luv.preet's questions

I am developing an API in which I need the paths of multiple files from a remote server. The number of files varies from 100 - 500. The files are in different folders.

So, I am looping over and over again, like 10-50(depending on number of files) times, and then getting paths using ssh in my python api.

But I want an optimized solution for this problem. Right now, I am making ssh connection as many times as loop goes, which is slow, and also not the best thing to do.

I was thinking of copying the /var/lib/mlocate/mlocate.db of remote server daily in my local machine and then find the path using locate command using this db, If that is possible. OR Like storing the remote directory index in my local machine which I can query more fastly.

What are other better ways to achieve this?

I want to connect my libre calc sheet to postgresql so that it gets updated automatically after some interval.

So, I click sheet > Link to external Data, then the connector appears,

I have entered the correct URL(I think so).

But the OK button remains disabled. I cannot click OK. I also don't know the available tables/ranges option.

What am I doing wrong?

I am installing hackintosh in virtualbox on ubuntu, but when I try to start, I get above error.

Here are some more details,

Result Code:   NS_ERROR_FAILURE (0x80004005)
Component: ConsoleWrap
Interface: IConsole {872da645-4a9b-1727-bee2-5585105b9eed}

How can I solve this issue?

I am trying to transfer some file through scp. But it is not working,

scp "vogo_psql_db\:2017-08-30T18\:00\:01.762662" "[email protected]:db-dump"
ssh: Could not resolve hostname vogo_psql_db\\: Name or service not known

The problem is I think that due to colons, it is taking vogo_psql_db as hostname even though the escape chars have been used.

What should be done to solve this ?

I know what this command does. It sets up swap area on a device or in a file.
The device being a partition, and file being a simple file.

But I want to know what it does internally on the disk.

Correct me here if I am wrong, Swap is slower than RAM, but faster than simple Disk for operations.

This is my question.

What changes does mkswap makes on disk? I mean, the swap area is still on disk (be it a partition or a file), right?, then how come it works faster than rest of the disk? It must be doing some changes on the disk? What are these changes?

EDIT - It is in reference to this video, https://www.youtube.com/watch?v=uxiis-K6v7w

here it is being told that data accessed frequently is moved to the outer area of the disk. So, I guess mkswap might move the swap partition/file to some outer area of the disk.

I was just messing around with the system to learn how to extend swap memory. Then I came to know about /dev/zero. It is refered to as brother of /dev/null(I read it somewhere).

Then I ran man zero,

The discription says,

       Reads  from /dev/null always return end of file (i.e., read(2) returns 0), whereas reads from /dev/zero always return bytes contain‐
   ing zero ('\0' characters).

What is the difference between this end of file and bytes containing zero? Aren't they same thing?

I have a plugin for alerting on elasticsearch.

This is run manually everytime

python -m elastalert.elastalert --verbose --rule example_frequency.yaml   

By this way have to dedicate a screen to it and I want it to run in the background.

This task can be done in 2 ways, either I use Supervisor or systemd.

All I have to do is write a script for any of these.

But what should I actually use in production ?

What are the pros and cons of both these ?

This is in reference to this question.

I wanted to log everytime someone logged in via ansible.

I put my script in ~/.bash_profile file. Here is the script,

#!/bin/bash

now=$(date +"%T")
echo "$USER logged in at: $now" >> /home/alphauser/ssh-entries.log

I put this at the end of ~/.bash_profile,

bash ~/ssh-login-entry.sh

However, ansible uses the non-interactive shell, so my script was not executed when I logged in via ansible as ~/.bash_profile is executed only when a new interactive shell is started.

Then I pasted this at the end of .bashrc file, but still it did not work.

How can I log entries when someone logs in via non-interactive shell ? I mean where should I place this script so that it should be executed everytime someone logs in via ansible ?

I hope the question is clear.

When I log in as root,

luvpreet@DHARI-Inspiron-3542:/$ sudo su

I am getting something like this:

\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\u@\h:\w\$ 

What does this mean and why am I seeing it?

EDIT - Here are some more helpful things asked for in comments,

luvpreet@DHARI-Inspiron-3542:/$ sudo su -
DHARI-Inspiron-3542#  
DHARI-Inspiron-3542# exit
luvpreet@DHARI-Inspiron-3542:/$ sudo -i
\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\u@\h:\w\$ 
\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\u@\h:\w\$ exit
luvpreet@DHARI-Inspiron-3542:/$ echo $0
/bin/bash

EDIT 2 - refered to comments made

luvpreet@DHARI-Inspiron-3542:/$ ls -1 /root/.[a-zA-Z0-9]*
ls: cannot access '/root/.*': Permission denied
luvpreet@DHARI-Inspiron-3542:/$ sudo su -
DHARI-Inspiron-3542# ls -1d /root/.[a-zA-Z0-9]*
/root/.bash_history
/root/.bashrc
/root/.bashrc.bk
/root/.profile
/root/.profile.bk
/root/.viminfo
/root/.zcompdump

