From 9ea54ae09053d94cad41724c806633c38cb90ae4 Mon Sep 17 00:00:00 2001
From: Gabriel Scherer <gabriel.scherer@gmail.com>
Date: Sat, 24 Mar 2018 18:40:46 +0100
Subject: [PATCH] a backup script to send backups over email when changed

This is a home-grown backup script that I built for a local HotCRP
installation, but I thought that it may be useful to others as
well. The backup strategy is as follows:

- a backups/ directory at the project root with at most two backups saved,
  the last one (`new`) and the one before that (`old`)

- whenever the script runs, it moves `new` to `old` and creates a new
  `new` file using the `backupdb.sh` script; then it compares `new`
  and `old`, and sends the new backup by email if it differs from the
  old one

- if a `passphrase` file exists in the `backups` directory, then
  it is used as a passphrase for `gpg` to encrypt the backup file

Finally, note that we are careful to pass `--skip-dump-date` to
`mysqldump`, otherwise the backups would always differ.

The script is careful to `cd` to its base directory, so that it can be
invoked directly from a crontab.
---
 lib/mail-backup-if-new.sh | 79 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)
 create mode 100644 lib/mail-backup-if-new.sh

diff --git a/lib/mail-backup-if-new.sh b/lib/mail-backup-if-new.sh
new file mode 100644
index 0000000000..62b6778364
--- /dev/null
+++ b/lib/mail-backup-if-new.sh
@@ -0,0 +1,79 @@
+#!/bin/bash
+
+# backups/ has special 'new' and 'old' symbolic links to the last
+# backup and the one-before-last backup.  We use them to check whether
+# the new backup is identical to the previous backup. If it isn't, we
+# send an email with the backup.
+
+# If a backups/passphrase file exists, its content is used as a
+# passphrase to encrypt the backup file before sending it by email.
+
+# we assume the script is in the 'lib/' directory of a HotCRP install,
+# move to the root of the project before running the code
+cd "$( dirname "${BASH_SOURCE[0]}" )"/..
+
+BACKUP=backup-$(date +%F-%T).txt
+PASSPHRASE_FILE=passphrase
+
+if [ -z "$BACKUP_EMAIL" ]
+then
+   echo "You must define a BACKUP_EMAIL environment variable"
+   echo "with the email address to send backups to."
+   exit 2
+fi
+
+dump () {
+  # --skip-dump-date is necessary, otherwise a timestamp
+  # is included in the backup and the files always differ
+  ./lib/backupdb.sh --skip-dump-date > backups/$BACKUP
+}
+
+# assumes that 'dump ()' ran and that we are in 'backups'
+init () {
+  ln -s old $BACKUP
+  ln -s new $BACKUP
+}
+
+# assumes that 'dump ()' ran and that we are in 'backups'
+age () {
+  rm $(readlink old)
+  mv new old
+  ln -s $BACKUP new
+}
+
+email () {
+  echo "New backup $(readlink new) differs from $(readlink old), so it is attached" \
+    | mailx -s "[hotcrp-mlocaml2017-postproceedings] new backup" -a $BACKUP_FILE $BACKUP_EMAIL
+}
+
+# assumes that 'dump ()' ran and that we are in 'backups'
+email_if_changed () {
+  echo "comparing the new backup $BACKUP and the previous one $(readlink old)"
+  cmp new old && exit 0
+  
+  if [[ -f $PASSPHRASE_FILE ]]
+  then
+      echo "encrypting the backup according to backups/$PASSPHRASE_FILE"
+      gpg --batch --passphrase-file $PASSPHRASE_FILE --symmetric $BACKUP
+      BACKUP_FILE=$BACKUP.gpg
+      email
+      rm $BACKUP_FILE
+  else
+      BACKUP_FILE=$BACKUP
+      email
+  fi
+}
+
+mkdir_if_missing () {
+ if [[ ! -d backups ]]
+ then
+     mkdir -p backups
+     init
+ fi
+}
+
+dump
+cd backups
+age
+email_if_changed
+