Building a killer script in bash

Question

I've been trying to learn the syntax of logic statements in bash, how to do if/else, pipes and stuff. I'm trying to build a bash script, but I fail miserably after 3 hours of not getting how this stuff works.

Now I need this little script, I'll try to explain it using a generalized code, or call it whatever you want. Here you go:

while variable THRESHOLD = 10

{
if netstat -anltp contains a line with port 25565
then set variable THRESHOLD to 0 and variable PROCNUM to the process number,
else add 1 to variable THRESHOLD
sleep 5 seconds
}
kill the process No. PROCNUM
restart the script

Basically, what it does is, that once the socket closes, after a few tries, it kills the process which was listening on that port.

I'm pretty sure it's possible, but I can't figure out how to do it properly. Mostly because I don't understand pipes and am not really familiar with grep. Thank you for your help, in advance.

what's to understand about pipes? they take the output from one program and feed it as input to another. — Marc B, Jun 24 '13 at 17:19
Oh. Well that feels awkward. What if I want to feed it to a binary as a specific argument? Let's say binary -a passedtext OR binary -d passedtext -a somethingelse ? — HajdaCZ, Jun 24 '13 at 17:21
use `xargs`: it takes the data being piped in and feeds to a specified app as command line arguments. e.g. `echo 'somefile.txt' | xargs rm -f` will actually execute `rm -f somefile.txt`. — Marc B, Jun 24 '13 at 17:27

c-is-best · Answer 1 · 2013-06-24T19:52:38.270

    #!/bin/bash
    # write a little function
    function do_error {
        echo "$@" 1>&2
        exit 1
    }
    # make the user pass in the path to the executable
    if [ "$1" == "" ]; then
        do_error "USAGE: `basename $0` <path to your executable>"
    fi
    if [ ! -e $1 ]; then
        do_error "Unable to find executable at $1"
    fi
    if [ ! -x $1 ]; then
        do_error "$1 is not an executable"
    fi
    PROC="$1"
    PROCNAME=`basename $PROC`

    # forever
    while [ 1 ]; do
        # check whether the process is up
        proc=`ps -ef | grep $PROCNAME 2>/dev/null`
        # if it is not up, start it in the background (unless it's a daemon)
        if [ "$proc" == "" ]; then
            $PROC &
        fi
        # reinitialize the threshold
        threshold=0
        # as long as we haven't tried 10 time, continue trying
        while [ threshold -lt 10 ]; do
            # run netstat, look for port 25565, and see if the connection is established. 
            # it would be better to checks to make sure
            # that the process we expect is the one that established the connection
            output=`netstat -anp | grep 25565 | grep ESTABLISHED 2>/dev/null`
            # if netstat found something, then our process was able to establish the connection
            if [ "$output" != "" ]; then
                threshold = 0
            else
                # increment the threshold
                threshold=$((threshold + 1))
            fi
            # i would sleep for one second
            sleep 1
        done
        kill -9 $PROCNUM
    done

Thanks. I haven't actually run this, so if you encounter any errors you can let me know and I'll fix them. — c-is-best, Jun 24 '13 at 19:54

score 1 · Accepted Answer · answered Jun 24 '13 at 19:51

Don't want be offensive, but if you can write a "generalized" program all you need is learn th syntax of the while, if for bash and read the man pages of the grep and kill and so on...

And the pipes are the same as in your garden. Having two things: tap and pond. You can fill your pond with many ways (e.g. with rain). Also, you can open your tap getting water. But if you want fill the pond with the water from a tap, need a pipe. That's all. Syntax:

tap | pond

the output from a tap
connect with a pipe
to the (input) of the pond

e.g.

netstat | grep

the output from a netstat
connect with a pipe
to the input of the grep

that's all magic... :)

About the syntax: You tagged your question as bash.

So googling for a bash while syntax will show to you, this Beginners Bash guide

http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_09_02.html

to, and you can read about the if in the same website.

Simply can't believe than after 3 hours you cannot understand basic while and if syntax to write your program with a bash syntax - especially, when you able write an "generalized" program...

is is not to hard (with modifying the 1st example in the above page) to write:

THRESHOLD="0"
while [ $THRESHOLD -lt 10 ]
do
    #do the IF here
    THRESHOLD=$[$THRESHOLD+1]
done

and so on...

Building a killer script in bash

2 Answers2