How to check for the first element in an Array

Question

I have the following Array Loop set up. It simply loops through a string and checks to see if each item matches a letter and sets some variables accordingly.

#!/usr/bin/env bash
IN="ItemName -a -b"

itemA=0
itemB=0
itemI=0

while IFS=' -' read -ra ARR; do
    for x in "${ARR[@]}"; do
        echo "x = $x"
        y=${x:0:1}
        echo "y = $y"
        case $y in
            "a") itemA=1 ;;
            "b") itemB=1 ;;
            "I") itemI=1 ;;
        esac
    done
done <<< "$IN"

echo "itemA is $itemA
echo "itemB is $itemB"
echo "itemI is $itemI" 

However, for the first element ("itemName"), I need to set this separately, so that it isn't checked as part of the case stemement. Ideally I want to end up with:

itemName is ItemName
itemA is 1
itemB is 1
itemI is 0

How can I check for the first element? I tried

if [ ${ARR[0]} ] ; then
    itemName=$x
else
 . . . 
fi  

but ended up with none of the variables getting set.

Answer 1

I don't see why you're using a while loop. Loose the while and your problems disappear:

#!/usr/bin/env bash
IN="ItemName -a -b"
itemA=0
itemB=0
itemI=0

## Read the array, no loop required
IFS=' -' read -ra ARR <<<"$IN"
## Save the 1st element as itemN
itemN=${ARR[0]}
## Remove itemN from the array
unset ARR[0]
## Now loop over the rest
for x in "${ARR[@]}"; do
    y=${x:0:1}
    case $y in
        "a") itemA=1 ;;
        "b") itemB=1 ;;
        "I") itemI=1 ;;
        esac
done

echo "itemN is $itemN"
echo "itemA is $itemA"
echo "itemB is $itemB"
echo "itemI is $itemI" 

Running the script above prints:

itemN is ItemName
itemA is 1
itemB is 1
itemI is 0

Answer 2

I don't know if this is necessarily the best way, but it works:

#!/usr/bin/env bash
IN="ItemName -a -b"

itemName=0
itemA=0
itemB=0
itemI=0

while IFS=' -' read -ra ARR; do
    for x in "${ARR[@]:1:0}"; do
        itemName=$x
    done

    for x in "${ARR[@]:1}"; do
        y=${x:0:1}
        case $y in
            "a") itemA=1 ;;
            "b") itemB=1 ;;
            "I") itemI=1 ;;
        esac
    done
done <<< "$IN"

echo "itemName is $itemName"
echo "itemA is $itemA
echo "itemB is $itemB"
echo "itemI is $itemI" 

Basically, I processed the Array twice. Once to get just the first Item, and then again to get all items except the first item.

Answer 3

I would use more arrays. This answer relies on bash version 4 for associative arrays.

IFS=' -' read -ra words <<<"$IN"
declare -A items

for i in "${words[@]}"; do
    [[ $i == "ItemName" ]] && continue
    items["${i:0:1}"]=1
done

for key in "${!items[@]}"; do
    echo "item $key is ${items[$key]}"
done

item a is 1
item b is 1

If you need to set some default values for the items array:

declare -A items=([a]=0 [b]=0 [c]=0)

Then you would see

item a is 1
item b is 1
item c is 0