Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
16.3. Applications
Clever use of I/O redirection permits parsing and stitching together snippets of command output (see Example
11−6). This permits generating report and log files.
Example 16−11. Logging events
#!/bin/bash
# logevents.sh, by Stephane Chazelas.
# Event logging to a file.
# Must be run as root (for write access in /var/log).
ROOT_UID=0
E_NOTROOT=67
# Only users with $UID 0 have root privileges.
# Non−root exit error.
if [ "$UID" −ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
FD_DEBUG1=3
FD_DEBUG2=4
FD_DEBUG3=5
# Uncomment one of the two lines below to activate script.
# LOG_EVENTS=1
# LOG_VARS=1
log() # Writes time and date to log file.
{
echo "$(date) $*" >&7 # This *appends* the date to the file.
# See below.
}
case $LOG_LEVEL in
1) exec 3>&2 4> /dev/null 5> /dev/null;;
2) exec 3>&2 4>&2 5> /dev/null;;
3) exec 3>&2 4>&2 5>&2;;
*) exec 3> /dev/null 4> /dev/null 5> /dev/null;;
esac
FD_LOGVARS=6
if [[ $LOG_VARS ]]
then exec 6>> /var/log/vars.log
else exec 6> /dev/null
fi
# Bury output.
FD_LOGEVENTS=7
if [[ $LOG_EVENTS ]]
then
Chapter 16. I/O Redirection 251