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#!/bin/bash
# Cleanup, version 3

# Warning:
# -------
# This script uses quite a number of features that will be explained
#+ later on.
# By the tim you've finished the first half of the book,
#+ there should be nothing mysterious about it.

LOG_DIR=/var/log
ROOT_UID=0     # Only users with $UID 0 have root privileges.
LINES=50       # Default number of lines saved.
E_XCD=66       # Can't change directory?
E_NOTROOT=67   # Non-root exit error.

# Run as root, of course.
if [ "$UID" -ne "$ROOT_UID" ]
then
  echo "Must be root to run this script."
  exit $E_NOTROOT
fi


if [ -n "$1" ]
# Test if command line argument present (non-empty).
then
  lines=$1
else
  lines=$LINES  # Default, if not specified on command line.
fi

cd $LOG_DIR

if [ `pwd` != "$LOG_DIR" ]     # or   if [ "$PWD" != "$LOG_DIR" ]
                              # Not in /var/log?
then
  echo "Can't change to $LOG_DIR."
  exit $E_XCD
fi     # Doublecheck if in right directory, before messing with log file.

tail -$lines messages > mesg.temp     # Saves last section of message log file.
mv mesg.temp messages     # Become new log directory.

cat /dev/null > wtmp     # ': > wtmp' and '> wtmp' have the same effect.
echo "Logs cleaned up."

exit 0
# A zero return value from the script upon exit
#+ indicates success to the shell.