The Code

#!/bin/sh

# backup - Creates either a full or incremental backup of a set of
#     defined directories on the system. By default, the output
#     file is saved in /tmp with a timestamped filename, compressed.
#     Otherwise, specify an output device (another disk, or a
#     removable storage device).

usageQuit()
{
  cat << "EOF" >&2
Usage: $0 [-o output] [-i|-f] [-n]
  -o lets you specify an alternative backup file/device
  -i is an incremental or -f is a full backup, and -n prevents
  updating the timestamp if an incremental backup is done.
EOF
   exit 1
}

compress="bzip2"                # change for your favorite compression app
inclist="/tmp/backup.inclist.$(date +%d%m%y)"
 output="/tmp/backup.$(date +%d%m%y).bz2"
 tsfile="$HOME/.backup.timestamp"
  btype="incremental"           # default to an incremental backup
  noinc=0                       # and an update of the timestamp

trap "/bin/rm -f $inclist" EXIT

while getopts "o:ifn" arg; do
  case "$arg" in
    o ) output="$OPTARG";        ;;
    i ) btype="incremental";     ;;
    f ) btype="full";            ;;
    n ) noinc=1;                 ;;
    ? ) usageQuit                ;;
  esac
done

shift $(($OPTIND - 1))

echo "Doing $btype backup, saving output to $output"

timestamp="$(date +'%m%d%I%M')"

if [ "$btype" = "incremental" ] ; then
  if [ ! -f $tsfile ] ; then
    echo "Error: can't do an incremental backup: no timestamp file" >&2
    exit 1
  fi
  find $HOME -depth -type f -newer $tsfile -user ${USER:-LOGNAME} | \
    pax -w -x tar | $compress > $output
  failure="$?"
else
  find $HOME -depth -type f -user ${USER:-LOGNAME} | \
    pax -w -x tar | $compress > $output
  failure="$?"
fi

if [ "$noinc" = "0" -a "$failure" = "0" ] ; then
  touch -t $timestamp $tsfile
fi
exit 0

How It Works

For a full system backup, the pax command does all the work, piping its output to a compression program (bzip2 by default) and then to an output file or device. An incremental backup is a bit more tricky because the standard version of tar doesn't include any sort of modification time test, unlike the GNU version of tar. The list of files modified since the previous backup is built with find and saved in the inclist temporary file. That file, emulating the tar output format for increased portability, is then fed to pax directly.

Choosing when to mark the timestamp for a backup is an area in which many backup programs get messed up, typically marking the "last backup time" when the program has finished the backup, rather than when it started. Setting the timestamp to the time of backup completion can be a problem if any files are modified during the backup process (which can take quite a while if the backup is being fed to a tape device). Because files modified under this scenario would have a last-modified date older than the timestamp date, they would not be backed up the next night.

However, timestamping before the backup takes place is wrong too, because if the backup fails, there's no way to reverse the updated timestamp. Both of these problems are avoided by saving the date and time before the backup starts (in the timestamp variable), but applying the value of $timestamp to $tsfile using the -t flag to touch only after the backup has succeeded.

Running the Script

This script has a number of options, all of which can be ignored to perform the default incremental backup based on the timestamp for the last incremental backup. The flags allow you to specify a different output file or device (-o output), to choose a full backup (-f), to actively choose an incremental backup (-i), or to prevent the timestamp file from being updated in the case of an incremental backup (-n).

The Results

$ backup
Doing incremental backup, saving output to /tmp/backup.140703.bz2

As you would expect, the output of a backup program isn't very scintillating. But the resulting compressed file is sufficiently large that it shows plenty of data is within:

$ ls -l /tmp/backup*
-rw-r--r--  1 taylor  wheel  61739008 Jul 14 07:31 backup.140703.bz2