Zig update script - only works when zig is already installed

zig-up.sh

#!/usr/bin/env bash

# NOTE: This script requires the 'jq' tool.

TMP_DIR=/tmp/zig-update
if [ -z "$ZIG_DIR" ]; then 
    ZIG_DIR=`dirname $(which zig)`
fi
TARGET=$(zig env | jq --raw-output .target | sed 's/^\([a-z0-9_\-]*\).*/\1/g')
VERSION=$(zig env | jq --raw-output .version)
JSON=$(curl -s https://ziglang.org/download/index.json)
TARBALL_URL=$(echo $JSON | jq --raw-output ".master[\"$TARGET\"].tarball")
MASTER_VERSION=$(echo $JSON | jq --raw-output .master.version)

if [ "$TARBALL_URL" = "null" ]; then 
    echo "Cannot find a tarball for $TARGET"
    exit 1
fi

if [ "$MASETR_VERSION" = "$VERSION" ]; then 
    echo "$VERSION is already installed"
    exit 1
fi

mkdir -p $TMP_DIR

echo "Downloading tarball..."

if curl "$TARBALL_URL" | tar -xJC $TMP_DIR; then 
    BASENAME=$(basename $TARBALL_URL ".tar.xz")
    rm -rf $ZIG_DIR
    mv "$TMP_DIR/$BASENAME" $ZIG_DIR
else
    echo "Failed to update"
fi

VERSION=$(zig env | jq --raw-output .version)
echo "Current version is $VERSION"

# Clean up
rm -rf $TMP_DIR