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Accepted
Janith
Asked: 2018-12-08 01:39:45 +0800 CST

Assigned variable for pipeline is not working

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I have a shell script to check port status. After checking the port if it is busy then will kill it. 

#check port and kill
checkPort(){
     check=$(sudo netstat -nlpt | grep 2020 | awk '{print $6}')
    word="LISTEN"
    killProcess=$(sudo netstat -nlpt | grep 2020 | awk '{print $7}' |
                  sed 's/.\{5\}$//' | sort -u |  xargs -t kill -9)
    if [[ "$check" == "$word"  ]];
    then
        echo "killing the port 2020"
        $killProcess
     else
        echo "Not Listen"
    fi
}

When I run the $killProcess variable I am getting errors. But if I put the full command as sudo netstat -nlpt | grep 2020 | awk '{print $7}' | sed 's/.\{5\}$//' | sort -u | xargs -t kill -9 inside the if condition, it is working fine. What would be the mistake here?

The errors I get:

kill -9 

Usage:
 kill [options] <pid|name> [...]

Options:
 -a, --all              do not restrict the name-to-pid conversion to processes
                        with the same uid as the present process
 -s, --signal <sig>     send specified signal
 -q, --queue <sig>      use sigqueue(2) rather than kill(2)
 -p, --pid              print pids without signaling them
 -l, --list [=<signal>] list signal names, or convert one to a name
 -L, --table            list signal names and numbers

 -h, --help     display this help and exit
 -V, --version  output version information and exit

I put xargs -i kill -kill {} instead of xargs kill -9. Works fine. Is it a good practice?

scripts bash

1 Answers

  1. Best Answer

    The syntax var=$(command) will run command and assign its output to the variable $var. This means that this line in your script:

    killProcess=$(sudo netstat -nlpt | grep 2020 | awk '{print $7}' |
              sed 's/.\{5\}$//' | sort -u |  xargs -t kill -9)

    will always run this pipeline which ends with the kill command, and assign its output to $killprocess. Since the kill command has no output, this line later will never do anything:

    $killProcess

    What you want to do is killProcess="sudo netstat ... | xargs -t kill -9. But there are other issues with your command as well. First of all, it will match any lines containing 2020, but you only want cases where 2020 is the port. What if it is the PID instead?

    Here is a working version of your function with a few changes:

    checkPort(){
  ## Get the target port. If none is given, default to 2020
  targetPort=${1:-2020}

  ## Collect the PID(s) that are listening on the port in the
  # array 'pids'.
  pids=( $(sudo netstat -nlpt | awk "\$4~/:$targetPort\$/{print \$7}" |
           sed 's|/.*||'))
  ## If no processes were listening, move on
  if [[ -z ${pids[0]} ]];
  then
    echo "No process listening on port $targetPort"
  ## If any processes were listening, kill them.
  else
    echo "killing process(es) listening on port $targetPort"
    ## First, try to kill the process gracefully
    kill "${pids[@]}"
    ## Check if any are still running
    for pid in "${pids[@]}"; do
      ## If it is still running
      if kill -0 "$pid"; then
        ## Kill it with fire
        kill -9 "$pid"
      fi
    done
  fi
}

    This can now take a parameter, so run it as checkport to check port 2020, and checkport 22 will check port 22. It also first tries to kill gracefully (ou should always avoid kill -9 unless absolutely necessary) and only uses kill -9 if that fails.

    • 2