diff --git a/ChangeLog b/ChangeLog
index 7b348b2..9e22a9d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+btrbk-current
+
+  * ssh_filter_btrbk.sh uses new exit statuses on failures (1 when the
+    SSH command was rejected, 2 when the program’s arguments could not
+    be parsed).
+  * ssh_filter_btrbk.sh’s logging output format has changed slightly.
+
 btrbk-0.32.5
 
   * Correct handling of zero-size raw info file (close #491).
diff --git a/ssh_filter_btrbk.sh b/ssh_filter_btrbk.sh
index d4ea193..ef97c02 100755
--- a/ssh_filter_btrbk.sh
+++ b/ssh_filter_btrbk.sh
@@ -1,9 +1,12 @@
-#!/bin/bash
+#!/bin/sh
 
-set -e
-set -u
 
-export PATH=/sbin:/bin:/usr/sbin:/usr/bin
+# initialise and sanitise the shell execution environment
+unset -v IFS
+export LC_ALL=C
+export PATH='/usr/bin:/bin'
+
+set -e -u
 
 enable_log=
 restrict_path_list=
@@ -12,48 +15,86 @@ allow_exact_list=
 allow_rate_limit=1
 allow_stream_buffer=1
 allow_compress=1
-compress_list="gzip|pigz|bzip2|pbzip2|bzip3|xz|lzop|lz4|zstd"
+compress_list='gzip|pigz|bzip2|pbzip2|bzip3|xz|lzop|lz4|zstd'
 
 # note that the backslash is NOT a metacharacter in a POSIX bracket expression!
 option_match='-[a-zA-Z0-9=-]+'   # matches short as well as long options
-file_match_sane='/[0-9a-zA-Z_@+./-]*' # matches file path (equal to $file_match in btrbk < 0.32.0)
+file_match_sane='/[0-9a-zA-Z_@+./-]*' # matches file path (equal to ${file_match} in btrbk < 0.32.0)
 file_match="/[^']*" # btrbk >= 0.32.0 quotes file arguments: match all but single quote
 file_arg_match="('${file_match}'|${file_match_sane})" # support btrbk < 0.32.0
 
+is_pathname_absolute()
+{
+    # Checks whether a string is an absolute pathname (that is: one that is non-
+    # empty and starts with either exactly one or more than two `/`).
+
+    local pathname="$1"
+
+    [ "${pathname}" != '//' ] || return 1
+    [ -n "${pathname##//[!/]*}" ] || return 1
+    [ -z "${pathname##/*}" ] || return 1
+    [ -n "${pathname}" ] || return 1
+
+    return 0
+}
+
+print_normalised_pathname()
+{
+    # Normalises a pathname given via the positional parameter #1 as follows:
+    # * Folds any >=3 leading `/` into 1.
+    #   POSIX specifies that implementations may treat exactly 2 leading `//`
+    #   specially and therefore such are not folded here.
+    # * Folds any >=2 non-leading `/` into 1.
+    # * Strips any trailing `/`.
+
+    local pathname="$1"
+
+    printf '%s' "${pathname}" | sed -E 's%^///+%/%; s%(.)//+%\1/%g; s%/+$%%'
+}
+
 log_cmd()
 {
-    if [[ -n "$enable_log" ]]; then
-        logger -p $1 -t ssh_filter_btrbk.sh "$2 (Name: ${LOGNAME:-<unknown>}; Remote: ${SSH_CLIENT:-<unknown>})${3:+: $3}: $SSH_ORIGINAL_COMMAND"
+    local priority="$1"
+    local authorisation_decision="$2"
+    local reason="${3-}"
+
+    if [ -n "${enable_log}" ]; then
+        logger -p "${priority}" -t ssh_filter_btrbk.sh "${authorisation_decision} (Name: ${LOGNAME:-<unknown>}; Connection: ${SSH_CONNECTION:-<unknown>})${reason:+: ${reason}}: ${SSH_ORIGINAL_COMMAND}"
     fi
 }
 
 allow_cmd()
 {
-    allow_list="${allow_list}|$1"
+    local cmd="$1"
+
+    allow_list="${allow_list}|${cmd}"
 }
 
