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#!/bin/sh
set -eu
prev_hash=
prev_bk_file=
# Back up a database and output a SHA-256 hash of its SQL dump.
backup()
{
local db_file="${1}"
local bk_file="${2}"
shift 2
sqlite3 "${db_file}" ".backup ${bk_file}~"
sqlite3 "${bk_file}~" '.dump' | sha256sum | cut -d ' ' -f 1
return 0
}
# Read previous backup's hash and file name.
# Returns 0 if the new backup's hash is equal to the previous backup's hash, or
# 1 otherwise.
compare_hashes()
{
local db_file="${1}"
local bk_hash="${2}"
shift 2
{ read -r prev_hash prev_bk_file 0<"${db_file}.bakhash"; } 2>/dev/null \
|| :
case "${prev_hash}" in "${bk_hash}") return 0;; esac
return 1
}
# Make a symbolic link with the new backup file name to the previous backup
# file.
# No path resolution is performed, so either absolute paths should be used or
# all backup files should be in the same directory.
# The linking of the final backup file and the unlinking of the temporary file
# are not atomic, but as long as they're done in this order, the only possible
# consequence is that the linking is done but an extraneous temporary file
# remains.
link_to_prev()
{
local bk_file="${1}"
shift 1
ln -s "${prev_bk_file}.xz" "${bk_file}.xz"
rm "${bk_file}~"
return 0
}
# Compress and save a backup file.
save_backup()
{
local db_file="${1}"
shift 1
xz "${bk_file}~"
mv "${bk_file}~.xz" "${bk_file}.xz"
return 0
}
# Save a backup file's name and hash for future comparison and linking.
save_hash()
{
local db_file="${1}"
local bk_file="${2}"
local bk_hash="${3}"
shift 3
printf '%s %s\n' "${bk_hash}" "${bk_file}" 1>"${db_file}.bakhash~"
mv "${db_file}.bakhash~" "${db_file}.bakhash"
return 0
}
usage()
{
printf 'Usage: %s <database-file> <backup-file>\n' "${0}"
return 0
}
main()
{
local db_file=
local bk_file=
local bk_hash=
if [ ${#} -ne 2 ]; then
usage 1>&2
return 1
fi
db_file="${1}"
bk_file="${2}"
shift 2
bk_hash="$(backup "${db_file}" "${bk_file}")"
if compare_hashes "${db_file}" "${bk_hash}"; then
link_to_prev "${bk_file}"
else
save_backup "${bk_file}"
save_hash "${db_file}" "${bk_file}" "${bk_hash}"
fi
return 0
}
main "${@}"
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