Author: root

What you do when you are not sure of the command you are running especially in case of complex commands which uses a lot of options. We use man pages to get some help in such situation. Some of the other options may include commands like ‘help‘, ‘whereis‘ and ‘whatis‘. But all has their Pros and Cons.

While going through man pages for options and help, the description in man pages are too lengthy to understand specially in short span of time.

Similarly, ‘help‘ command may not give you desired output.

A ‘whereis‘ command hardly tells anything other than the location of Installed Binaries (May be Important at time).

A ‘whatis‘ command gives strict and one liner answer which is not much helpful other than acknowledging the purpose of the command, Moreover it never says a single word about the available options.

We have used all these options till date to solve our issue in the dilemma but here comes an interactive cheat-sheet application ‘cheat‘ which is going to lead all the rest.

What is cheat?

Cheat is an interactive cheat-sheet application released under GNU General Public License for Linux Command line users which serves the purpose of showing, use cases of a Linux command with all the options and their short yet understandable function.

Installing ‘Cheat’ in Linux Systems

‘Cheat‘ has two major dependency – ‘python‘ and ‘pip‘. Make sure you have installed python and pip before installing ‘cheat‘ on the system.

Install Python

# apt-get install Python	(On Debian based Systems)
# yum install python		(On RedHat based Systems)

Install Pip

# apt-get install python-pip 	(On Debian based Systems)
# yum install python-pip 	(On RedHat based Systems)

NOTE: pip is an easy install replacement and is intended to be an improved Python package installer.

Download and Install Cheat

We will be downloading ‘cheat’ from Git. Make sure you have package ‘git’ installed, if not better install this first.

# apt-get install git	(On Debian based Systems)
# yum install git	(On RedHat based Systems)

Next, install the required python dependencies by running following command.

# pip install docopt pygments

Now, clone the Git repository of cheat.

# git clone https://github.com/chrisallenlane/cheat.git

Move to the cheat directory and run ‘setup.py‘ (a python script).

# cd cheat
# python setup.py install

If installation goes smoothly, you should be able to see a cheat version installed on the system.

# cheat -v 

cheat 2.0.9

Required Configuration for Cheat

You must have an ‘EDITOR‘ environment variable set in ‘~/.bashrc’ file. Open the user ‘.bashrc‘ file and add the following line to it.

export EDITOR=/usr/bin/nano

You can use your favourite editor here in place of ‘nano‘. Save the file and logout. Again Login to make the changes taken into effect.

Next, add the cheat autocompletion feature to enable command-line autocompletion for different shells. To enable autocompletion, simply clone the ‘cheat.bash‘ script and copy the script to the appropriate path in your system.

# wget https://github.com/chrisallenlane/cheat/raw/master/cheat/autocompletion/cheat.bash 
# mv cheat.bash /etc/bash_completion.d/

NOTE: The team has uploaded other shell’s auto completion scrip to Git, which may be cloned and used in case of respective Shell. Use the following link for other shell’s auto completion script.

1. Auto Completion Script for Various Shells

Optionally, you can also enable syntax highlighting, if desired. To active syntax highlighting feature, add a CHEATCOLORS environment variable in your ‘.bashrc‘ file.

export CHEATCOLORS=true

The Cheat application default program only serves the basic and most used commands. The content of cheat-sheet resides at location ~/.cheat/. Manual Cheatsheets can be added to this location to make the application rich.

# cheat -e xyz

This will open xyz cheat-sheet if available. If not it will create one. The cheat-sheet will be opened in the default EDITOR, we set in .bashrc in the configuration stage, above.

Usage of Cheat with Some Commands

A tarball may be *.gz or *.bz2 or *.zip or *.xz. So, what option to be used where?

I never run dd command, no matter how much sure I am about the command before consulting and cross checking it at more than one location. The things seems to be easy now.

A ‘uname‘ command help.

A short ifconfig command line tutorial, in action.

A ‘top‘ command, one of the most important command for Admin and Normal User.

How about Cheating the cheat command (though the other sense)? Get a list of available commands, the cheat-sheet of which is installed in the System.

Search Cheat-sheet with specific keyword.

See the location of built-in cheat-sheets for all the commands.

$ cheat -d 

/home/avi/.cheat 
/usr/local/lib/python2.7/dist-packages/cheat/cheatsheets

Copy the in-built cheat-sheet to your native directory.

# cp /usr/local/lib/python2.7/dist-packages/cheat/cheatsheets/* /home/avi/.cheat/

Conclusion

This wonderful project is a life Saviour in many-a-situation. It just gives you information that is required, nothing extra, nothing vague and to the point. This is a must tool for everyone. Easy to build, easy to install, easy to run and easy to understand, this project seems promising.

