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