Add a node script that shrinks a container's root disk
CI / pytest and ruff (push) Successful in 5m19s
CI / pytest and ruff (push) Successful in 5m19s
scripts/pct-shrink-rootfs runs as root on the Proxmox node and keeps the VMID. Without --yes it only measures and prints the plan. It adds a temporary mp9 of the new size (Proxmox allocates and formats it), rsyncs the root disk into it, and swaps it into rootfs under Proxmox's config flock. The config is written like file_set_contents does on pmxcfs (tmp file, then rename), after checking that rootfs and mp9 did not change during the copy. The old volume is kept as the first free unusedK and the exact delete command is printed; pct rescan is not used, so other orphans of the VMID are not pulled in. It refuses snapshots and pending sections, a lock, template, protection, HA, replication, any mount point, and sizes the data does not fit (10 % headroom on top of the ext4 reserve and metadata, and an inode check). On an rsync failure the new volume is freed by its volid. Tests run the real script against stub pct, df, rsync, flock, ha-manager, pvesr and id. README documents both shrink routes. Design: docs/plans/lxc-shrink.md (section B). Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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#!/usr/bin/env bash
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# Shrink a container's root disk by copying it into a smaller volume. Run as root
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# on the Proxmox node. Without --yes it only measures and prints the plan.
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# Spec: docs/plans/lxc-shrink.md (section B).
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set -euo pipefail
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CONF_DIR=${PCT_CONF_DIR:-/etc/pve/lxc}
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BACKUP_DIR=${PCT_SHRINK_BACKUP_DIR:-/root}
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GIB=1073741824
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ctid="" conf="" mounted=0 new_volid=""
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die() { echo "pct-shrink-rootfs: $*" >&2; exit 1; }
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usage() { die "usage: pct-shrink-rootfs [--yes] <vmid> <new-size-GiB>"; }
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conf_get() { awk -v k="$1" 'index($0, k ": ") == 1 { print substr($0, length(k) + 3); exit }' "$conf"; }
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refuse_if() { if grep -Eq "$1" "$conf"; then die "$2"; fi; }
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# The value of the unusedK line that holds exactly this volid (no regex on the volid).
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unused_key_of() {
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awk -v v="$1" '/^unused[0-9]+: / { if (substr($0, index($0, ": ") + 2) == v) { print substr($0, 1, index($0, ":") - 1); exit } }' "$conf"
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}
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size_bytes() {
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[[ $1 =~ ^([0-9]+)([KMGT]?)$ ]] || die "cannot read the current rootfs size '$1'"
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local m=1
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case ${BASH_REMATCH[2]} in K) m=1024 ;; M) m=$((1024 ** 2)) ;; G) m=$GIB ;; T) m=$((1024 ** 4)) ;; *) ;; esac
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echo $((BASH_REMATCH[1] * m))
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}
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manual_cleanup() {
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cat >&2 <<EOT
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pct-shrink-rootfs: 'pct unmount $ctid' failed. Nothing else was touched. Clean up by hand:
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pct unmount $ctid
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pct unlock $ctid # if the lock remains
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EOT
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}
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unmount_or_die() {
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mounted=0
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pct unmount "$ctid" || { manual_cleanup; exit 1; }
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}
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on_exit() {
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local rc=$?
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trap - EXIT
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if ((mounted)); then unmount_or_die; fi
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if ((rc != 0)) && [[ -n $new_volid ]]; then
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echo "pct-shrink-rootfs: the new volume $new_volid is still in the config as mp9;" \
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"remove it with 'pct set $ctid --delete mp9' and then the unusedK that holds it." >&2
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fi
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exit "$rc"
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}
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# Runs inside flock. Writes like PVE's file_set_contents: "<file>.tmp.<pid>" in the same
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# pmxcfs directory, then rename. Re-checks both volids under the lock before writing.