This Git project has added a wonderful gag which I am not going to explain but leave on you to interpret.

That’s all for now. I’ll be here again with another interesting article you people will love to read. Till then stay tuned and connected to Tecmint. Don’t forget to provide us with your valuable feedback in the comment section below.

Don’t Miss: Understanding Shell Commands Easily Using “Explain Shell” Script

Source

Boxes is a simple, configurable command line program which can draw any kind of box around its input text. It filters text and draws shapes around it – it’s practically a text filter. In fact it is designed to be integrated with your editor as a text filter (supports Vim default). It can draw shapes ranging from simple boxes to complex ASCII art.

In this article, we will learn how to use the boxes utility to draw shapes in the Linux terminal.

How to Install Boxes Utility in Linux

To install the boxes utility in Linux, use the appropriate command for your distribution.

$ sudo apt install boxes  [On Debian/Ubuntu]
$ sudo yum install boxes  [On CentOS/RHEL]
$ sudo dnf install boxes  [On Fedora]

Now that you have boxes installed, note that it uses the $HOME/.boxes user specific configuration file or the /etc/boxes/boxes-config system-wide configuration file.

Let’s have some Linux terminal fun.

To see the default boxes design, simply provide some input text to it as shown.

$ echo "Hey, this is Tecmint.com! Thanks for following us." | boxes

/******************************************************/
/* Hey, this is Tecmint.com! Thanks for following us. */
/******************************************************/

To specify another design, use the -d flag as shown.

$ echo "Hey, this is Tecmint.com! Thanks for following us." | boxes -d boy

                        .-"""-.
                       / .===. \
                       \/ 6 6 \/
                       ( \___/ )
  _________________ooo__\_____/_____________________
 /                                                  \
| Hey, this is Tecmint.com! Thanks for following us. |
 \______________________________ooo_________________/
                       |  |  |
                       |_ | _|
                       |  |  |
                       |__|__|
                       /-'Y'-\
                      (__/ \__)

To align or position text inside the box, use the -a flag. Let’s demonstrate how this works with the following example (where the c means center).

$ echo "Hey, this is Tecmint.com! Thanks for following us." | boxes -d diamonds

       /\          /\          /\          /\          /\
    /\//\/\    /\//\/\    /\//\/\    /\//\/\    /\//\/\
 /\//\\///\/\//\\///\/\//\\///\/\//\\///\/\//\\///\/\
//\\//\/\///\\//\/\///\\//\/\///\\//\/\///\\//\/\///\
\//\/Hey, this is Tecmint.com! Thanks for following us.  \/\//
 \/                                                          \/
 /\                                                          /\
//\                                                        //\
\//                                                        \//
 \/                                                          \/
 /\                                                          /\
//\/\                                                    /\//\
\///\/\//\\///\/\//\\///\/\//\\///\/\//\\///\/\//\\//
 \/\///\\//\/\///\\//\/\///\\//\/\///\\//\/\///\\//\/
    \/\//\/    \/\//\/    \/\//\/    \/\//\/    \/\//\/
       \/          \/          \/          \/          \/

$ echo "Hey, this is Tecmint.com! Thanks for following us." | boxes -d diamonds -a c

       /\          /\          /\          /\          /\
    /\//\/\    /\//\/\    /\//\/\    /\//\/\    /\//\/\
 /\//\\///\/\//\\///\/\//\\///\/\//\\///\/\//\\///\/\
//\\//\/\///\\//\/\///\\//\/\///\\//\/\///\\//\/\///\
\//\/                                                    \/\//
 \/                                                          \/
 /\                                                          /\
//\   Hey, this is Tecmint.com! Thanks for following us.   //\
\//                                                        \//
 \/                                                          \/
 /\                                                          /\
//\/\                                                    /\//\
\///\/\//\\///\/\//\\///\/\//\\///\/\//\\///\/\//\\//
 \/\///\\//\/\///\\//\/\///\\//\/\///\\//\/\///\\//\/
    \/\//\/    \/\//\/    \/\//\/    \/\//\/    \/\//\/
       \/          \/          \/          \/          \/

In the Christmas season, you can use the santa design to send your family and friends happy holiday messages, for example.

$ echo "Tecmint.com wishes you a Merry Christmas and a Happy New Year 2019" | boxes -d santa

                                 .-"``"-.
                                /______; \
                               {_______}\|
                               (/ a a \)(_)
                               (.-.).-.)
  _______________________ooo__(    ^    )___________________________
 /                             '-.___.-'                            \
| Tecmint.com wishes you a Merry Christmas and a Happy New Year 2019 |
 \________________________________________ooo_______________________/
                               |_  |  _|  jgs
                               \___|___/
                               {___|___}
                                |_ | _|
                                /-'Y'-\
                               (__/ \__)

To list all available designs/styles, run the following command.

