element then another one square brackets subscript
to close square bracket equals value two and so forth you can have as many of those as you want
I've just put two in my example. So I've rewritten the last example where we set each value
into the array fruits by doing one per line and there's a compound statement a compound assignment
which sets them all in one line. I want to read this out because it's pretty much the same as before
but laid out differently if you like then again the subscript equals part is optional if what
is doing is starting is zero and going up in in incremental steps incrementing by one each time
so you can write it as fruit equals open parenthesis then in quote apple space in quote pair
space in quote grape close parenthesis so that's there's been signed to element 0102 and what's
happening is bash is seeing each of these items in this parenthesis list and is simply incrementing
the index all by itself before it is it assigns it to an array element there's also a way in which
you can take an already existing index array and append to it by typing in this particular case
fruits plus equals open parenthesis quote banana close quote close parenthesis I should say and
that depends to it so it sticks banana on as the the fourth element one with the index of three
remember zero is started zero plus equals operator is I think a fairly recent addition to bash
some people have seen and had to done this myself forget but the plus in front of the equals so this
is not the same thing and if you were to to type fruit equals open parenthesis banana to an array
that had already been loaded with some data it's going to clear it completely and just add the one
element the zero element will be banana so watch out for that another way to define an index to
array is with the declare built-in command now I'm going to look at declaring a bit more detail
later on in this group of shows but I'm not not going to go into great depth today but anyway declare
space hyphen lowercase a space and then a name in fact a list of names will create a bunch of array
variables which are indexed arrays declare can be used to create other things so you need to say
what type of thing you're creating and that's what this hyphen lowercase a does so that's in
indexed arrays so they can be really simple or you can be more formal about it and do use declare
however when it comes to creating an associative array you have to start with the declare you can't
do the explicit indexing stuff because how could you the the indices or subscripts are strengths
any old string you care to come up with so there is no implicit thing you can do it has to be an
explicit definition but the initialization of an element is done pretty much the same way name
square brackets subscript close square bracket equals value the subscript that you put in between
the square brackets doesn't need to be quoted if it contains a space but other characters which are
not just simple spaces and letters numbers might need quotes so I've got an example here where
we declare I didn't say that you had to use a hyphen capital A to define an associative array I
skipped over that bit so if you did declare space hyphen capital A space capitals then
got an example here where we're initializing capitals square brackets England is set to the
string London Scotland Edinburgh Wales Cardiff Northern Ireland Belfast Northern Ireland has got
a space in it and they haven't quoted it in here and that that's worked fine you can also use
compound assignment as we talked about with index stuff declare space hyphen capital S space
capitals and then you would do capitals equals open parenthesis then you've got to put the subscripts
so square brackets England's close square bracket equals London and so on and so I've quoted the
the values in all these cases because it's just wise too as I said before you can even stick those
two things together and you can declare you populate the array at the time you declare it so
so after capitals in the declare command you can just put equals and then in parenthesis the list
of things you're you're setting up I've got an example here that shows it you can do the same with
the indexed arrays as well I didn't mention that earlier so I've got an example here of how you
would create an associative array called char chrs in which I'm assuming you would be storing
certain characters and when you look them up you would get back and a textual description of
what the character is so I've got declare space minus capital A space chrs then you set the
an element chrs open square brackets quote I've used a single quote open square bracket so that's
the character close quote then another close square bracket equals and then the string open square
bracket but you'd have to quote those square brackets that you're using as the subscripts
not too surprisingly because there's this lots of scope for confusion there so how do you get hold
of the contents of an array having initialized it in some way or another and there are other ways
that you can initialize it than we've seen so far but I'm not going to go into those in this
episode they'll come later so one of the simplest ways of visualizing the contents of either type
of array is using declare with the argument hyphen lowcase p and this what this does it just
generates a string which you could save away somewhere which is actually the command you use
or would could use to create the array in the first place so we've got for example the three
arrays that we've been talking about so far declare space hyphen p lowcase p that is space
fruits space capital space chrs and what you get back is a bunch of declares declares space hyphen
a fruits equals and because this this is a lowcase a and because the this one's an index array you
just see the parenthesized list of subscripts and values if you did it if you um look at the
declare hyphen capital A capitals then you see the um the list of subscripts and values
starting with square brackets quotes northern island quote close square bracket equal
bell fast in quotes you get the general idea hopefully the ordering that you get stuff back in
when you you run this command and really you use this if you were saving the structure of an
an array maybe to rebuild it later on in a script or something or in fact you might have been
populating it from some data and you want to save it away back it up the order which you get
stuff back is is always in index order when it's an index to array but when it's an associative array
there is no defined order we just happen to get them back in the order we're seeing here and
bash when it reports them back quotes the strings the subscript strings in an associative array
but you can emit most of them when you're doing itself as long as you're sensible about it
so that's one way of seeing what's in an array but when you're doing you're developing stuff
the usual way that you access array elements is with the syntax dollar because you're
expanding a variable but this time you put an open curly bracket the name of the array
then in square brackets a subscript which can be an expression or a constant close
square brackets close curly bracket so that's how you access an array element
the curly brackets you need them in order to avoid any conflict with bashes
file name expansion operators now when I first saw this I wondered why why can't you simply
write something like dollar fruits square brackets one that would be the logical thing to do
but the problem is that the way bash is structured that would be parsed as the variable dollar