 allow_exact_cmd()
 {
-    allow_exact_list="${allow_exact_list}|$1"
+    local cmd="$1"
+
+    allow_exact_list="${allow_exact_list}|${cmd}"
 }
 
 reject_and_die()
 {
-    local reason=$1
-    log_cmd "auth.err" "btrbk REJECT" "$reason"
-    echo "ERROR: ssh_filter_btrbk.sh: ssh command rejected: $reason: $SSH_ORIGINAL_COMMAND" 1>&2
-    exit 255
+    local reason="$1"
+
+    log_cmd 'auth.err' 'btrbk REJECT' "${reason}"
+    printf 'ERROR: ssh_filter_btrbk.sh: ssh command rejected: %s: %s\n' "${reason}" "${SSH_ORIGINAL_COMMAND}" >&2
+    exit 1
 }
 
 run_cmd()
 {
-    log_cmd "auth.info" "btrbk ACCEPT"
-    eval " $SSH_ORIGINAL_COMMAND"
+    log_cmd 'auth.info' 'btrbk ACCEPT'
+    eval " ${SSH_ORIGINAL_COMMAND}"
 }
 
 reject_filtered_cmd()
 {
-    if [[ -n "$restrict_path_list" ]]; then
+    if [ -n "${restrict_path_list}" ]; then
 	# match any of restrict_path_list,
 	# or any file/directory (matching file_match) below restrict_path
 	path_match="'(${restrict_path_list})(${file_match})?'"
@@ -66,7 +107,7 @@ reject_filtered_cmd()
     # btrbk >= 0.32.0 quotes files, allow both (legacy)
     path_match="(${path_match}|${path_match_legacy})"
 
-    if [[ -n "$allow_compress" ]]; then
+    if [ -n "${allow_compress}" ]; then
         decompress_match="(${compress_list}) -d -c( -[pT][0-9]+)?"
         compress_match="(${compress_list}) -c( -[0-9])?( -[pT][0-9]+)?"
     else
@@ -76,8 +117,8 @@ reject_filtered_cmd()
 
     # rate_limit_remote and stream_buffer_remote use combined
     # "mbuffer" as of btrbk-0.29.0
-    if [[ -n "$allow_stream_buffer" ]] || [[ -n "$allow_rate_limit" ]]; then
-        mbuffer_match="mbuffer -v 1 -q( -s [0-9]+[kmgKMG]?)?( -m [0-9]+[kmgKMG]?)?( -[rR] [0-9]+[kmgtKMGT]?)?"
+    if [ -n "${allow_stream_buffer}" ] || [ -n "${allow_rate_limit}" ]; then
+        mbuffer_match='mbuffer -v 1 -q( -s [0-9]+[kmgKMG]?)?( -m [0-9]+[kmgKMG]?)?( -[rR] [0-9]+[kmgtKMGT]?)?'
     else
         mbuffer_match=
     fi
@@ -87,31 +128,39 @@ reject_filtered_cmd()
     stream_in_match="(${decompress_match} \| )?(${mbuffer_match} \| )?"
     stream_out_match="( \| ${mbuffer_match})?( \| ${compress_match}$)?"
 
+    # `grep`’s `-q`-option is not used as it may cause an exit status of `0` even
+    # when an error occurred.
+
     allow_stream_match="^${stream_in_match}${allow_cmd_match}${stream_out_match}"
-    if [[ $SSH_ORIGINAL_COMMAND =~ $allow_stream_match ]] ; then
+    if printf '%s' "${SSH_ORIGINAL_COMMAND}" | grep -E "${allow_stream_match}" >/dev/null 2>/dev/null; then
         return 0
     fi
 
     exact_cmd_match="^(${allow_exact_list})$";
-    if [[ $SSH_ORIGINAL_COMMAND =~ $exact_cmd_match ]] ; then
+    if printf '%s' "${SSH_ORIGINAL_COMMAND}" | grep -E "${exact_cmd_match}" >/dev/null 2>/dev/null; then
         return 0
     fi
 
