Addressed some of the shellcheck whinings -- primarily to not declare…

… and assign at the same time

Also made that script executable since it is a script with shebang -- should be
good like that

Here is exemplar list of complaints from shellcheck.  Not sure if "local" was actually needed here.
The mapfile ones are still to be done

    In contrib/incremental_backup/dump.sh line 82:
        LIST+=($(echo "$CHANNEL_LIST_JSON" | $JQ_B -r 'map(select(.is_im == true)) | .[] | .user, .id'))
                   ^-- SC2207 (warning): Prefer mapfile or read -a to split command output (or quote to avoid splitting).

    In contrib/incremental_backup/dump.sh line 127:
        local META_JSON=$(<"$META_FILE")
                  ^-------^ SC2155 (warning): Declare and assign separately to avoid masking return values.

    In contrib/incremental_backup/dump.sh line 129:
        local PREVIOUS_DATE="$(echo "$META_JSON" | $JQ_B -r '.last_updated | select(. != null)')"
                  ^-----------^ SC2155 (warning): Declare and assign separately to avoid masking return values.

    In contrib/incremental_backup/dump.sh line 156:
        local NEW_MESSAGE_COUNT=$($JQ_B -r '.messages | length' "$CHANNEL_FILE")
                  ^---------------^ SC2155 (warning): Declare and assign separately to avoid masking return values.

    In contrib/incremental_backup/dump.sh line 165:
                local MERGED_CONTENT=$($JQ_B -s '.[0] as $o1 | .[1] as $o2 | ($o1 + $o2) | .messages = ($o1.messages + $o2.messages)' "$CHANNEL_FILE_OLD" "$CHANNEL_FILE")
                                  ^------------^ SC2155 (warning): Declare and assign separately to avoid masking return values.
                                                            ^-- SC2016 (info): Expressions don't expand in single quotes, use double quotes for that.

    In contrib/incremental_backup/dump.sh line 174:
        local TOTAL_MESSAGE_COUNT=$($JQ_B -r '.messages | length' "$CHANNEL_FILE")
                  ^-----------------^ SC2155 (warning): Declare and assign separately to avoid masking return values.

    In contrib/incremental_backup/dump.sh line 190:
        LIST+=( $(im_channels) )
                    ^------------^ SC2207 (warning): Prefer mapfile or read -a to split command output (or quote to avoid splitting).

    In contrib/incremental_backup/dump.sh line 192:
        LIST+=( $(group_channels) )
                    ^---------------^ SC2207 (warning): Prefer mapfile or read -a to split command output (or quote to avoid splitting).

contrib/incremental_backup/dump.sh

@@ -49,6 +49,7 @@ INCLUDE_GROUP_CHANNEL_NAME_REGEX=(
 
 ### Qualify needed binaries
 JQ_B=$(which jq)
+# shellcheck disable=SC2181
 [ $? -ne 0 ] && echo "Please be sure jq is available in your PATH. https://stedolan.github.io/jq/" && exit 1
 SLACKDUMP_B="./slackdump"
 [ ! -x "$SLACKDUMP_B" ] && echo "Please be sure slackdump is located at '$SLACKDUMP_B'. https://github.com/rusq/slackdump" && exit 1
@@ -71,7 +72,7 @@ function get_list {
 
 # Echo out all channel names. This excludes any direct message groups and 1-1 messages.
 function channel_names {
-	echo "$CHANNEL_LIST_JSON" | $JQ_B -r '[ map(select( (.name_normalized != "") and (.name_normalized | startswith("mpdm-") | not) ) ) | .[] | .name_normalized ] | sort | .[]'
+	echo "$CHANNEL_LIST_JSON" | "$JQ_B" -r '[ map(select( (.name_normalized != "") and (.name_normalized | startswith("mpdm-") | not) ) ) | .[] | .name_normalized ] | sort | .[]'
 }
 
