rrsync replacement without perl, useful for limited environment like CoreOS

rrysnc.sh

#!/bin/sh

# Copyright (C) 2004 Joe Smith <js-cgi@inwap.com>
# Copyright (C) 2004-2015 Wayne Davison <wayned@samba.org>
# Copyright (C) 2016-2018 YOSHIOKA Takuma <tashioka.256@gmail.com>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# see https://www.samba.org/ftp/unpacked/rsync/support/rrsync for reference
# bash version: https://gist.github.com/lo48576/d120d139e091f2338d35e1de85f9315f

# This script is based on `support/rrsync` perl script in rsync-3.1.2.

USAGE="Use command=\"$0 [-ro|-wo] SUBDIR\"
	in front of lines in ${HOME}/.ssh/authorized_keys"
LOGFILE="${HOME}/logs/rrsync.log"
CMDNAME="$0"

# Log the given message.
die() {
	DATETIME="$(LANG=C date '+%F %T')"
	echo "[$DATETIME] $CMDNAME: $1" >>"$LOGFILE"
	echo >>"$LOGFILE"
	exit 2
}

# Die with the given error message.
debug() {
	DATETIME="$(LANG=C date '+%F %T')"
	echo "[$DATETIME][DEBUG] $1" | tr '\n' '\0' | sed -e 's/\0/\n[DEBUG]'"$DATETIME"'/g' >>"$LOGFILE"
}

#debug ">>>>>>>>>>>>>>>>>>>>>>>>>"
#debug "$SSH_ORIGINAL_COMMAND"

# Get "only" option.
# It would be "w" for `-wo` (write only), "r" for `-ro` (read only), or empty string (neither `-ro` nor `-wo`).
ONLY=""
while [ "x$1" = "x-ro" -o "x$1" = "x-wo" ] ; do
	if [ "x$1" = "x-ro" ] ; then
		if [ "x$ONLY" = "xw" ] ; then
			die "the -ro and -wo options conflict."
		fi
		ONLY="r"
		shift
	else
		if [ "x$ONLY" = "xr" ] ; then
			die "the -ro and -wo options conflict."
		fi
		ONLY="w"
		shift
	fi
done
# Now "$ONLY" has "r", "w" or "".

# Ensure the subdirectory is given.
if [ $# -lt 1 ] ; then
	die "No subdirectory specified.\n${USAGE}"
fi
ALLOWED_DIR="$1"
cd "$ALLOWED_DIR" || die 'Failed to change current directory'

# Ensure the rrsync is invoked via sshd.
if [ -z "$SSH_ORIGINAL_COMMAND" ] ; then
	die "Not invoked via sshd.\n${USAGE}"
fi

# Ensure the rrsync is invoked in server mode via remote rsync.
if ! echo "$SSH_ORIGINAL_COMMAND" | grep -sqE '^rsync\s+--server' ; then
	die "SSH_ORIGINAL_COMMAND='$SSH_ORIGINAL_COMMAND' is not 'rsync --server' or '--server' is not specified first."
fi

# Check whether the rsync server should be sender or receiver.
AM_SENDER=0
if echo "$SSH_ORIGINAL_COMMAND" | grep -sqE '^rsync\s+--server\s+--sender\s' ; then
	AM_SENDER=1
	# The local rsync is sender mode, i.e. files should be sent from local to remote.
	if [ "x$ONLY" = "xw" ] ; then
		die "Sending to read-only server not allowed."
	fi
else
	# The local rsync is receiver mode, i.e. files should be sent from remote to local.
	if [ "x$ONLY" = "xr" ] ; then
		die "Reading to write-only server not allowed."
	fi
fi

# Command line sanity check.
set -f
set -- $SSH_ORIGINAL_COMMAND
# For each argument:
CHECK_TYPE=0
while [ $# -ge 1 ] ; do
	# Collapse continuous slashes in the argument.
	ARG="$(echo "$1" | sed -e 's!///*!/!g')"

	# Ensure the argument has no ".." as path component.
	if echo "$ARG" | grep -sqE '(^(-[a-zA-Z0-9-]+=?)?|/)\.\.(/|$)' ; then
		debug "command line: ${SSH_ORIGINAL_COMMAND}\ndangerous argument: \`$1\`"
		die "Do not use .. in option argument, anchor the path at the root of your restricted dir."
	fi

	# Ensure the argument has no absolute path.
	if echo "$ARG" | grep -sq '^/' ; then
		debug "command line: ${SSH_ORIGINAL_COMMAND}\ndangerous argument: \`$1\`"
		die "Do not use absolute path in argument, anchor the path at the root of your restricted dir."
	fi

	# values copied from https://www.samba.org/ftp/unpacked/rsync/support/rrsync -> $short_no_arg less $short_disabled
	if echo "$ARG" | grep -sqE '^-([ACDEHIJKLORSWXbcdgklmnoprtuvxyz]*s[ACDEHIJKLORSWXbcdgklmnoprtuvxyz]*(e\d*\.\w*)?$)'; then
		debug "command line: ${SSH_ORIGINAL_COMMAND}\ndangerous argument: --protect-args / -s"
		die "--protect-args is not allowed as we need to inspect the target path"
	fi
	shift
done

#debug "$SSH_ORIGINAL_COMMAND"

# Arguments are safer!
# Execute rsync.
exec $SSH_ORIGINAL_COMMAND