fruits or the variable fruits being expanded because of the dollar on the front and the square
bracketed number will be regarded as a glob range expression you know those things where you have
open square brackets one hyphen nine or something like that so that would not give the result
that that's wanted I did a few tests to see what would happen if you did that given I was working
in an environment where I had the various arrays that we've been looking at so far what would happen
so I echoed dollar fruits square brackets one and I get back the expansion of fruits and you get
back the first element followed by square brackets one which is pretty well meaningful meaning
less sorry not meaningful as what I was going to say and that's partly because if you
use the name of an array without any subscript then that's interpreted by bash as the element with
index 0 I get the first first element and an index to array it also has the same meaning when
you come to an associative array which I think is a bit odd the following example though is showing
if you declared a associative array so it's a hyphen capital A and I've called it hash and in it
I've created an element with subscript of low case a value 42 low case b subscript value 97 and a
value subscript of zero and I put the string what is this question mark that's all in parentheses
if I echo the array hash just by giving it dollar hash what I get back is that bash has interpreted
hash as being the first element or the zero element or indeed even more weird the element which
has the index zero well it just happens to be one is the string zero but you get back the string
what is this now that's very strange so I think I'm trying to prove here that you should not
emit the curly brackets because you're going to be in a whole whole painful area which you're going
to regret I think because you struggle to work at what an earth is happening if you put the curly
brackets around this stuff then you would type echo space and then in double quotes don't
strictly need double quotes but it's always wise to put them in dollar open curly bracket fruits
open square bracket one closed square bracket close curly bracket close quotes and you would get
back the word pair because that's the element with index of one so that's how you access an individual
array element if you want to do something where you look at all of the elements of an array
then instead of using a subscript which is a number or a string two special subscripts which
you can use the first one is an at sign and the second one is an asterisk there's an example here
where we're echoing fruits using the subscript of an at we get back the four four elements
apple pair grape and banana we get them back in order now the difference between the at sign
and the asterisk is only apparent when you use the expression in double quotes and only when the
thing you're using it with cares about about the outcome if you use an asterisk the array elements
are returned as a single word and they're separated by whatever the first character of the
IFS variable is we looked at this in an earlier episode i don't remember what number it was i didn't
make an over but the ifs or IFS variable defines it's the interfield separator and it's usually
space unless you fiddle with it so you're going to get back the list of things apple pair grape
and banana in that particular instance but they're all going to be stuck together in one word
that is as if they were in one string if you use the at sign the array elements are returned as
a list of separate words and this thing about words here is a little bit odd but i've
gone an example coming up it's a downloadable one which attempts to demonstrate what this actually
means and it's really relevant when you are expanding an array in a loop so the downloadable script
is called bash16 underscore ex1.sh there's only one one script in this associated with this show
this time and it does some stuff and it uses the ever beloved user share dict words to generate
random words i've listed it in the notes and i'll just go through the the components of it
first of all we declare an array the array is an associate no it's not it's an index to array
so we're using a hyphen lowercase a and we are calling it words and we populate this array
in a for loop so we've got four word that's the variable it's going to be receiving
each individual thing for word in and then we've got a command substitution which is
sort of stuff i've been doing in previous examples gripping stuff out of user share dict words
when this case i'm gripping out everything that's not a capitalized word and it's not
a possessive i've talked about this before so i won't go into huge detail that is being
piped to the shuff command with the argument the option hyphen n with the number five so that
all that does is to randomly pick five words out of what's being returned by the grip so after
that closed square bracket we've got a semicolon and the do then what's happening is words the array
words is being appended to so it's words plus equals open parenthesis and then in quotes
we have this double quotes i should say dollar word so every time around the loop a word is being
added to the array and there's the done part of the loop so that's it that's loop that's
chunking through this list of five words and adding them to the array this is not the best way to
initialize an array i should say but we haven't looked at better ways yet then the next bit is a loop
which uses the asterisk as the index when it expands the array so we echo the fact that we're
doing this now we've got another full loop four word in and then in double quotes we've got dollar
open curly bracket words that array name a square open square bracket asterisk closed square bracket
close curly bracket double quote that's a quoted reference to the whole array but it's using the
asterisk as we said before that's followed by semicolon and do i like to stick the two on the same
line we echo the words and we get back then the next one next case is a another full loop doing
just about exactly the same thing except that it's using an an at sign as the index so i won't
explain it any more than that now when it's run it comes back with the list of five random words
and i've run it and put the results into the notes and you you get the the message using and then
the the format that's using what's using the asterisk or the at sign and then you get the five words
in the case we use an asterisk the words are written out on one line and that's because the loop
went round only once because the the variable word was filled with all four elements of the array
which were to then print it out the loop the echo and the loop print it out the other one which
uses an at gets back each word each element of the array into the variable word each time around
the loop and it it it errates five times so it gets back the same it's the same array that we're
looking at the same set of five words but it gets them one at a time so it calls echo five times
so they come out one per line for five hopefully that there's some some things in that script
that you might find useful it doesn't it anything useful really but it demonstrates the difference
between these forms i think it's quite important took me a long while to fully understand what
this meant so i've tried to condense the meaning into this script for you to to take away we leave it
there then and carry on looking at this business of accessing railaments in the next episode hopefully
you found that useful and we'll be back for the next one bye bye
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