$ boxes -l

59 Available Styles in "/etc/boxes/boxes-config":
-------------------------------------------------

ada-box
(public domain), coded by Neil Bird <neil.bird@rdel.co.uk>:

    ---------------
    --           --
    --           --
    ---------------


ada-cmt
(public domain), coded by Neil Bird <neil.bird@rdel.co.uk>:

    --
    -- regular Ada
    -- comments
...

It supports line justification, box size specification, text padding, indentation, use of regular expressions and much more.

Valentine’s day coming closer, and you wanted to impress your girlfriend or wife in a Linux way, then use boxes as shown.

$ echo -e "\n\tMe: Will you be my Valentine?\n\tGirl: No way\n\tMe: sudo will you be my Valentine?\n\tGirl: Yes..yes..yes! Let's go!" | boxes -d boy

                        .-"""-.
                       / .===. \
                       \/ 6 6 \/
                       ( \___/ )
          _________ooo__\_____/_____________
         /                                  \
        |                                    |
        | Me: Will you be my Valentine?      |
        | Girl: No way                       |
        | Me: sudo will you be my Valentine? |
        | Girl: Yes..yes..yes! Let's go!     |
         \______________________ooo_________/
                       |  |  |
                       |_ | _|
                       |  |  |
                       |__|__|
                       /-'Y'-\
                      (__/ \__)

For more information and examples, go to http://boxes.thomasjensen.com/examples.html.

Boxes is a command line utility that draws a box around its input text. In this article, we will learn how to install and use boxes utility to draw shapes in the Linux terminal. Use the feedback form below to share your thoughts about it.

Source

As an alternative to cron job scheduler, the at command allows you to schedule a command to run once at a given time without editing a configuration file.

The only requirement consists of installing this utility and starting and enabling its execution:

# yum install at              [on CentOS based systems]
$ sudo apt-get install at     [on Debian and derivatives]

Next, start and enable the at service at the boot time.

--------- On SystemD ---------
# systemctl start atd
# systemctl enable atd

--------- On SysVinit ---------
# service atd start
# chkconfig --level 35 atd on

Once atd is running, you can schedule any command or task as follows. We want to send 4 ping probes to www.google.com when the next minute starts (i.e. if it’s 22:20:13, the command will be executed at 22:21:00) and report the result through an email (-m, requires Postfix or equivalent) to the user invoking the command:

# echo "ping -c 4 www.google.com" | at -m now + 1 minute

If you choose to not use the -m option, the command will be executed but nothing will be printed to standard output. You can, however, choose to redirect the output to a file instead.

In addition, please note that at not only allows the following fixed times: now, noon (12:00), and midnight (00:00), but also custom 2-digit (representing hours) and 4-digit times (hours and minutes).

For example,

To run updatedb at 11 pm today (or tomorrow if the current date is greater than 11 pm), do:

# echo "updatedb" | at -m 23

To shutdown the system at 23:55 today (same criteria as in the previous example applies):

# echo "shutdown -h now" | at -m 23:55

You can also delay the execution by minutes, hours, days, weeks, months, or years using the + sign and the desired time specification as in the first example.

Time specifications are subject to the POSIX standard.

Summary

As a rule of thumb, use at instead of cron job scheduler whenever you want to run a command or execute a given task at a well-defined time only once. For other scenarios, use cron.

Source

Breaking News! Microsoft is Creating Linux-based Smartphone OS

Last updated April 1, 2019

Microsoft, the king of desktop operating systems, haven’t had much luck with the mobile operating systems. It’s Windows-based mobile operating systems Windows Mobile and Windows Phone, both failed miserably and have been discontinued.

But that hasn’t deterred Microsoft from trying its hands in the lucrative mobile market again.

Microsoft’s yet another attempt on mobile OS

The unconfirmed news is that Microsoft is creating a Linux-based mobile operating system with a special focus on protecting users’ privacy.

We have put special attention on user privacy. There will be no data collection through Cortana. The OS will not be updated without user permission and above all, the updates won’t require you to restart your system several times. That itself is our biggest achievement till date.

Don Jhoe, Microsoft Product Manager

Reports are not confirmed but the project is internally called “Mazure”. The name could be changed as the project progresses.

The Mazure OS will be completely open source. This shows the firm commitment to open source development from Microsoft. This is another effort from Microsoft to give back to the community by open sourcing essential tools like Windows calculator app.

Targeting 4 billion strong mobile OS market

There are over 4 billion mobile devices in the world. A tech-giant like Microsoft cannot simply give up on this big a market.

