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#!/bin/bash
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# Author: Ben Wong <ben@wongs.net>
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# Created on: <2001-08-08 11:56:03 hackerb9>
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# Time-stamp: <2002-11-20 14:13:11 bbb>
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# onlogdo: a hack to handle a common problem: when a certain message
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# shows up in the log file, an action should be performed. E.g.: when
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# a PC-card network device is inserted, it should be ifconfig’d or
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# perhaps dhclient should be run. This program can do that in a fairly
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# reasonable way.
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### FIRST, THE HELPER FUNCTIONS ###
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function showmanpage () {
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cat <<‘EOF’
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NAME
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onlogdo – monitor a log file and execute commands based on patterns
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SYNOPSIS
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onlogdo <logfile> <pattern> <command>
[ <pattern> <command> …
]
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logfile: a file that gets appended to (e.g., /var/log/messages),
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pattern: a string (can use bash‘s extended pathname globbing syntax),
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command: the command to run when the previous pattern is seen.
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DESCRIPTION
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Onlogdo is a hack to handle a common problem: when a certain
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message shows up in a log file, an action should be performed.
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E.g.: when a PC-card network device is inserted, it should be
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ifconfig’d or perhaps dhclient should be run. This program can do
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that in a fairly reasonable way.
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Onlogdo continuously reads lines as they are appended to a log
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file. When a line matches a given pattern, the command associated
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with that pattern is run in the background. The patterns
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recognized are standard pathname globbing (*, ?, []) plus bash‘s
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extended pattern matching (extglob) syntax (see bash(1)).
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The log file is reopened properly, if the log file being read is
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rotated. Also, since it blocks when waiting for new lines, onlogdo
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takes up almost no CPU time.
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PATTERN MATCHING SYNTAX
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Don’t worry about learning the pattern matching the first time you
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read this manual; you‘ll rarely need it, unless you’re anal about
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being as succinct as possible. (Like the author).
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The following is an excerpt from bash‘s man page; see bash(1) for
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full details. In the following description, a pattern-list is a
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list of one or more patterns separated by a |. Composite patterns
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may be formed using one or more of the following sub-patterns:
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* Matches any string, including the null string.
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? Matches any single character.
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[…] Matches any one of the enclosed characters. A pair of
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characters separated by a hyphen denotes a range
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expression. If the first character following the [ is a
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! or a ^ then any character not enclosed is matched.
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?(pattern-list)
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Matches zero or one occurrence of the given patterns
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*(pattern-list)
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Matches zero or more occurrences of the given patterns
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+(pattern-list)
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Matches one or more occurrences of the given patterns
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@(pattern-list)
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Matches exactly one of the given patterns
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!(pattern-list)
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Matches anything except one of the given patterns
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EXAMPLES
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Pipelines are okay:
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onlogdo /var/log/messages ‘*‘ ‘sleep 5; echo olleH | rev‘
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Wildcards match filenames as usual in a command:
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onlogdo /var/log/messages ‘Segfault‘ ‘rm /*.core‘
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Multiple pattern and command pairs are allowed:
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onlogdo /var/log/messages \
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‘ep0 at pcmcia‘ ‘/etc/rc.d/dhclient start‘ \
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‘ep0 detached‘ ‘/etc/rc.d/dhclient stop‘ \
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‘wi0 at pcmcia‘ ‘/etc/rc.d/dhclient start‘ \
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‘wi0 detached‘ ‘/etc/rc.d/dhclient stop‘
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Bash’s extended pathname globbing to do the same as above:
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onlogdo /var/log/messages \
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‘@(ep|wi)[0-9] at pcmcia’ ‘/etc/rc.d/dhclient start’ \
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‘@(ep|wi)[0-9] detached’ ‘/etc/rc.d/dhclient stop’
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A useful example for NetBSD/hpcmips:
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onlogdo /var/log/messages ‘hpcapm: resume’ ‘sleep 1; xrefresh’
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Under NetBSD/hpcmips-1.5.1, the X server screen is overwritten by the
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console on an APM resume. I put the last “onlogdo” example in my
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system xinitrc, so xrefresh will be run automatically. (The sleep
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is there to wait for the console to finish junking up the screen).
