lsblk -n -o NAME,SIZE,TYPE /dev/sd* 2>/dev/null works on cmd line, not in script file

Question

I am writing a script to detect and format USB drives for Raspberry Pi. I got the script to work perfectly for sd cards inserted via a USB adapter. It failed when I tried a usb3 HDD. Isolating the failure indicates a problem with wildcard expansion and redirection processing in the script.

When I execute lsblk -n -o NAME,SIZE,TYPE /dev/sd* 2>/dev/null

in a root terminal, the output is empty without any USB device and appropriate when one is inserted:

sda     3.7G disk
└─sda1  3.7G part
sda1    3.7G part

When I run it this way in a script:

#!/bin/bash
xtra='/dev/sd* 2>/dev/null'
DETECTOR="lsblk -n -o NAME,SIZE,TYPE $xtra"

zenity --info --text="Remove all USB devices" --width=620 --height=200
not_in=$($DETECTOR)
zenity --info --text="Insert USB device to format" --width=420 --height=100
usb_in=$($DETECTOR)

# Get the difference which is inserted USB device and partitions on it
delta=$(diff <(echo "$not_in") <(echo "$usb_in"))
drive=(${delta//[^[:ascii:]]/})  # Delete the non-ASCII characters

echo -e "$not_in\n$usb_in\n\n${drive[@]}\n\n$delta"

What is get is:

lsblk: /dev/sd*: not a block device
lsblk: 2>: not a block device
lsblk: /dev/null: not a block device
lsblk: 2>: not a block device
lsblk: /dev/null: not a block device

sda     3.7G disk
└─sda1  3.7G part
sda1    3.7G part

1c1,3 < --- > sda 3.7G disk > sda1 3.7G part > sda1 3.7G part

1c1,3
< 
---
> sda     3.7G disk
> └─sda1  3.7G part
> sda1    3.7G part

I don't understand how to get only the inserted device and its partitions in an array variable ($drive). There is something about how the lsblk command is being interpreted that is different between the command line invocation where the bash expands sd* properly and directs stderr to /dev/null and how it runs in the script context.

I've read similar issues related to how different shells interpret wildcards & redirections, but this only involves bash.

The only workaround I could come up with requires creating temporary files, which isn't elegant or appealing:

Answer 1

#!/bin/bash
DETECTOR="lsblk -n -o NAME,SIZE,TYPE /dev/sd*"

zenity --info --text="Remove all USB devices" --width=620 --height=200
$DETECTOR > noSD 2>/dev/null
zenity --info --text="Insert USB device to format" --width=420 --height=100
$DETECTOR > sdIn 2>/dev/null

# Get the difference which is inserted USB device and partitions on it
delta=$(diff noSD sdIn | tr -d \> | grep sd)
drive=(${delta//[^[:ascii:]]/})  # Delete the non-ASCII characters

not_in=$(cat noSD)
usb_in=$(cat sdIn)
echo -e "Not in:$not_in\nIn:$usb_in\n\nArray:${drive[@]}\n\nDiff:\n$delta"