A backup script

backup.sh

#!/bin/sh

set -e

test "$#" -ge 2 || {
	echo >&2 'usage: backup DIR DEST [ROOT]'
	echo >&2 '  Make a backup of DIR in DEST/DATE/PATH, where:'
	echo >&2 "    * DATE is the today's date in the format YYYY-MM-DD."
	echo >&2 '    * PATH is the path to DIR relative to ROOT (ROOT defaults to /).'
	echo >&2
	echo >&2 '  If there are previous backups in DEST, unmodified files will be'
	echo >&2 '  hard linked to the latest backup.'
	exit 1
}

dest="$2"
today="$(date -I)"
root="${3:-/}"
dir="$(realpath --relative-base="$root" "$1")"


# Find the last backup of $dir, to hard link if possible.
for backup in "$dest"/[0-9][0-9][0-9][0-9]-[0-1][0-9]-[0-3][0-9]; do
	test -d "$backup/$dir" && test "$(basename "$backup")" != "$today" && {
		last=$backup
	}
done

test -n "$last" && {
	echo 'Hard linking last backup found at '"$last/$dir"
	mkdir -p "$dest/$today/$dir"
	cp -alTn "$last/$dir" "$dest/$today/$dir"
}

test -n "$last" || {
	echo 'No previous backups of '"$dir"
}


echo 'Syncing...'
rsync -aRF --delete-excluded --delete --info=progress2 "$root/./$dir" "$dest/$today"


# Notes

# Copying $src to $dest might mean:
#   - "make $dest = $src" or "copy to"
#   - "make $dest/$src = $src" or "copy into"

# cp will "copy into" if the destination exists and is a directory.
# Otherwise it will "copy to" it, even overwriting the target if is is a regular file.
# The -T flag says to always "copy to".

# rsync will "copy to" if the source argument has a trailing slash.
# If it doesn't rsync will "copy into".