The world of mobile operating systems is dominated by Android and iOS and many experts think that it is saturated and a new player doesn’t stand a chance.

But Linux-based KaiOS came into the market in 2017 and created a niche for itself in just one year.

This moderate success of KaiOS perhaps gave Microsoft the idea to launch its own mobile operating system based on Linux.

Lately, a number of open source mobile operating system projects have come up. Almost all of them use Linux kernel underneath.

Linux-based smartphones are also a hot-topic in tech world. Librem5 and Necuno have already announced Linux-based smartphones with focus on privacy.

Microsoft has taken the same idea and started its own Linux-based mobile OS “Mazure” with a promise to protect user’s privacy.

You can find more information about this project on its extremely confidential website below.

Source

A system administrator’s routine tasks include configuring, maintaining, troubleshooting, and managing servers and networks within data centers. There are numerous tools and utilities in Linux designed for the administrative purposes.

In this article, we will review some of the most used command-line tools and utilities for network management in Linux, under different categories. We will explain some common usage examples, which will make network management much easier in Linux.

Table of Contents
ifconfig Command	ip Command	ifup Command	ethtool Command	ping Command
traceroute Command	mtr Command	route Command	nmcli Command	netstat Command
ss Command	nc Command	nmap Command	host Command	dig Command
nslookup Command	tcpdump Command	Wireshark Utility	bmon Tool	iptables Firewall
firewalld	UFW Firewall

This list is equally useful to full-time network engineers.

Network Configuration, Troubleshooting and Debugging Tools

1. ifconfig Command

ifconfig is a command line interface tool for network interface configuration and also used to initialize an interfaces at system boot time. Once a server is up and running, it can be used to assign an IP Address to an interface and enable or disable the interface on demand.

It is also used to view the status IP Address, Hardware / MAC address, as well as MTU (Maximum Transmission Unit) size of the currently active interfaces. ifconfig is thus useful for debugging or performing system tuning.

Here is an example to display status of all active network interfaces.

$ ifconfig

enp1s0    Link encap:Ethernet  HWaddr 28:d2:44:eb:bd:98  
          inet addr:192.168.0.103  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::8f0c:7825:8057:5eec/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:169854 errors:0 dropped:0 overruns:0 frame:0
          TX packets:125995 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:174146270 (174.1 MB)  TX bytes:21062129 (21.0 MB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:15793 errors:0 dropped:0 overruns:0 frame:0
          TX packets:15793 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1 
          RX bytes:2898946 (2.8 MB)  TX bytes:2898946 (2.8 MB)

To list all interfaces which are currently available, whether up or down, use the -a flag.

$ ifconfig -a 	

To assign an IP address to an interface, use the following command.

$ sudo ifconfig eth0 192.168.56.5 netmask 255.255.255.0

To activate an network interface, type.

$ sudo ifconfig up eth0

To deactivate or shut down an network interface, type.

$ sudo ifconfig down eth0

Note: Although ifconfig is a great tool, it is now obsolete (deprecated), its replacement is ip command which is explained below.

2. IP Command

ip command is another useful command line utility for displaying and manipulating routing, network devices, interfaces. It is a replacement for ifconfig and many other networking commands. (Read our article “What’s Difference Between ifconfig and ip Command” to learn more about it.)

The following command will show the IP address and other information about an network interface.

$ ip addr show

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 28:d2:44:eb:bd:98 brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.103/24 brd 192.168.0.255 scope global dynamic enp1s0
       valid_lft 5772sec preferred_lft 5772sec
    inet6 fe80::8f0c:7825:8057:5eec/64 scope link 
       valid_lft forever preferred_lft forever
3: wlp2s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 38:b1:db:7c:78:c7 brd ff:ff:ff:ff:ff:ff
...

To temporarily assign IP Address to a specific network interface (eth0), type.

$ sudo ip addr add 192.168.56.1 dev eth0

To remove an assigned IP address from an network interface (eth0), type.

$ sudo ip addr del 192.168.56.15/24 dev eth0

To show the current neighbour table in kernel, type.

$ ip neigh

192.168.0.1 dev enp1s0 lladdr 10:fe:ed:3d:f3:82 REACHABLE

3. ifup, ifdown, and ifquery command

ifup command actives a network interface, making it available to transfer and receive data.

$ sudo ifup eth0

ifdown command disables a network interface, keeping it in a state where it cannot transfer or receive data.

$ sudo ifdown eth0

ifquery command used to parse the network interface configuration, enabling you to receive answers to query about how it is currently configured.

$ sudo ifquery eth0

4. Ethtool Command

ethtool is a command line utility for querying and modifying network interface controller parameters and device drivers. The example below shows the usage of ethtool and a command to view the parameters for the network interface.

