| Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
|---|---|---|
| Prev | Chapter 16. I/O Redirection | Next |
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 16-12. Logging events
1 #!/bin/bash
2 # logevents.sh, by Stephane Chazelas.
3
4 # Event logging to a file.
5 # Must be run as root (for write access in /var/log).
6
7 ROOT_UID=0 # Only users with $UID 0 have root privileges.
8 E_NOTROOT=67 # Non-root exit error.
9
10
11 if [ "$UID" -ne "$ROOT_UID" ]
12 then
13 echo "Must be root to run this script."
14 exit $E_NOTROOT
15 fi
16
17
18 FD_DEBUG1=3
19 FD_DEBUG2=4
20 FD_DEBUG3=5
21
22 # Uncomment one of the two lines below to activate script.
23 # LOG_EVENTS=1
24 # LOG_VARS=1
25
26
27 log() # Writes time and date to log file.
28 {
29 echo "$(date) $*" >&7 # This *appends* the date to the file.
30 # See below.
31 }
32
33
34
35 case $LOG_LEVEL in
36 1) exec 3>&2 4> /dev/null 5> /dev/null;;
37 2) exec 3>&2 4>&2 5> /dev/null;;
38 3) exec 3>&2 4>&2 5>&2;;
39 *) exec 3> /dev/null 4> /dev/null 5> /dev/null;;
40 esac
41
42 FD_LOGVARS=6
43 if [[ $LOG_VARS ]]
44 then exec 6>> /var/log/vars.log
45 else exec 6> /dev/null # Bury output.
46 fi
47
48 FD_LOGEVENTS=7
49 if [[ $LOG_EVENTS ]]
50 then
51 # then exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log)
52 # Above line will not work in Bash, version 2.04.
53 exec 7>> /var/log/event.log # Append to "event.log".
54 log # Write time and date.
55 else exec 7> /dev/null # Bury output.
56 fi
57
58 echo "DEBUG3: beginning" >&${FD_DEBUG3}
59
60 ls -l >&5 2>&4 # command1 >&5 2>&4
61
62 echo "Done" # command2
63
64 echo "sending mail" >&${FD_LOGEVENTS} # Writes "sending mail" to fd #7.
65
66
67 exit 0 |