cleanup_3.sh

#!/bin/bash

# Cleanup, version 3.

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

LOG_DIR=/var/log

ROOT_UID=0      # Only users with $UID 0 have root priveleges.
LINES=50        # Default number of lines saved.
E_XCD=86        # Can't change directory?
E_NOTROOT=87    # 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 whether command-line argument is present (non-empty).
then
    lines=$1
else
    lines=$LINES # Default, if not specified on command-line.
fi

# Stephane Chazelas suggests the following,
#+ as a better way of checking command-line arguments,
#+ but this is still a bit advalnced for this stage of the tutorial.
#
#
#
#   E_WRONGARGs=85 # Non-numerical argument (bad argument format).
#   
#   case "$1" in 
#   ""      ) lines=50;;
#   *[!0-9]*) echo "Usage: `basename $0` lines-to-cleanup";
#   exit $E_WRONGARGS;;
#   *       ) lines=$1;;
#   esac
#
#* Skip ahead to "Loops" chapter to decipher all this.


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.

# Far more efficient is:
#
# cd /var/log || {
#   echo "Connot change to necessary directory." >$2
#   exit $E_XCD;
#}

tail -n $lines messages > mesg.temp # Save last seccion of message log file.
mv mesg.temp messages               # Rename it as system log file.

# cat /dev/null > messages
#* No longer needed, as the above method is safer.

cat /dev/null > wtmp  # ': > wtmp' and '> wtmp' have the same effect.

echo "Log files cleaned up."

# Note that there are other log files in /var/log not affected
#+ by this script.

exit 0

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