$ sudo ethtool enp0s3

Settings for enp0s3:
	Supported ports: [ TP ]
	Supported link modes:   10baseT/Half 10baseT/Full 
	                        100baseT/Half 100baseT/Full 
	                        1000baseT/Full 
	Supported pause frame use: No
	Supports auto-negotiation: Yes
	Advertised link modes:  10baseT/Half 10baseT/Full 
	                        100baseT/Half 100baseT/Full 
	                        1000baseT/Full 
	Advertised pause frame use: No
	Advertised auto-negotiation: Yes
	Speed: 1000Mb/s
	Duplex: Full
	Port: Twisted Pair
	PHYAD: 0
	Transceiver: internal
	Auto-negotiation: on
	MDI-X: off (auto)
	Supports Wake-on: umbg
	Wake-on: d
	Current message level: 0x00000007 (7)
			       drv probe link
	Link detected: yes

5. Ping Command

ping (Packet INternet Groper) is a utility normally used for testing connectivity between two systems on a network (Local Area Network (LAN) or Wide Area Network (WAN)). It use ICMP (Internet Control Message Protocol) to communicate to nodes on a network.

To test connectivity to another node, simply provide its IP or host name, for example.

$ ping 192.168.0.103

PING 192.168.0.103 (192.168.0.103) 56(84) bytes of data.
64 bytes from 192.168.0.103: icmp_seq=1 ttl=64 time=0.191 ms
64 bytes from 192.168.0.103: icmp_seq=2 ttl=64 time=0.156 ms
64 bytes from 192.168.0.103: icmp_seq=3 ttl=64 time=0.179 ms
64 bytes from 192.168.0.103: icmp_seq=4 ttl=64 time=0.182 ms
64 bytes from 192.168.0.103: icmp_seq=5 ttl=64 time=0.207 ms
64 bytes from 192.168.0.103: icmp_seq=6 ttl=64 time=0.157 ms
^C
--- 192.168.0.103 ping statistics ---
6 packets transmitted, 6 received, 0% packet loss, time 5099ms
rtt min/avg/max/mdev = 0.156/0.178/0.207/0.023 ms

You can also tell ping to exit after a specified number of ECHO_REQUEST packets, using the -c flag as shown.

$ ping -c 4 192.168.0.103

PING 192.168.0.103 (192.168.0.103) 56(84) bytes of data.
64 bytes from 192.168.0.103: icmp_seq=1 ttl=64 time=1.09 ms
64 bytes from 192.168.0.103: icmp_seq=2 ttl=64 time=0.157 ms
64 bytes from 192.168.0.103: icmp_seq=3 ttl=64 time=0.163 ms
64 bytes from 192.168.0.103: icmp_seq=4 ttl=64 time=0.190 ms

--- 192.168.0.103 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3029ms
rtt min/avg/max/mdev = 0.157/0.402/1.098/0.402 ms

6. Traceroute Command

Traceroute is a command line utility for tracing the full path from your local system to another network system. It prints number of hops (router IP’s) in that path you travel to reach the end server. It is an easy-to-use network troubleshooting utility after ping command.

In this example, we are tracing the route packets take from the local system to one of Google’s servers with IP address 216.58.204.46.

$ traceroute 216.58.204.46

traceroute to 216.58.204.46 (216.58.204.46), 30 hops max, 60 byte packets
 1  gateway (192.168.0.1)  0.487 ms  0.277 ms  0.269 ms
 2  5.5.5.215 (5.5.5.215)  1.846 ms  1.631 ms  1.553 ms
 3  * * *
 4  72.14.194.226 (72.14.194.226)  3.762 ms  3.683 ms  3.577 ms
 5  108.170.248.179 (108.170.248.179)  4.666 ms 108.170.248.162 (108.170.248.162)  4.869 ms 108.170.248.194 (108.170.248.194)  4.245 ms
 6  72.14.235.133 (72.14.235.133)  72.443 ms 209.85.241.175 (209.85.241.175)  62.738 ms 72.14.235.133 (72.14.235.133)  65.809 ms
 7  66.249.94.140 (66.249.94.140)  128.726 ms  127.506 ms 209.85.248.5 (209.85.248.5)  127.330 ms
 8  74.125.251.181 (74.125.251.181)  127.219 ms 108.170.236.124 (108.170.236.124)  212.544 ms 74.125.251.181 (74.125.251.181)  127.249 ms
 9  216.239.49.134 (216.239.49.134)  236.906 ms 209.85.242.80 (209.85.242.80)  254.810 ms  254.735 ms
10  209.85.251.138 (209.85.251.138)  252.002 ms 216.239.43.227 (216.239.43.227)  251.975 ms 209.85.242.80 (209.85.242.80)  236.343 ms
11  216.239.43.227 (216.239.43.227)  251.452 ms 72.14.234.8 (72.14.234.8)  279.650 ms  277.492 ms
12  209.85.250.9 (209.85.250.9)  274.521 ms  274.450 ms 209.85.253.249 (209.85.253.249)  270.558 ms
13  209.85.250.9 (209.85.250.9)  269.147 ms 209.85.254.244 (209.85.254.244)  347.046 ms 209.85.250.9 (209.85.250.9)  285.265 ms
14  64.233.175.112 (64.233.175.112)  344.852 ms 216.239.57.236 (216.239.57.236)  343.786 ms 64.233.175.112 (64.233.175.112)  345.273 ms
15  108.170.246.129 (108.170.246.129)  345.054 ms  345.342 ms 64.233.175.112 (64.233.175.112)  343.706 ms
16  108.170.238.119 (108.170.238.119)  345.610 ms 108.170.246.161 (108.170.246.161)  344.726 ms 108.170.238.117 (108.170.238.117)  345.536 ms
17  lhr25s12-in-f46.1e100.net (216.58.204.46)  345.382 ms  345.031 ms  344.884 ms

