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I'm writing simple script in bash, to analyze some logs in real time and wondering about how to approach the fact, that every few seconds I have to find place in the file that I finished reading previous time. Right now I am doing something like this:

LOG_FILE=path_to_file
DELAY=1   #time between refresh
LINES=100 #lines to read at one cycle

LAST=$(tail -n 1 $LOG_FILE)      

IFS=$'n'
while true;
do
    clear;
    found=0
    LOG=$(tail -n $LINES $LOG_FILE)
    for line in $LOG
    do
        if [ $line = $LAST ]; then 
            found=1
            continue
        fi        
        if [ $found = 0 ]; then
            continue
        fi
        #Analyzing counting nd stuff.
        echo "$stuff"
    done
    LAST=$line
    sleep $DELAY;
done

So every cycle I am fetching some number of lines from the end of file and looking for the one that was the last in the previous run. This will work quite ok until in one cycle more that defined number of lines will be added. I can always say something like LINES=10000 but in this case there will be tousands of useless runs just to determine if yet I found the last line from previous run. I'm wondering if I can do it somewhat more efficient?

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Using the tail -f - for a simple "waits for another lines" isn't a solution? –  jm666 2 mins ago
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2 Answers

up vote 0 down vote

Using tail -f, you can keep the file open and read new lines as they are logged. Use a while / read loop on tail -f.

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up vote 0 down vote

I think you are looking for sth like this:

#!/bin/bash
GAP=10     #How long to wait
LOGFILE=$1 #File to log to

if [ "$#" -ne "1" ]
    then
    echo "USAGE: ./watch-log.sh <file with absolute path>"
exit 1
fi


#Get current long of the file
len=`wc -l $LOGFILE | awk '{ print $1 }'`
echo "Current size is $len lines."

while :
do
    if [ -N $LOGFILE ] 
        then
        echo "`date`: New Entries in $LOGFILE: "
        newlen=`wc -l $LOGFILE | awk ' { print $1 }'`
        newlines=`expr $newlen - $len`
        tail -$newlines $LOGFILE
        len=$newlen
        fi
sleep $GAP
done
exit 0
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