EDIT 3 -

luvpreet@DHARI-Inspiron-3542:/etc/ansible$ sudo find /root /etc/ -maxdepth 1 -type f -exec grep -n PS1 {} +
/root/.zcompdump:1201:'-value-,PS1,-default-' '_ps1234'
/root/.zcompdump:1207:'-value-,RPS1,-default-' '_ps1234'
/root/.bashrc:60:    PS1='${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '
/root/.bashrc:62:    PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '
/root/.bashrc:69:    PS1="\[\e]0;${debian_chroot:+($debian_chroot)}\u@\h: \w\a\]$PS1"
/root/.bashrc.bk:9:[ -z "$PS1" ] && return
/root/.bashrc.bk:56:    PS1='${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '
/root/.bashrc.bk:58:    PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '
/root/.bashrc.bk:65:    PS1="\[\e]0;${debian_chroot:+($debian_chroot)}\u@\h: \w\a\]$PS1"
/etc/profile:7:if [ "$PS1" ]; then
/etc/profile:9:    # The file bash.bashrc already sets the default PS1.
/etc/profile:10:    # PS1='\h:\w\$ '
/etc/profile:16:      PS1='# '
/etc/profile:18:      PS1='$ '
/etc/bash.bashrc:7:[ -z "$PS1" ] && return
/etc/bash.bashrc:19:PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '

The ss command is very similar to netstat command but it is faster and more accurate than netstat.

monitoring-server@monitoring-server:~$ ss
Netid State      Recv-Q Send-      Local Address:Port    Peer Address:Port       
u_str ESTAB      0      0          * 34006              * 34007                

This is the first line of the ouptut. This command tells all the sockets open in the system.

Total on my system is 96 and 56 of them has u_str as Netidand rest have tcp.

What is this u_str here , Also please explain more about Recv-Q and Send-Q?

I am using Netdata , Influxdb and Grafana to monitor my system. I saw in the networking section that the number(around 80) of sockets opened by user "root" are much higher than the other users.

The below image tells the data for past 15 minutes, and it usually remains same with very little variations.

Is Something wrong with my system OR this is usual on ubuntu systems ? Moreover Is there any way I can see the sockets opened by users ?

Some extra info,

alphauser@AlphaServer:~$ sudo netstat nap | wc -l
228
alphauser@AlphaServer:~$ netstat -an | grep ESTABLISHED | wc -l
22

I created a cronjob in the following way to run a script. The script started a service only if it was down.

Here is the script,

#!/bin/bash
service=influxdb

if (( $(ps -ef | grep -v grep | grep $service | wc -l) > 0 ))
then
echo "$service is running!!!"
else
service $service start
fi

I created a cron job like this,

alphauser@AlphaServer:~$ sudo crontab -e

And then added this line

* * * * * bash /home/alphauser/influx-start.sh > /home/alphauser/output-influx-start.txt

I stored the output in a file just to check its output.

The service stopped and now it was the time for cron to show its magic. But it failed to start the service. I saw the output file and this was written in that,

Starting influxdb...
influxdb process was unable to start [ FAILED ]

then I removed this cronjob from by sudo crontab -r.

I added this line at the end of etc/crontab file i.e,

* * * * * root bash /home/alphauser/influx-start.sh > /home/alphauser/outputinflux.txt

and it worked. The service started and This was the output,

influxdb is running!!!

I want to know that why it failed with sudo crontab -e but it worked with etc/crontab file.

The authentication of sudo cannot be the issue because I added it in the sudo crontab and by chance if that would have been the case, it would have said You must be root to run this script.

systemd gives us the systemctl commands suite which is mostly used to enable services to start at boot time. We can also start, stop, reload, restart and check status of services with the help of systemctl.

We can do, for example, sudo systemctl enable service_name, and service_name will automatically start at boot time. We can also disable services not to start at boot time.

Is the only difference between the service and systemctl commands that systemctl can be used to enable the start of services at run time? Can we use systemctl on any service? What other significant differences are there?

There are 7 terminals that can be used in our system. tty7 is GUI based.

How can I check which tty I'm currently using?

How to check which Byte Order my system is Using ?

Little Endian OR Big Endian ?

I have installed ubuntu server in Virtual Machine. I started exploring about the network, and read a book for it.

It says that my DNS server is given in /etc/resolv.conf file.

This is my server's IP.

luvpreet@server16:~$ hostname -I
192.168.122.62 

This is the /etc/resolv.conf file,

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 192.168.122.1

Well, DNS server maps the human readable addresses to IP addresses. In my main machine(not in VM)

tcp        0      0 192.168.122.1:53        0.0.0.0:*               LISTEN      2332/dnsmasq    

The DNS server is running.

But how it works, I have no idea that how this IP address i.e, 192.168.122.1 is serving as my DNS Server. Please explain its working in detail and also the flow by which this DNS server works.

I know how a DNS server works, it is like a phonebook which helps us to locate to a particular website. Where is the DNS cache? I want to know how this IP address is working as my DNS server ? Can I bind any IP address as my DNS server ?If yes, How to do that ? If no, why ?

Sorry if this is dumb but I am not getting this.

When using terminal, it saves the previously executed commands. When I press the Up arrow button, it tells us that which commands I have run previously.

Suppose, I am using my friend's system and start using it's command-line. Now, my friend will get to know what commands I have been running. But I don't know want him to know this.

Is there a way I can stop terminal from saving these commands for the current session ?

(Just suppose this scenario for this question. I am not a cheater)

I was monitoring my system using Netdata today, and noticed that, some kind of X apps are using more CPU than other applications.

Look clearly in the image, X is taking a larger CPU% time than others. Not only CPU, but everything like Disk, Swap, Net, Memory and Processes.

I searched for it on the web, but I couldn't get a clear view. Please explain. Thank you.

echo $$ 

It tells the PID of the shell. So, if I kill this shell using

kill 123 #suppose 123 is PID of shell

Nothing happens actually, I tried it. I again ran

echo $$

and the shell was running under same PID. Why the shell is not stopped ? What may happen if the shell is killed ?

The terminal is running when we open it.

luvpreet@DHARI-Inspiron-3542:/$

I have just opened it. So, when I press Ctrl+C, why doesn't it kill itself and close the terminal??