7. MTR Network Diagnostic Tool

MTR is a modern command-line network diagnostic tool that combines the functionality of ping and traceroute into a single diagnostic tool. Its output is updated in real-time, by default until you exit the program by pressing q.

The easiest way of running mtr is to provide it a host name or IP address as an argument, as follows.

$ mtr google.com
OR
$ mtr 216.58.223.78

Sample Output

tecmint.com (0.0.0.0)                                   Thu Jul 12 08:58:27 2018
First TTL: 1

 Host                                                   Loss%   Snt   Last   Avg  Best  Wrst StDev
 1. 192.168.0.1                                         0.0%    41    0.5   0.6   0.4   1.7   0.2
 2. 5.5.5.215                                           0.0%    40    1.9   1.5   0.8   7.3   1.0
 3. 209.snat-111-91-120.hns.net.in                      23.1%    40    1.9   2.7   1.7  10.5   1.6
 4. 72.14.194.226                                       0.0%    40   89.1   5.2   2.2  89.1  13.7
 5. 108.170.248.193                                     0.0%    40    3.0   4.1   2.4  52.4   7.8
 6. 108.170.237.43                                      0.0%    40    2.9   5.3   2.5  94.1  14.4
 7. bom07s10-in-f174.1e100.net                          0.0%    40    2.6   6.7   2.3  79.7  16.

You can limit the number of pings to a specific value and exit mtr after those pings, using the -c flag as shown.

$ mtr -c 4 google.com

8. Route Command

route is a command line utility for displaying or manipulating the IP routing table of a Linux system. It is mainly used to configure static routes to specific hosts or networks via an interface.

You can view Kernel IP routing table by typing.

$ route

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         gateway         0.0.0.0         UG    100    0        0 enp0s3
192.168.0.0     0.0.0.0         255.255.255.0   U     100    0        0 enp0s3
192.168.122.0   0.0.0.0         255.255.255.0   U     0      0        0 virbr0

There are numerous commands you can use to configure routing. Here are some useful ones:

Add a default gateway to the routing table.

$ sudo route add default gw <gateway-ip>

Add a network route to the routing table.

$ sudo route add -net <network ip/cidr> gw <gateway ip> <interface>

Delete a specific route entry from the routing table.

$ sudo route del -net <network ip/cidr>

9. Nmcli Command

Nmcli is an easy-to-use, scriptable command-line tool to report network status, manage network connections, and control the NetworkManager.

To view all your network devices, type.

$ nmcli dev status

DEVICE      TYPE      STATE      CONNECTION         
virbr0      bridge    connected  virbr0             
enp0s3      ethernet  connected  Wired connection 1 

To check network connections on your system, type.

$ nmcli con show

Wired connection 1  bc3638ff-205a-3bbb-8845-5a4b0f7eef91  802-3-ethernet  enp0s3 
virbr0              00f5d53e-fd51-41d3-b069-bdfd2dde062b  bridge          virbr0 

To see only the active connections, add the -a flag.

$ nmcli con show -a

Network Scanning and Performance Analysis Tools

10. Netstat Command

netstat is a command line tool that displays useful information such as network connections, routing tables, interface statistics, and much more, concerning the Linux networking subsystem. It is useful for network troubleshooting and performance analysis.

Additionally, it is also a fundamental network service debugging tool used to check which programs are listening on what ports. For instance, the following command will show all TCP ports in listening mode and what programs are listening on them.