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swap_config() {
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local conf=$1 old=$2 new=$3 size=$4 backup=$5 tmp=$1.tmp.$$
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cp "$conf" "$backup"
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if ! awk -v old="$old" -v new="$new" -v size="$size" '
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{ key = substr($0, 1, index($0, ":") - 1); val = substr($0, index($0, ": ") + 2) }
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key ~ /^unused[0-9]+$/ { used[substr(key, 7) + 0] = 1 }
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key == "mp9" { split(val, m, ","); ok_new = (m[1] == new); next }
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key == "rootfs" {
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n = split(val, a, ",")
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ok_old = (a[1] == old)
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out = new; seen = 0
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for (i = 2; i <= n; i++) {
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if (substr(a[i], 1, 5) == "size=") { out = out ",size=" size; seen = 1 }
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else out = out "," a[i]
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}
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if (!seen) out = out ",size=" size
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print "rootfs: " out; next
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}
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{ print }
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END { if (!ok_old || !ok_new) exit 3; k = 0; while (k in used) k++; print "unused" k ": " old }
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' "$conf" > "$tmp"; then
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rm -f "$tmp"
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echo "pct-shrink-rootfs: config changed during the copy (rootfs or mp9 differs); not swapped" >&2
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return 1
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fi
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mv -f "$tmp" "$conf"
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}
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export -f swap_config
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free_new_volume() {
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local key vol=$new_volid
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pct set "$ctid" --delete mp9
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new_volid="" # no longer mp9: the on_exit hint would be wrong
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key=$(unused_key_of "$vol")
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[[ -n $key ]] || die "cannot find $vol among the unused volumes; delete it by hand"
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pct set "$ctid" --delete "$key"
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}
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main() {
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local yes=0 size root rootfs old_volid storage cur_bytes used iused out mp9 new_size backup key
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while [[ ${1:-} == --* ]]; do
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[[ $1 == --yes ]] || usage
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yes=1
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shift
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done
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[[ $# -eq 2 ]] || usage
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ctid=$1 size=$2
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[[ $ctid =~ ^[0-9]+$ ]] || die "vmid must be a number"
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[[ $size =~ ^[1-9][0-9]{0,8}$ ]] || die "new size must be a positive integer (GiB)"
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[[ $(id -u) -eq 0 ]] || die "must run as root on the node"
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command -v pct > /dev/null || die "pct not found; run this on a Proxmox node"
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conf=$CONF_DIR/$ctid.conf
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[[ -f $conf ]] || die "no config $conf"
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[[ $(pct status "$ctid") == "status: stopped" ]] || die "container $ctid must be stopped"
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refuse_if '^\[' "config has a snapshot or pending section"
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refuse_if '^lock:' "container is locked"
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refuse_if '^template: 1$' "container is a template"
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refuse_if '^protection: 1$' "container has protection set"
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refuse_if '^(mp[0-9]+|lxc\.mount\.entry):' "container has a mount point (mpN or lxc.mount.entry); not supported"
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out=$(ha-manager status)
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if grep -Eq "(^| )ct:$ctid( |\$)" <<< "$out"; then die "container is HA-managed"; fi
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out=$(pvesr list)
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if grep -Eq "^$ctid-" <<< "$out"; then die "container has a replication job"; fi
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rootfs=$(conf_get rootfs)
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old_volid=${rootfs%%,*}
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[[ $old_volid == *:* && $old_volid != /* ]] || die "rootfs is not a volume: $old_volid"
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storage=${old_volid%%:*}
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[[ $rootfs =~ (^|,)size=([^,]+) ]] || die "rootfs has no size= option"
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cur_bytes=$(size_bytes "${BASH_REMATCH[2]}")
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((size * GIB < cur_bytes)) || die "new size must be smaller than the current rootfs (${BASH_REMATCH[2]})"
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trap on_exit EXIT
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root=/var/lib/lxc/$ctid/rootfs
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mounted=1
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pct mount "$ctid"
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used=$(df -B1 --output=used "$root" | tail -n1 | tr -d ' ')
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iused=$(df -i --output=iused "$root" | tail -n1 | tr -d ' ')
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unmount_or_die
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[[ $used =~ ^[0-9]+$ && $iused =~ ^[0-9]+$ ]] || die "cannot read df output"
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echo "Container $ctid, rootfs $old_volid ($storage): $used bytes, $iused inodes used."
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echo "Config dir: $CONF_DIR; backup dir: $BACKUP_DIR"
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# 5 % root reserve + ~2 % ext4 metadata; mkfs.ext4 default is one inode per 16 KiB.
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((used * 1100 <= size * GIB * 930)) || die "data too full for ${size}G: need used * 1.1 <= 93 % of the disk"
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((iused * 11 <= size * 65536 * 10)) || die "too many inodes for ${size}G: $iused used, ${size}G gives $((size * 65536))"
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if ((!yes)); then
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echo "Plan: allocate ${size}G on $storage, rsync the root disk into it, swap it into rootfs."
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echo "The old volume is kept as unusedK. Re-run with --yes to do it."
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return 0
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fi
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pct set "$ctid" --mp9 "$storage:$size,mp=/.shrink-new"
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mp9=$(conf_get mp9)
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[[ -n $mp9 ]] || die "mp9 not found after pct set"
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new_volid=${mp9%%,*}
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[[ ,$mp9, =~ ,size=([^,]+), ]] || die "mp9 has no size= option: $mp9"
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new_size=${BASH_REMATCH[1]}
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mounted=1
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pct mount "$ctid"
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if ! rsync -aHAX --numeric-ids --whole-file --sparse --one-file-system \
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--exclude=/.shrink-new "$root/" "$root/.shrink-new/"; then
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unmount_or_die
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free_new_volume
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die "rsync failed; the new volume was removed and the config is unchanged"
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fi
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unmount_or_die
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backup=$BACKUP_DIR/pct-shrink-$ctid-$(date +%Y%m%d-%H%M%S).conf
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flock "/run/lock/lxc/pve-config-$ctid.lock" \
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bash -c 'set -euo pipefail; swap_config "$@"' _ "$conf" "$old_volid" "$new_volid" "$new_size" "$backup"
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new_volid=""
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key=$(unused_key_of "$old_volid")
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cat <<EOT
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Done. rootfs is now on the new ${new_size} volume; config backup: $backup
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The old root disk is kept as '$key: $old_volid'.
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Start the container and check it. Then free the old volume:
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pct set $ctid --delete $key
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rsync does not copy chattr flags (+i, +a); re-apply them in the container if you used any.
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EOT
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}
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if [[ "${BASH_SOURCE[0]}" == "$0" ]]; then
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main "$@"
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fi
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