Entire array is echoed, but not elements of it - Shell Script

Question

I have this structure in Shell Script BASH

i=0
while read linha_reg
do
    REG_ADDRESS=$(echo $linha_reg | cut -f1 -d ' ')
    [ $REG_ADDRESS -lt 16] && T[$i]=$REG_ADDRESS || S[$i]=$REG_ADDRESS
    i=$((i+1))
done < register.txt

while read linha_inst
do
    RS=$(echo $linha_inst | cut -f4 -d ' ')
    RS=$(echo $RS | cut -f1 -d ',')

    RS_INDEX=${RS:2:2}
    RS_LETTER=${RS:1:1}

    [ "$RS_LETTER" == "s" ] && echo ${S[$RS_INDEX]} 
done < instruction.txt

With this snippet I have only a blank space in the echo ${S[$RS_INDEX]} when I execute it on Terminal. However, if I write

echo ${S[@]}

It outputs the entire array, as requested by the echo. If I write

echo ${S[3]}

Doesn't work also.

I'm lost because I don't know what explains this and I want it to return only a single element, as the big snippet does but it doesn't output by unknown reasons.

Cheers

Also, you increment $i whether you assign to T or S; that will leave gaps in the array you don't assign to. — chepner
– chepner, Commented Aug 27, 2014 at 18:52
Check how many elements you have in S with echo ${#S[@]} to verify you have more than 1 element. Then you know how many to expect. You may also want to post example lines of register.txt and instruction.txt. Using echo | cut ... is a wonky way to do what should be done by substring extraction. — David C. Rankin
– David C. Rankin, Commented Aug 27, 2014 at 19:07
oh -- your indexes are not 0-7! You need 2 different counters for T[x] and S[y] in order to keep you indexes straight. use i for S and say j for T. — David C. Rankin
– David C. Rankin, Commented Aug 27, 2014 at 19:21
To keep the indexes straight, so something like i=0; j=0; .. [ $REG_ADDRESS -lt 16] && { T[$i]=$REG_ADDRESS; ((i++)); } || { S[$j]=$REG_ADDRESS; ((j++)); } --or-- declare -a T; declare -a S and then use T+=$REG_ADDRESS and S+=$REG_ADDRESS — David C. Rankin
– David C. Rankin, Commented Aug 27, 2014 at 19:29
@DavidC.Rankin T+=($REG_ADDRESS), otherwise you simply perform string concatenation on ${T[0]}. — chepner
– chepner, Commented Aug 27, 2014 at 19:39

l0b0 · Accepted Answer · 2014-08-27 21:08:12Z

1

To add to an array, use var+=(value) like @chepner suggested. Using var[$i]=value results in sparse arrays, where some indexes are undefined.

Also:

Use More Quotes™!
In Bash you can use here strings like some_command <<< "$variable" to avoid echo.
When you do a && b || c, c is run if either a or b fails. To avoid this, use the standard
```
if a
then
    b
else
    c
fi
```
Use a single = with [ (see help test).

edited Aug 27, 2014 at 21:08

answered Aug 27, 2014 at 21:01

l0b0

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