$ sudo netstat -tnlp

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
tcp        0      0 0.0.0.0:587             0.0.0.0:*               LISTEN      1257/master         
tcp        0      0 127.0.0.1:5003          0.0.0.0:*               LISTEN      1/systemd           
tcp        0      0 0.0.0.0:110             0.0.0.0:*               LISTEN      1015/dovecot        
tcp        0      0 0.0.0.0:143             0.0.0.0:*               LISTEN      1015/dovecot        
tcp        0      0 0.0.0.0:111             0.0.0.0:*               LISTEN      1/systemd           
tcp        0      0 0.0.0.0:465             0.0.0.0:*               LISTEN      1257/master         
tcp        0      0 0.0.0.0:53              0.0.0.0:*               LISTEN      1404/pdns_server    
tcp        0      0 0.0.0.0:21              0.0.0.0:*               LISTEN      1064/pure-ftpd (SER 
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      972/sshd            
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      975/cupsd           
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN      1257/master         
tcp        0      0 0.0.0.0:8090            0.0.0.0:*               LISTEN      636/lscpd (lscpd -  
tcp        0      0 0.0.0.0:993             0.0.0.0:*               LISTEN      1015/dovecot        
tcp        0      0 0.0.0.0:995             0.0.0.0:*               LISTEN      1015/dovecot        
tcp6       0      0 :::3306                 :::*                    LISTEN      1053/mysqld         
tcp6       0      0 :::3307                 :::*                    LISTEN      1211/mysqld         
tcp6       0      0 :::587                  :::*                    LISTEN      1257/master         
tcp6       0      0 :::110                  :::*                    LISTEN      1015/dovecot        
tcp6       0      0 :::143                  :::*                    LISTEN      1015/dovecot        
tcp6       0      0 :::111                  :::*                    LISTEN      1/systemd           
tcp6       0      0 :::80                   :::*                    LISTEN      990/httpd           
tcp6       0      0 :::465                  :::*                    LISTEN      1257/master         
tcp6       0      0 :::53                   :::*                    LISTEN      1404/pdns_server    
tcp6       0      0 :::21                   :::*                    LISTEN      1064/pure-ftpd (SER 
tcp6       0      0 :::22                   :::*                    LISTEN      972/sshd            
tcp6       0      0 ::1:631                 :::*                    LISTEN      975/cupsd           
tcp6       0      0 :::25                   :::*                    LISTEN      1257/master         
tcp6       0      0 :::993                  :::*                    LISTEN      1015/dovecot        
tcp6       0      0 :::995                  :::*                    LISTEN      1015/dovecot        

To view kernel routing table, use the -r flag (which is equivalent to running route command above).

$ netstat -r

Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
default         gateway         0.0.0.0         UG        0 0          0 enp0s3
192.168.0.0     0.0.0.0         255.255.255.0   U         0 0          0 enp0s3
192.168.122.0   0.0.0.0         255.255.255.0   U         0 0          0 virbr0

Note: Although Netstat is a great tool, it is now obsolete (deprecated), its replacement is ss command which is explained below.

11. ss Command

ss (socket statistics) is a powerful command line utility to investigate sockets. It dumps socket statistics and displays information similar to netstat. In addition, it shows more TCP and state information compared to other similar utilities.

The following example show how to list all TCP ports (sockets) that are open on a server.

$ ss -ta

State      Recv-Q Send-Q                                        Local Address:Port                                                         Peer Address:Port                
LISTEN     0      100                                                       *:submission                                                              *:*                    
LISTEN     0      128                                               127.0.0.1:fmpro-internal                                                          *:*                    
LISTEN     0      100                                                       *:pop3                                                                    *:*                    
LISTEN     0      100                                                       *:imap                                                                    *:*                    
LISTEN     0      128                                                       *:sunrpc                                                                  *:*                    
LISTEN     0      100                                                       *:urd                                                                     *:*                    
LISTEN     0      128                                                       *:domain                                                                  *:*                    
LISTEN     0      9                                                         *:ftp                                                                     *:*                    
LISTEN     0      128                                                       *:ssh                                                                     *:*                    
LISTEN     0      128                                               127.0.0.1:ipp                                                                     *:*                    
LISTEN     0      100                                                       *:smtp                                                                    *:*                    
LISTEN     0      128                                                       *:8090                                                                    *:*                    
LISTEN     0      100                                                       *:imaps                                                                   *:*                    
LISTEN     0      100                                                       *:pop3s                                                                   *:*                    
ESTAB      0      0                                             192.168.0.104:ssh                                                         192.168.0.103:36398                
ESTAB      0      0                                                 127.0.0.1:34642                                                           127.0.0.1:opsession-prxy       
ESTAB      0      0                                                 127.0.0.1:34638                                                           127.0.0.1:opsession-prxy       
ESTAB      0      0                                                 127.0.0.1:34644                                                           127.0.0.1:opsession-prxy       
ESTAB      0      0                                                 127.0.0.1:34640                                                           127.0.0.1:opsession-prxy       
LISTEN     0      80                                                       :::mysql                                                                  :::*             
...

