So, without further ado,  here it goes:

#!/bin/bash 

[ $# -eq 0 ] && {
        echo "usage: salt <length>">&2
        exit
}
strings </dev/urandom | while read line; do
        echo $line | tr '\n\t ' $RANDOM:0:1 >> /tmp/.salt.$$
        salt=$(cat /tmp/.salt.$$)
        [ ${#salt} -ge $1 ] && salt=${salt:0:$1} && echo $salt && break
done
rm -f /tmp/.salt.$$

I had to use $1 and not a var, and echo the salt right from inside the while, because the ‘|’ creates another shell, so I can’t pass variables to or from the while in this case.

If you want to keep things simple, you can go perl and just do

#!/usr/bin/perl
use Crypt::Salt;
my $length = shift;
print salt($length);

But if you ever find yourself in a server with no cpan, the first option might prove useful

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Here’s the code:

#!/bin/bash 

timeout=$1
command=$2
shift 2

check_pid_name()
{
        command=$1
        childpid=$2
        [ -d /proc/$childpid ] || return 1
        [ $(grep -c $command /proc/$childpid/cmdline 2>/dev/null) -gt 0 ] && return 0 || return 1
}

[ -z "$command" ] && echo "Usage: safecmd   [args]">&2 && exit 1

safe_run()
{
        command=$1
        shift
        $command $*
        kill $$
}

safe_run $command $* &
childpid=$!
sleep $timeout 

check_pid_name $command $childpid && {
        kill $childpid
        sleep 0.1
        check_pid_name $command $childpid && kill -9 $childpid
        echo "$command $* timed out"
}

You can try it how like this, for example:

./safecmd 2 sleep 4

which will output

./safecmd.1: line 34: 11739 Terminated               safe_run $command $*
sleep 4 timed out

or ./safecmd 2 sleep 1

which will output

Terminated

Indicating that sleep 1 finished successfully

This is still missing something. You can’t test for the success of the command you pass safecmd, if it exits successfully you’ll see the same return code than if it would have exited with an error.

In order to improve this, as far as I can tell, you have to use two scripts, one just as the wrapper (to use one separate script instead of the function safe_run()). That’s the way this is handled in Highbase, and I’ll post a full example in my next post on this subject.

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In order to avoid this situations, here’s a small shell script that will execute any command with a guaranteed timeout:

#!/bin/bash

timeout=$1
command=$2

check_pid_name()
{
        command=$1
        childpid=$2
        [ -d /proc/$childpid ] || return 1
        [ $(grep -c $command /proc/$childpid/cmdline 2>/dev/null) -gt 0 ] && return 0 || return 1
}

[ -z "$command" ] && echo "I need a command to run">&2 && exit 1

[ $# -eq 2 ] && shift || shift 2

$command $* &
childpid=$!

sleep $timeout 

check_pid_name $command $childpid && {
        kill $childpid
        sleep 0.1
        check_pid_name $command $childpid && kill -9 $childpid
        echo "$command $* timed out"
}

As you can see, it’s quite simple. You just have to start the desired command in the background, sleep for the timeout, and then kill the process if it’s still running.

Yes, this script has a defect, all the commands will wait $timeout to run, but that’s easy to solve. I’ll post an improved (but slightly more complex) version in my next post.

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