From 0cd64b92465be64e1fb55acf407830a97b2d4af5 Mon Sep 17 00:00:00 2001
From: P. J. McDermott <pj@pehjota.net>
Date: Tue, 22 May 2018 18:02:14 -0400
Subject: bin/bakdb: New script

---
(limited to 'bin/bakdb')

diff --git a/bin/bakdb b/bin/bakdb
new file mode 100755
index 0000000..7ab7c8c
--- /dev/null
+++ b/bin/bakdb
@@ -0,0 +1,113 @@
+#!/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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