Create a BusyBox based Emergency Shell

eshell.sh

#!/bin/bash

###########################################################################
### Creates a BusyBox emergency shell in RAM
###
### Script by Jack Zielke <eshell@linuxcoffee.com>
### http://linuxcoffee.com/eshell
###
### BusyBox is maintained by Denys Vlasenko, and licensed under the GNU
### GENERAL PUBLIC LICENSE version 2.
### http://www.busybox.net/
###########################################################################
###
### THIS SOFTWARE IS PROVIDED "AS-IS" WITHOUT WARRANTY OF ANY KIND.
###
###########################################################################
### @version $Id: eshell.sh 258 2012-09-13 15:18:18Z jzielke $
###########################################################################

# Options are ramfs, tmpfs
# tmpfs can be swapped, ramfs will not be swapped
TYPE=ramfs

# These are used for creating and locating the permanent RAM disk
PERM_DIR=/mnt/eshell
PERM_BB=/bin/busybox

echo

make_ramdisk() {
  echo 'Calculating space'
  read SIZE < <("$1" du "$1")
  SIZE="${SIZE%%	$1}"	# That is a tab
  # Provide a few k for wiggle room
  SIZE=$[SIZE+9]
  read ID < <("$1" id -u)
  echo 'Creating '$SIZE'k RAM disk in '$2
  if [ $ID -eq 0 ]; then
    "$1" mount -t "$TYPE" -o size=${SIZE}k,mode=755 "$TYPE" "$2"
  else
     sudo "$1" mount -t "$TYPE" -o size=${SIZE}k,mode=755,uid=$ID "$TYPE" "$2"
     if [ "$TYPE" == "ramfs" ]; then
       sudo "$1" chown "$ID" "$2"
     fi
  fi
}

make_env() {
  "$1" cp "$1" "$2/busybox"
  echo 'Creating .profile'
  echo alias rootsh=\'sudo env PATH=\"\$PATH\" busybox ash\' >"$2/.profile"
  echo alias >>"$2/.profile"
  echo echo \'exit to cleanup\' >>"$2/.profile"
  echo echo \'rootsh for a root shell\' >>"$2/.profile"
  echo echo >>"$2/.profile"
  echo 'Building symlinks'
  cd "$2"
  { { ./busybox --list 2>/dev/null; } || { ./busybox --help | ./busybox sed -n '/Currently defined functions:/,$p' | ./busybox grep -v 'Currently defined functions:' | ./busybox tr -d ' \t\n' | ./busybox tr , '\n' | ./busybox grep -v 'busybox'; }; } | ./busybox xargs -n 1 ./busybox ln -s busybox
}

setup_eshell() {
  # Check if RAM disk is already mounted
  read OUTPUT < <($PERM_BB grep -- "$PERM_DIR" /proc/mounts)
  if [ -z "$OUTPUT" ]; then
    echo 'Creating eshell RAM disk'
    "$PERM_BB" mkdir -p "$PERM_DIR"
    make_ramdisk "$PERM_BB" "$PERM_DIR"
  fi
  make_env "$PERM_BB" "$PERM_DIR"
}

check_tty () {
  echo 'Checking for tty'
  "$1" tty >/dev/null
  if [ $? -ne 0 ]; then
    echo 'No tty found, did you mean to ssh -t ?'
    echo 'Press ctrl-c to stop or enter to continue'
    read JUNK
  fi
}

check_fallback() {
  read OUTPUT < <("$1" true 2>&1 >&-)
  if [ "$OUTPUT" == 'Using fallback suid method' ]; then
    echo 'To suppress "Using fallback suid method" warnings, as root:'
    echo 'touch /etc/busybox.conf'
    echo
  fi
}

run_eshell() {
  check_fallback "$1/busybox"
  cd /
  export PATH="$1:$PATH"
  echo 'Starting shell'
  ENV="$1/.profile" "$1/busybox" ash
  echo
}

show_help() {
echo 'eshell creates a BusyBox emergency shell in RAM.
There are 3 ways to use eshell.
1) eshell
	Without any parameters:
	This will set your PATH and run busybox ash as your shell.
	If a permanent RAM disk was previously setup it will be used.
	sudo access is required if the permanent RAM disk is not already set up.
	sudo is not part of busybox.  Run rootsh to sudo to root and retain
	the eshell PATH.
2) eshell <busybox>
	With the path to busybox as the parameter:
	This is useful if you have downloaded busybox and it is not in your path
	or you want to use a binary other than the first busybox in your path.
	If the path to busybox is provided the permanent shell will not be used.
3) eshell setup
	This will setup eshell in '$PERM_DIR' for later use.
	Run eshell setup from your system startup scripts.
'
  check_fallback busybox
}

if [ "$1" == '--help' -o "$1" == '-h' ]; then
  show_help
  exit
elif [ "$1" == "setup" ]; then
  setup_eshell
  exit
elif [ $# -eq 0 -a -e "$PERM_DIR/busybox" ]; then
  echo 'Using permanent RAM disk in '$PERM_DIR
  check_tty "$PERM_DIR/busybox"
  run_eshell "$PERM_DIR"
  exit
elif [ $# -eq 0 ]; then
  echo 'Locating busybox'
  read BB < <(command -v busybox)
else
  echo 'Using '$*' for busybox binary'
  BB="$*"
fi

if [ -n "$BB" ]; then
  # Should check that $BB is actually a busybox binary
  check_tty "$BB"
  echo 'Creating temp directory'
  read TMPDIR < <($BB mktemp -td 2>/dev/null || $BB mktemp -d /tmp/tmp.XXXXXX)
  make_ramdisk "$BB" "$TMPDIR"
  make_env "$BB" "$TMPDIR"
  run_eshell "$TMPDIR"
  echo 'Cleaning up'
  sudo "$BB" umount "$TMPDIR"
  "$BB" rmdir "$TMPDIR"
else
  echo >&2
  echo 'Error: Could not find busybox in '$PATH >&2
  echo 'Rerun '$0' providing the path to busybox as an argument' >&2
  echo 'Use -h or --help for help' >&2
  echo >&2
  exit 1
fi

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