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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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>Chapter 20. I/O Redirection</TD
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>20.3. Applications</H1
><P
>Clever use of I/O redirection permits parsing and stitching
	together snippets of command output (see <A
HREF="internal.html#READREDIR"
>Example 15-7</A
>). This permits
	 generating report and log files.</P
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>Example 20-12. Logging events</B
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>   1&nbsp;#!/bin/bash
   2&nbsp;# logevents.sh
   3&nbsp;# Author: Stephane Chazelas.
   4&nbsp;# Used in ABS Guide with permission.
   5&nbsp;
   6&nbsp;# Event logging to a file.
   7&nbsp;# Must be run as root (for write access in /var/log).
   8&nbsp;
   9&nbsp;ROOT_UID=0     # Only users with $UID 0 have root privileges.
  10&nbsp;E_NOTROOT=67   # Non-root exit error.
  11&nbsp;
  12&nbsp;
  13&nbsp;if [ "$UID" -ne "$ROOT_UID" ]
  14&nbsp;then
  15&nbsp;  echo "Must be root to run this script."
  16&nbsp;  exit $E_NOTROOT
  17&nbsp;fi  
  18&nbsp;
  19&nbsp;
  20&nbsp;FD_DEBUG1=3
  21&nbsp;FD_DEBUG2=4
  22&nbsp;FD_DEBUG3=5
  23&nbsp;
  24&nbsp;# === Uncomment one of the two lines below to activate script. ===
  25&nbsp;# LOG_EVENTS=1
  26&nbsp;# LOG_VARS=1
  27&nbsp;
  28&nbsp;
  29&nbsp;log()  # Writes time and date to log file.
  30&nbsp;{
  31&nbsp;echo "$(date)  $*" &#62;&#38;7     # This *appends* the date to the file.
  32&nbsp;#     ^^^^^^^  command substitution
  33&nbsp;                           # See below.
  34&nbsp;}
  35&nbsp;
  36&nbsp;
  37&nbsp;
  38&nbsp;case $LOG_LEVEL in
  39&nbsp; 1) exec 3&#62;&#38;2         4&#62; /dev/null 5&#62; /dev/null;;
  40&nbsp; 2) exec 3&#62;&#38;2         4&#62;&#38;2         5&#62; /dev/null;;
  41&nbsp; 3) exec 3&#62;&#38;2         4&#62;&#38;2         5&#62;&#38;2;;
  42&nbsp; *) exec 3&#62; /dev/null 4&#62; /dev/null 5&#62; /dev/null;;
  43&nbsp;esac
  44&nbsp;
  45&nbsp;FD_LOGVARS=6
  46&nbsp;if [[ $LOG_VARS ]]
  47&nbsp;then exec 6&#62;&#62; /var/log/vars.log
  48&nbsp;else exec 6&#62; /dev/null                     # Bury output.
  49&nbsp;fi
  50&nbsp;
  51&nbsp;FD_LOGEVENTS=7
  52&nbsp;if [[ $LOG_EVENTS ]]
  53&nbsp;then
  54&nbsp;  # exec 7 &#62;(exec gawk '{print strftime(), $0}' &#62;&#62; /var/log/event.log)
  55&nbsp;  # Above line fails in versions of Bash more recent than 2.04. Why?
  56&nbsp;  exec 7&#62;&#62; /var/log/event.log              # Append to "event.log".
  57&nbsp;  log                                      # Write time and date.
  58&nbsp;else exec 7&#62; /dev/null                     # Bury output.
  59&nbsp;fi
  60&nbsp;
  61&nbsp;echo "DEBUG3: beginning" &#62;&#38;${FD_DEBUG3}
  62&nbsp;
  63&nbsp;ls -l &#62;&#38;5 2&#62;&#38;4                             # command1 &#62;&#38;5 2&#62;&#38;4
  64&nbsp;
  65&nbsp;echo "Done"                                # command2 
  66&nbsp;
  67&nbsp;echo "sending mail" &#62;&#38;${FD_LOGEVENTS}
  68&nbsp;# Writes "sending mail" to file descriptor #7.
  69&nbsp;
  70&nbsp;
  71&nbsp;exit 0</PRE
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