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SEE ALSO
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tail(1), bash(1)
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BUGS
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If this had been written in Perl it would have used “normal”
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regexp syntax instead of pathname expansion. Bash‘s extensions
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make the patterns as powerful as regexps, but more recondite.
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There is no (documented) way for a command to refer to specifics
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in the line that matched. E.g., if the pattern was “@(ep|wi)0”,
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the command wouldn’t know if the line contained ep0 or wi0.
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The author has spent way too much time perfecting a kludge. No
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matter what you‘re using onlogdo for, there’s probably a more
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“correct” way to do it. On the other hand, onlogdo is widely
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applicable and easy to use, so at least you won‘t be wasting much
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time while doing it the “wrong” way.
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There should be a real man page.
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AUTHOR
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Ben Wong <Benjamin.Wong@cc.gatech.edu>
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HISTORY
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Onlogdo started life as a one line kludge to work around a bug in
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the interaction between APM and the X server in NetBSD-1.5/hpcmips
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in September of 2001.
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EOF
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}
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### USAGE AND ARGUMENT SANITY CHECKING ###
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if [[ $# -lt 3 ]]; then
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showmanpage
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exit 1
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fi
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# Emacs’s syntax highlighting doesn‘t handle single ticks (‘) properly.
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# Check if -v (verbose) flag was given.
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if [[ “$1” == “-v” ]]; then
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ifverbose=echo # Echo commands verbosely, if -v.
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shift
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else
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ifverbose=: # Usually just run a no-op.
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fi
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# Make sure the log file is readable.
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if [[ ! -r “$1” ]]; then
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echo “onlogdo: \”$1\” is not readable” >&2
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exit 1
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fi
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if [[ -d “$1” ]]; then
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echo “onlogdo: \”$1\” is a directory” >&2
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exit 1
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fi
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### SIGNAL HANDLING AND EXIT CLEANUP ###
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# Run cleanup function when shell exits.
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trap cleanup EXIT
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cleanup () {
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echo “onlogdo: cleaning up…” >&2
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if [[ “$TAILPID” ]]; then
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kill $TAILPID # Kill the bg tail process.
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fi
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rm -f “$PIPE” # Delete the named pipe.
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rmdir “$PIPEDIR”
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return 0
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}
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### MAIN ROUTINE STARTS HERE ###
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# Don’t expand wildcards in pattern arguments as filenames.
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set -o noglob
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# Use bash’s extended pattern matching operators
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shopt -s extglob
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# Create a pipe to read the log file a line at a time;
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# Use mktemp for security.
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PIPEDIR=$(mktemp -d /tmp/onlogdo.XXXXXX)
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PIPE=“$PIPEDIR/$(echo $1 | sed ‘s#^/##; s#/#.#g’)” # var.log.messages
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rm -f $PIPE
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mknod $PIPE p
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tail -Fn0 $1 >$PIPE & # FSF tail: “tail –retry -fn0”
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TAILPID=$!
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exec 5< $PIPE
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# Copy the positional paramaters in to a variable that can be indexed.
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params=(“$0” “$@”)
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# Repeatedly read from the pipe, process each line.
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while :; do
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while read REPLY <&5; do
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$ifverbose -e “\nline: \”$REPLY\”“
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for ((i=2; i<$#; i+=2)); do
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pattern=${params[$i]}
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command=${params[$((i+1))]}
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$ifverbose “pattern: \”$pattern\”“
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if [[ -z “${REPLY##*$pattern*}” ]]; then
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$ifverbose “*MATCHED*”
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$ifverbose “command: \”$command\”“
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set +o noglob # Allow pathname matching in commands
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eval $command &
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set -o noglob # No pathname expansion for patterns
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fi
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done
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done
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# We get here whenever the pipe first blocks (returns EOF)
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sleep 1 # Don’t loop too quickly
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done
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### MAIN ROUTINE ENDS HERE ###
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# Note: if we ever get here, the cleanup() function will be called
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# automatically since we’re trapping on signal 0.