 # Echo out all 1-1 direct message channels as user_name channel_id pairs
@@ -84,7 +85,8 @@ function im_channels {
 		local USER_ID=${LIST[0]}
 		local CHANNEL_ID=${LIST[1]}
 		LIST=( "${LIST[@]:2}" )
-		local USER_NAME=$(echo "$USER_LIST_JSON" | $JQ_B -r "map(select(.id == \"${USER_ID}\")) | .[].name")
+		local USER_NAME
+		USER_NAME=$(echo "$USER_LIST_JSON" | $JQ_B -r "map(select(.id == \"${USER_ID}\")) | .[].name")
 		# Filter the list by username, as per the configured INCLUDE_USER_NAME_REGEX list 
 		for NAME_REGEX in "${INCLUDE_USER_NAME_REGEX[@]+"${INCLUDE_USER_NAME_REGEX[@]}"}"; do
 			if [[ "${USER_NAME}" =~ $NAME_REGEX ]]; then
@@ -122,9 +124,11 @@ function dump {
 		echo "{}" > "$META_FILE"
 	fi
 	# Read the meta into memory
-	local META_JSON=$(<"$META_FILE")
+	local META_JSON
+	META_JSON=$(<"$META_FILE")
 
-	local PREVIOUS_DATE="$(echo "$META_JSON" | $JQ_B -r '.last_updated | select(. != null)')"
+	local PREVIOUS_DATE
+	PREVIOUS_DATE="$(echo "$META_JSON" | $JQ_B -r '.last_updated | select(. != null)')"
 	local FROM_FLAG=""
 	if [ "$PREVIOUS_DATE" != "" ]; then
 		FROM_FLAG="-dump-from $PREVIOUS_DATE"
@@ -151,7 +155,8 @@ function dump {
 	echo "Dumping messages from \"$PREVIOUS_DATE\" to \"$CURRENT_DATE\""
  	$SLACKDUMP_B -download -r json "$FROM_FLAG" "$TO_FLAG" -base "$BASE_DIR" "$CHANNEL_ID" > "$LOG_FILE" 2>&1
 
- 	local NEW_MESSAGE_COUNT=$($JQ_B -r '.messages | length' "$CHANNEL_FILE")
+ 	local NEW_MESSAGE_COUNT
+	NEW_MESSAGE_COUNT=$($JQ_B -r '.messages | length' "$CHANNEL_FILE")
  	echo "Found '$NEW_MESSAGE_COUNT' new message(s)."
 
 	# If we have an old file...
@@ -160,7 +165,9 @@ function dump {
 		# and remove the old file
 		if [ "$NEW_MESSAGE_COUNT" -gt 0 ]; then
 			# See https://stackoverflow.com/a/75597380/397210
-			local MERGED_CONTENT=$($JQ_B -s '.[0] as $o1 | .[1] as $o2 | ($o1 + $o2) | .messages = ($o1.messages + $o2.messages)' "$CHANNEL_FILE_OLD" "$CHANNEL_FILE")
+			local MERGED_CONTENT
+			# shellcheck disable=SC2016
+			MERGED_CONTENT=$($JQ_B -s '.[0] as $o1 | .[1] as $o2 | ($o1 + $o2) | .messages = ($o1.messages + $o2.messages)' "$CHANNEL_FILE_OLD" "$CHANNEL_FILE")
 			echo "$MERGED_CONTENT" > "$CHANNEL_FILE"
 			rm -f "$CHANNEL_FILE_OLD"
 		else
@@ -169,7 +176,8 @@ function dump {
 			mv "$CHANNEL_FILE_OLD" "$CHANNEL_FILE"
 		fi
 	fi
-	local TOTAL_MESSAGE_COUNT=$($JQ_B -r '.messages | length' "$CHANNEL_FILE")
+	local TOTAL_MESSAGE_COUNT
+	TOTAL_MESSAGE_COUNT=$($JQ_B -r '.messages | length' "$CHANNEL_FILE")
 	echo "Total messages for '$CHANNEL_NAME' channel: $TOTAL_MESSAGE_COUNT"
 
 	# Update the last updated date