Add ability to use random data or zeroes

README.md

@@ -6,8 +6,9 @@ device after it has been cleanly removed from its pool.
 
 ## What It Erases
 
-The script performs 10 overwrite passes with zeroes over the first 4 MiB and
+The script performs 10 overwrite passes over the first 4 MiB and the last
-the last 4 MiB of the target block device. These regions include:
+4 MiB of the target block device. It uses zeroes by default and can use random
+data when requested. These regions include:
 
 - MBR data and GPT primary partition table at the beginning of a disk.
 - GPT backup partition table at the end of a disk.
@@ -41,6 +42,17 @@ chmod +x erase-zfs-metadata.sh
 sudo ./erase-zfs-metadata.sh /dev/sdX
 ```
 
+Use `--source random` to overwrite with data from `/dev/urandom` instead of
+the default zeroes:
+
+```bash
+sudo ./erase-zfs-metadata.sh --source random /dev/sdX
+```
+
+The only accepted source values are `zeroes` and `random`. Random overwrites
+are typically slower and do not erase additional parts of the device; both
+modes overwrite the same first and last 4 MiB regions.
+
 Replace `/dev/sdX` with the actual device, such as `/dev/sdb`,
 `/dev/nvme0n1`, or a device-mapper path. Do not use the disk containing the
 running system.

erase-zfs-metadata.sh

@@ -10,11 +10,16 @@ readonly WIPE_BYTES=$((WIPE_MIB * MIB))
 
 usage() {
     cat <<'EOF'
-Usage: sudo ./erase-zfs-metadata.sh /dev/<device>
+Usage: sudo ./erase-zfs-metadata.sh [--source zeroes|random] /dev/<device>
 
 Overwrites the first and last 4 MiB of the specified block device ten times.
 This destroys MBR/GPT partition tables and all conventional ZFS vdev labels.
 The device must not be mounted or part of an imported ZFS pool.
+
+Options:
+    --source zeroes  Write zeroes from /dev/zero (default).
+    --source random  Write random data from /dev/urandom.
+    -h, --help        Show this help message.
 EOF
 }
 
@@ -23,11 +28,49 @@ die() {
     exit 1
 }
 
-[[ $# -eq 1 ]] || { usage >&2; exit 2; }
+write_source=/dev/zero
-device=$1
+source_name=zeroes
+device=
+
+while [[ $# -gt 0 ]]; do
+    case $1 in
+        --source)
+            [[ $# -ge 2 ]] || die '--source requires zeroes or random.'
+            case $2 in
+                zeroes)
+                    write_source=/dev/zero
+                    source_name=zeroes
+                    ;;
+                random)
+                    write_source=/dev/urandom
+                    source_name=random
+                    ;;
+                *)
+                    die "invalid source '$2'; use zeroes or random."
+                    ;;
+            esac
+            shift 2
+            ;;
+        -h|--help)
+            usage
+            exit 0
+            ;;
+        -*)
+            die "unknown option '$1'."
+            ;;
+        *)
+            [[ -z $device ]] || die 'specify exactly one block device.'
+            device=$1
+            shift
+            ;;
+    esac
+done
+
+[[ -n $device ]] || { usage >&2; exit 2; }
 
 [[ $EUID -eq 0 ]] || die 'run as root (for example, with sudo).'
 [[ -b $device ]] || die "'$device' is not a block device."
+[[ -r $write_source ]] || die "'$write_source' is not readable."
 
 for command in blockdev dd findmnt lsblk zpool; do
     command -v "$command" >/dev/null 2>&1 || die "required command '$command' was not found."
@@ -50,16 +93,16 @@ size_bytes=$(blockdev --getsize64 "$device")
 
 last_offset=$((size_bytes - WIPE_BYTES))
 
-printf '\nWARNING: this permanently destroys partition tables and ZFS metadata on:\n  %s\n\n' "$device"
+printf '\nWARNING: this permanently destroys partition tables and ZFS metadata on:\n  %s\nusing %s data.\n\n' "$device" "$source_name"
 lsblk --output NAME,SIZE,TYPE,MOUNTPOINTS "$device"
 read -r -p "Type exactly 'ERASE $device' to continue: " confirmation
 [[ $confirmation == "ERASE $device" ]] || die 'confirmation did not match; no data was changed.'
 
 for ((pass = 1; pass <= PASSES; pass++)); do
     printf 'Pass %d/%d...\n' "$pass" "$PASSES"
-    dd if=/dev/zero of="$device" bs="$MIB" count="$WIPE_MIB" conv=fsync,notrunc status=none
+    dd if="$write_source" of="$device" bs="$MIB" count="$WIPE_MIB" conv=fsync,notrunc status=none
-    dd if=/dev/zero of="$device" bs=1 count="$WIPE_BYTES" seek="$last_offset" conv=fsync,notrunc status=none
+    dd if="$write_source" of="$device" bs=1 count="$WIPE_BYTES" seek="$last_offset" conv=fsync,notrunc status=none
 done
 
 sync
-printf 'Completed %d overwrite passes on %s.\n' "$PASSES" "$device"
+printf 'Completed %d %s overwrite passes on %s.\n' "$PASSES" "$source_name" "$device"