Bash - expanding variable nested in variable

Question

Noble StackOverflow readers,

I have a comma seperated file, each line of which I am putting into an array. Data looks as so...

25455410,GROU,AJAXa,GROU1435804437
25455410,AING,EXS3d,AING4746464646
25455413,TRAD,DLGl,TRAD7176202067

There are 103 lines and I am able to generate the 103 arrays without issue.

n=1; while read -r OrdLine; do
    IFS=',' read -a OrdLineArr${n} <<< "$OrdLine"
    let n++
done < $WkOrdsFile

HOWEVER, I can only access the arrays as so...

echo "${OrdLineArr3[0]}  <---Gives 25455413

I cannot access it with the number 1-103 as a variable - for example the following doesn't work...

i=3
echo "${OrdLineArr${i}[0]}

That results in...

./script2.sh: line 24: ${OrdLineArr${i}[0]}: bad substitution

I think that the answer might involve 'eval' but I cannot seem to find a fitting example to borrow. If somebody can fix this then the above code makes for a very easy to handle 2d array replacement in bash!

Thanks so much for you help in advance!

Dan

Possibly duplicate of stackoverflow.com/questions/6724056/…. — Chandranshu
– Chandranshu, Commented Nov 17, 2013 at 1:22
Bash supports only one-dimensional arrays and you would be better off using Perl for this task. If you really have to use Bash check out tldp.org/LDP/abs/html/arrays.html specifically example 27.17 — MattSizzle
– MattSizzle, Commented Nov 17, 2013 at 1:28
@ruakh - the indirect expansion within a function worked a treat. Is a very nice paradigm for 2-d arrays when forced to use bash to process them! Thanks so much! — user3000596
– user3000596, Commented Nov 20, 2013 at 11:30

ruakh · Accepted Answer · 2013-11-17 18:10:07Z

1

You can use indirect expansion. For example, if $key is OrdLineArr4[7], then ${!key} (with an exclamation point) means ${OrdLineArr4[7]}. (See §3.5.3 "Shell Parameter Expansion" in the Bash Reference Manual, though admittedly that passage doesn't really explain how indirect expansion interacts with arrays.)

I'd recommend wrapping this in a function:

function OrdLineArr () {
    local -i i="$1"          # line number (1-103)
    local -i j="$2"          # field number (0-3)
    local key="OrdLineArr$i[$j]"
    echo "${!key}"
}

Then you can write:

echo "$(OrdLineArr 3 0)"        # prints 25455413
i=3
echo "$(OrdLineArr $i 0)"       # prints 25455413

This obviously isn't a total replacement for two-dimensional arrays, but it will accomplish what you need. Without using eval.

answered Nov 17, 2013 at 18:10

ruakh

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5 Comments

gniourf_gniourf Over a year ago

This is definitely the best answer, yet, the echo statement at the end of the function OrdLineArr will remove trailing newlines if any. If this is a concern, use a global variable for the return value of that function, e.g., OrdLineArr_ret=${!key}.

gniourf_gniourf Over a year ago

Another quick comment, you can have access to the full array with aryp=OrdLineArr[@]: then "${!aryp}" will expand to the full array.

ruakh Over a year ago

@gniourf_gniourf: Re: trailing newlines: Yeah, I thought about that (it's one of the reasons I don't consider this a total replacement for two-dimensional arrays), but the OP's array obviously will never have newlines in its elements, so I considered it O.K.

ruakh Over a year ago

@gniourf_gniourf: By the way, it's actually not the echo that removes the trailing newlines, but rather the command-substitution that I wrapped OrdLineArr in when I called it. If ${OrdLineArr3[0]} ends in a sequence of newlines, then OrdLineArr 3 0 will properly print those newlines, but echo "$(OrdLineArr 3 0)" will discard them. (That's a bit pedantic, but it means that a sufficiently motivated caller could work around it by writing something like foo="$(OrdLineArr 3 1 ; echo .)" ; foo="${foo%.}".)

gniourf_gniourf Over a year ago

If the goal is to put it in a variable then the motivated caller will discard this function altogether and instead create one that takes a variable name as an extra parameter and will return the value in this variable with printf -v.

William Pursell · Accepted Answer · 2013-11-17 01:33:25Z

0

eval is usually a bad idea, but you can do it with:

eval echo "\${OrdLineArr$i[0]}"

answered Nov 17, 2013 at 1:33

William Pursell

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Comments

chepner · Accepted Answer · 2013-11-17 17:52:13Z

0

I would store each line in an array, but split it on demand:

readarray OrdLineArr < $WkOrdsFile
...
OrdLine=${OrdLineArr[i]}
IFS=, read -a Ord <<< "$OrdLine"

However, bash isn't really equipped for data processing; it's designed to facilitate process and file management. You should consider using a different language.

answered Nov 17, 2013 at 17:52

chepner

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