-    reject_and_die "disallowed command${restrict_path_list:+ (restrict-path: \"${restrict_path_list//|/\", \"}\")}"
+    local formatted_restrict_path_list="$(printf '%s' "${restrict_path_list}" | sed 's/|/", "/g')"
+    reject_and_die "disallowed command${restrict_path_list:+ (restrict-path: \"${formatted_restrict_path_list}\")}"
 }
 
 
 # check for "--sudo" option before processing other options
 sudo_prefix=
-for key; do
-    [[ "$key" == "--sudo" ]] && sudo_prefix="sudo -n "
-    [[ "$key" == "--doas" ]] && sudo_prefix="doas -n "
+for key in "$@"; do
+    if [ "${key}" = '--sudo' ]; then
+        sudo_prefix='sudo -n '
+    fi
+    if [ "${key}" = '--doas' ]; then
+        sudo_prefix='doas -n '
+    fi
 done
 
-while [[ "$#" -ge 1 ]]; do
+while [ "$#" -ge 1 ]; do
     key="$1"
 
-    case $key in
+    case "${key}" in
       -l|--log)
           enable_log=1
           ;;
@@ -121,7 +170,12 @@ while [[ "$#" -ge 1 ]]; do
           ;;
 
       -p|--restrict-path)
-          restrict_path_list="${restrict_path_list}|${2%/}"  # add to list while removing trailing slash
+          # check whether the pathname is absolute
+          if ! is_pathname_absolute "$2"; then
+              reject_and_die "pathname \"$2\" given to the \"--restrict-path\"-option is not absolute"
+          fi
+
+          restrict_path_list="${restrict_path_list}|$(print_normalised_pathname "$2")"
           shift # past argument
           ;;
 
@@ -161,8 +215,8 @@ while [[ "$#" -ge 1 ]]; do
           ;;
 
       *)
-          echo "ERROR: ssh_filter_btrbk.sh: failed to parse command line option: $key" 1>&2
-          exit 255
+          printf 'ERROR: ssh_filter_btrbk.sh: failed to parse command line option: %s\n' "${key}" >&2
+          exit 2
           ;;
     esac
     shift
@@ -173,15 +227,15 @@ done
 allow_exact_cmd "${sudo_prefix}btrfs subvolume (show|list)( ${option_match})* ${file_arg_match}";
 allow_cmd "${sudo_prefix}readlink"                    # resolve symlink
 allow_exact_cmd "${sudo_prefix}test -d ${file_arg_match}" # check directory (only for compat=busybox)
-allow_exact_cmd "cat /proc/self/mountinfo"            # resolve mountpoints
-allow_exact_cmd "cat /proc/self/mounts"               # legacy, for btrbk < 0.27.0
+allow_exact_cmd 'cat /proc/self/mountinfo'            # resolve mountpoints
+allow_exact_cmd 'cat /proc/self/mounts'               # legacy, for btrbk < 0.27.0
 
 # remove leading "|" on alternation lists
-allow_list=${allow_list#\|}
-allow_exact_list=${allow_exact_list#\|}
-restrict_path_list=${restrict_path_list#\|}
+allow_list="${allow_list#\|}"
+allow_exact_list="${allow_exact_list#\|}"
+restrict_path_list="${restrict_path_list#\|}"
 
-case "$SSH_ORIGINAL_COMMAND" in
+case "${SSH_ORIGINAL_COMMAND}" in
     *\.\./*)  reject_and_die 'directory traversal'  ;;
     *'
 '*)           reject_and_die 'unsafe character LF'  ;;
@@ -193,7 +247,7 @@ case "$SSH_ORIGINAL_COMMAND" in
     *\<*)     reject_and_die 'unsafe character "<"' ;;
     *\>*)     reject_and_die 'unsafe character ">"' ;;
     *\`*)     reject_and_die 'unsafe character "`"' ;;
-    *\|*)     [[ -n "$allow_compress" ]] || [[ -n "$allow_rate_limit" ]] || [[ -n "$allow_stream_buffer" ]] || reject_and_die 'unsafe character "|"' ;;
+    *\|*)     [ -n "${allow_compress}" ] || [ -n "${allow_rate_limit}" ] || [ -n "${allow_stream_buffer}" ] || reject_and_die 'unsafe character "|"' ;;
 esac
 
 reject_filtered_cmd