purge-file.sh

#!/bin/bash
#
#  Usage:
#
#   ./purge-file.sh <snapshot_path> "<file_or_relative_path>"
#
#  Example:
#
#   ./purge-file.sh /mnt/btrfs/snapshots/home "Downloads/*.iso"
#
#     This will remove all *.iso files from all snapshots in /mnt/btrfs/snapshots/home.*
#     such as a name like "home.20180121T070102-0500", it looks for
#     /mnt/btrfs/snapshots/home.*/**/Downloads/*.iso, precisely
#
#     You can also specify a file only, and the -path option to `find` will automatically
#     remove itself
#
#
set -eu
# set -x
SUBVOL_PATH="$1"
FILE="$2"
DIR="${2%/*}" && [[ $FILE == "$DIR" ]] && unset DIR

is_ro() {
    # Usage: is_ro <btrfs_subvolume>
    # Exits true/false if a subvolue is read-only
    [[ $(sudo btrfs property get "$(df -h "$1" | awk 'NR>1 { print $6 }')" ro) != 'ro=false' ]]
}

echo_wrap() {
    echo "$*"; eval "$*"
}

is_btrfs_subvolume() {
    local path inode fstype
    path="$1"
    inode=$( stat --format='%i' "$path" )
    fstype=$( stat -f --format='%T' "$path" )
    (
        (( inode == 256 )) && [[ $fstype == "btrfs" ]]
    ) && return 0 || return 1
}

purge_file() {
    local file="$1" # ex: /mnt/btrfs/snapshots/home.20180121T070102-0500/file.tar.gz
    local suffix="${file#${SUBVOL_PATH}}" # ex: .20180121T070102-0500/file.tar.gz
    local snapshot="${SUBVOL_PATH}${suffix%%/*}" # ex: /mnt/btrfs/snapshots/home.20180121T070102-0500
    local was_read_only=0

    is_btrfs_subvolume "$snapshot" || { echo "File not in a btrfs snapshot"; exit 1; }

    if is_ro "$snapshot"; then
        echo_wrap sudo btrfs property set "$snapshot" ro false &&
        was_read_only=1
    fi

    echo_wrap sudo rm -v -i "${file:?ERROR NO FILE}"

    if (( was_read_only )); then
        echo_wrap sudo btrfs property set "$snapshot" ro true
    fi
}

# Activate/check sudo early
sudo -v || { echo "Needs sudo"; exit 1; }

# Get an array of all the files we want to remove from the snapshots
echo "Finding files in subvol ${SUBVOL_PATH}*..."
mapfile -t FILE_LIST < <(
    sudo find "${SUBVOL_PATH}."* \
        -xdev ${DIR:+-path "${SUBVOL_PATH}*/**/${DIR}/\*"} \
        -name "${FILE##*/}"
)
echo "Done."

(( ${#FILE_LIST[@]} <= 0 )) && { echo "No files to delete"; exit 0; }

echo "Removing files.."
for file in "${FILE_LIST[@]}"; do
    purge_file "$file"
done
echo "Done."