Category: Uncategorized

Async waterfall example nodejs

Posted on by admin

To avoid callback hell , a very useful tool for structuring calls in a sequence and make sure the steps pass the data to the next step.

var async = require('async');

async.waterfall(
    [
        function(callback) {
            callback(null, 'Yes', 'it');
        },
        function(arg1, arg2, callback) {
            var caption = arg1 +' and '+ arg2;
            callback(null, caption);
        },
        function(caption, callback) {
            caption += ' works!';
            callback(null, caption);
        }
    ],
    function (err, caption) {
        console.log(caption);
        // Node.js and JavaScript Rock!
    }
);

nJoy 😉

Node.js Start script or run command and handoff to OS (no waiting)

Posted on by admin

Sometimes you want to run something in the OS from your node code but you do not want to follow it or have a callback posted on your stack. The default for child_process spawning is to hold on to a handle and park a callback on the stack. however there is an option which is called detached.

var spawn = require('child_process').spawn;
spawn('/usr/scripts/script.sh', ['param1'], {
    detached: true
});

You can even setup the environment of the call to add stdio and stderr pipes to the call and connect them to OS fs descriptors like this:

var fs = require('fs'),
    spawn = require('child_process').spawn,
    out = fs.openSync('./out.log', 'a'),
    err = fs.openSync('./err.log', 'a');

spawn('/usr/scripts/script.sh', ['param1'], {
    stdio: [ 'ignore', out, err ], // piping stdout and stderr to out.log
    detached: true
}).unref();

The unref disconnects the process. from the parent it equates to a disown in shell.

Thanks to O/P : https://stackoverflow.com/questions/25323703/nodejs-execute-command-in-background-and-forget

Also Ref: https://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options

and

https://github.com/nodejs/node-v0.x-archive/issues/9255

nJoy 😉

Identify OS on remote host

Posted on by admin

For nmap to even make a guess, nmap needs to find at least 1 open and 1 closed port on a remote host. Using the previous scan results, let us find out more about the host 192.168.0.115:

# nmap -O -sV 192.168.0.115

Output:

Starting Nmap 7.80 ( https://nmap.org ) at 2020-10-02 12:21 CEST
Nmap scan report for 192.168.0.115
Host is up (0.00023s latency).
Not shown: 991 closed ports
PORT      STATE SERVICE     VERSION
22/tcp    open  ssh         OpenSSH 5.1 (protocol 2.0)
80/tcp    open  http        Apache httpd 2.2.19 ((Unix) mod_ssl/2.2.19 OpenSSL/0.9.8zf DAV/2)
111/tcp   open  rpcbind     2 (RPC #100000)
139/tcp   open  netbios-ssn Samba smbd 3.X - 4.X (workgroup: WORKGROUP)
443/tcp   open  ssl/http    Apache httpd 2.2.19 ((Unix) mod_ssl/2.2.19 OpenSSL/0.9.8zf DAV/2)
445/tcp   open  netbios-ssn Samba smbd 3.X - 4.X (workgroup: WORKGROUP)
873/tcp   open  rsync       (protocol version 29)
2049/tcp  open  nfs         2-4 (RPC #100003)
49152/tcp open  upnp        Portable SDK for UPnP devices 1.6.9 (Linux 2.6.39.3; UPnP 1.0)
MAC Address: 00:26:2D:06:39:DB (Wistron)
Device type: general purpose
Running: Linux 2.6.X|3.X
OS CPE: cpe:/o:linux:linux_kernel:2.6 cpe:/o:linux:linux_kernel:3
OS details: Linux 2.6.38 - 3.0
Network Distance: 1 hop
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel:2.6.39.3


OS and Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 14.58 seconds

nJoy 😉

How to quit ESXi SSH and leave background tasks running

Posted on by admin

In Linux when a console session is closed most background jobs (^Z and bg %n) will stop running when the parent ( the ssh session ) is closed because the parent sends a SIGHUP to all its children when closing (properly). Some programs can catch and ignore the SIGHUP or not handle it at all hence passing to the root init parent. The disown command in a shell removes a background job from the list to send SIGHUPs to.

In ESXi there is no disown command. However there is a way to close a shell immediately without issuing the SIGHUPs :

exec </dev/null >/dev/null 2>/dev/null

The exec command will run a command and switch it out for the current shell. Also this command will make sure the stdio and stderr are piped properly.

nJoy 😉

Convert Large numbers to binary in Excel

Posted on by admin 
=DEC2BIN(MOD(QUOTIENT($A$1,16^7),16),4)&" "&DEC2BIN(MOD(QUOTIENT($A$1,16^6),16),4)&" "&DEC2BIN(MOD(QUOTIENT($A$1,16^5),16),4)&" "&DEC2BIN(MOD(QUOTIENT($A$1,16^4),16),4)&" "&DEC2BIN(MOD(QUOTIENT($A$1,16^3),16),4)&" "&DEC2BIN(MOD(QUOTIENT($A$1,16^2),16),4)&" "&DEC2BIN(MOD(QUOTIENT($A$1,16^1),16),4)&" "&DEC2BIN(MOD(QUOTIENT($A$1,16^0),16),4)

The above code will convert large numbers up to 32 bit.

nJoy 😉

SYSTEMD – controls

Posted on by admin

Starting with Ubuntu 15.04, Upstart will be deprecated in favor of Systemd. With Systemd to manage the services we can do the following:

systemctl start SERVICE – Use it to start a service. Does not persist after reboot

systemctl stop SERVICE – Use it to stop a service. Does not persist after reboot

systemctl restart SERVICE – Use it to restart a service

systemctl reload SERVICE – If the service supports it, it will reload the config files related to it without interrupting any process that is using the service.

systemctl status SERVICE – Shows the status of a service. Tells whether a service is currently running.

systemctl enable SERVICE – Turns the service on, on the next reboot or on the next start event. It persists after reboot.

systemctl disable SERVICE – Turns the service off on the next reboot or on the next stop event. It persists after reboot.

systemctl is-enabled SERVICE – Check if a service is currently configured to start or not on the next reboot.

systemctl is-active SERVICE – Check if a service is currently active.

systemctl show SERVICE – Show all the information about the service.

sudo systemctl mask SERVICE – Completely disable a service by linking it to /dev/null; you cannot start the service manually or enable the service.

sudo systemctl unmask SERVICE – Removes the link to /dev/null and restores the ability to enable and or manually start the service.

Just copied notes. 😉 nJoy

Using ngrok with React based websites

Posted on by admin

The problem :

When we have a react.js based site (hosted on port 8000) and ngrok it for development purposes we get

Invalid Host Header

The fix is to override the header on reflection:

ngrok http 8000 -host-header="localhost:8000"

 

That’s it

😉 nJoy

Clear Linux Cache

Posted on by admin

1. Clear PageCache only.

# sync; echo 1 > /proc/sys/vm/drop_caches

2. Clear dentries and inodes.

# sync; echo 2 > /proc/sys/vm/drop_caches

3. Clear PageCache, dentries and inodes.

# sync; echo 3 > /proc/sys/vm/drop_caches

nJoy 😉