Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
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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 |