I've made a Bash script to monitor some server log files for certain data and my method probably isn't the most efficient.

One section specifically bugs me is that I have to write a newline to the monitored log so that the same line wont be read over continually.

Feedback would be greatly appreciated!



# Creating the file
if [ ! -f "$onlinefile" ]; then
    touch $onlinefile
    echo "Name                  Date            Time" >> "$onlinefile"
if [ ! -f "$offlinefile" ]; then
    touch $offlinefile
    echo "Name                  Date            Time" >> "$offlinefile"

# Functions
function readfile {

# Login Variables
loginplayer=`tail -1 $serverlog | grep "[INFO]" | grep "joined the game" | awk '{print $4}'`
logintime=`tail -1 $serverlog | grep "[INFO]" | grep "joined the game" | awk '{print $2}'`
logindate=`tail -1 $serverlog | grep "[INFO]" | grep "joined the game" | awk '{print $1}'`

# Logout Variables
logoutplayer=`tail -1 $serverlog | grep "[INFO]" | grep "left the game" | awk '{print $4}'`
logouttime=`tail -1 $serverlog | grep "[INFO]" | grep "left the game" | awk '{print $2}'`
logoutdate=`tail -1 $serverlog | grep "[INFO]" | grep "left the game" | awk '{print $1}'`

# Check for Player Login
    if [ ! -z "$loginplayer" ]; then
        echo "$loginplayer          $logindate  $logintime" >> "$onlinefile"
        echo "Player $loginplayer login detected" >> "$serverlog"
        line=`grep -rne "$loginplayer" $offlinefile | cut -d':' -f1`
        if [ "$line" > 1 ]; then
            sed -i "$line"d $offlinefile
            unset loginplayer
                    unset line
# Check for Player Logout
    if [ ! -z "$logoutplayer" ]; then
        echo "$logoutplayer         $logoutdate $logouttime" >> "$offlinefile"
        echo "Player $loginplayer logout detected" >> "$serverlog"
        line=`grep -rne "$logoutplayer" $onlinefile | cut -d':' -f1`
        if [ "$line" > 1 ]; then
            sed -i "$line"d $onlinefile
            unset logoutplayer
            unset line

# Loop
while [ $index -lt 100 ]; do
  • \$\begingroup\$ A minor remark: I think ! -z is equivalent to -n. \$\endgroup\$ Jul 19, 2013 at 21:37

2 Answers 2


Your goal is to monitor $serverlog continuously, and update $onlinefile and $offlinefile accordingly. The fact that you repeatedly close and reopen $serverlog is problematic, not only for performance reasons, but as you remarked, you risk processing the same line endlessly. Therefore, your general strategy should be to keep the file open, like this:

tail -f "$serverlog" | do_all_processing_here

Note that grep "[INFO]" doesn't do what you intend; instead, it matches lines that contain any of the characters I, N, F, or O. You probably meant grep -F '[INFO]' — the -F causes grep to treat your pattern as a fixed string rather than a regular expression. Then, your structure would be:

tail -f "$serverlog" | grep -F '[INFO]' | do_more_processing_here

I'm going to guess that [INFO] would appear in the third field of your server log. If so, the efficient solution would be…

tail -f "$serverlog" | \
while read date time severity player message ; do
    case "$severity" in
        case "$message" in
          *joined the game*)
            echo "$player    $date  $time" >> "$onlinefile"
            sed -i -e "$(
                awk -v player="$player" '$1 == player { print NR "d;"}' "$offlinefile"
            )" "$offlinefile"
          *left the game*)
            echo "$player    $date  $time" >> "$offlinefile"
            sed -i -e "$(
                awk -v player="$player" '$1 == player { print NR "d;"}' "$onlinefile"
            )" "$onlinefile"
        esac # case $message
    esac # case $severity

It's also possible to skip awk and edit $offlinefile and $onlinefile directly with

sed -i "/^$player /d" "$offlinefile"

but there could be a vulnerability if $player contained special characters such as .*. A better solution, if you have GNU awk ≥ 4.1.0, is

awk -v player="$player" '$1 != player' -i inplace "$offlinefile"

With other versions of awk, you could use tempfile to help you perform the edit.


I have small suggestions for the function readfile to reduce the number of shell commands you need to run :

  1. Save the output of the following into a variable so it can be reused as shown in #2:

    last_line=$(tail -1 $serverlog | grep "[INFO]")
  2. As you are already using awk, use it to search and select columns both:

    login_time=$(echo $last_line | awk '/joined the game/ {print $2}')
  3. Since you are searching in a single file and not recursively, the -r flag to grep can be removed.


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