To display all active TCP connections together with their timers, run the following command.

$ ss -to

12 NC Command

NC (NetCat) also referred to as the “Network Swiss Army knife”, is a powerful utility used for almost any task related to TCP, UDP, or UNIX-domain sockets. It is used open TCP connections, listen on arbitrary TCP and UDP ports, perform port scanning plus more.

You can also use it as a simple TCP proxies, for network daemon testing, to check if remote ports are reachable and much more. Furthermore, you can employ nctogether with pv command to transfer files between two computers.

The following example, will show how to scan a list of ports.

$ nc -zv server2.tecmint.lan 21 22 80 443 3000

You can also specify a range of ports as shown.

$ nc -zv server2.tecmint.lan 20-90

The following example shows how to use nc to open a TCP connection to port 5000 on server2.tecmint.lan, using port 3000 as the source port, with a timeout of 10 seconds.

$ nc -p 3000 -w 10 server2.tecmint.lan 5000 

13. Nmap Command

Nmap (Network Mapper) is a powerful and extremely versatile tool for Linux system/network administrators. It is used gather information about a single host or explore networks an entire network. Nmap is also used to perform security scans, network audit and finding open ports on remote hosts and so much more.

You can scan a host using its host name or IP address, for instance.

$ nmap google.com 

Starting Nmap 6.40 ( http://nmap.org ) at 2018-07-12 09:23 BST
Nmap scan report for google.com (172.217.166.78)
Host is up (0.0036s latency).
rDNS record for 172.217.166.78: bom05s15-in-f14.1e100.net
Not shown: 998 filtered ports
PORT    STATE SERVICE
80/tcp  open  http
443/tcp open  https

Nmap done: 1 IP address (1 host up) scanned in 4.92 seconds

Alternatively, use an IP address as shown.

$ nmap 192.168.0.103

Starting Nmap 6.40 ( http://nmap.org ) at 2018-07-12 09:24 BST
Nmap scan report for 192.168.0.103
Host is up (0.000051s latency).
Not shown: 994 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
25/tcp   open  smtp
902/tcp  open  iss-realsecure
4242/tcp open  vrml-multi-use
5900/tcp open  vnc
8080/tcp open  http-proxy
MAC Address: 28:D2:44:EB:BD:98 (Lcfc(hefei) Electronics Technology Co.)

Nmap done: 1 IP address (1 host up) scanned in 0.13 seconds

Read our following useful articles on nmap command.

1. How to Use Nmap Script Engine (NSE) Scripts in Linux
2. A Practical Guide to Nmap (Network Security Scanner) in Kali Linux
3. Find Out All Live Hosts IP Addresses Connected on Network in Linux

DNS Lookup Utilities

14. host Command

host command is a simple utility for carrying out DNS lookups, it translates host names to IP addresses and vice versa.

$ host google.com

google.com has address 172.217.166.78
google.com mail is handled by 20 alt1.aspmx.l.google.com.
google.com mail is handled by 30 alt2.aspmx.l.google.com.
google.com mail is handled by 40 alt3.aspmx.l.google.com.
google.com mail is handled by 50 alt4.aspmx.l.google.com.
google.com mail is handled by 10 aspmx.l.google.com.

15. dig Command

dig (domain information groper) is also another simple DNS lookup utility, that is used to query DNS related information such as A Record, CNAME, MX Record etc, for example:

$ dig google.com

; <<>> DiG 9.9.4-RedHat-9.9.4-51.el7 <<>> google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 23083
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 13, ADDITIONAL: 14

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;google.com.			IN	A

;; ANSWER SECTION:
google.com.		72	IN	A	172.217.166.78

;; AUTHORITY SECTION:
com.			13482	IN	NS	c.gtld-servers.net.
com.			13482	IN	NS	d.gtld-servers.net.
com.			13482	IN	NS	e.gtld-servers.net.
com.			13482	IN	NS	f.gtld-servers.net.
com.			13482	IN	NS	g.gtld-servers.net.
com.			13482	IN	NS	h.gtld-servers.net.
com.			13482	IN	NS	i.gtld-servers.net.
com.			13482	IN	NS	j.gtld-servers.net.
com.			13482	IN	NS	k.gtld-servers.net.
com.			13482	IN	NS	l.gtld-servers.net.
com.			13482	IN	NS	m.gtld-servers.net.
com.			13482	IN	NS	a.gtld-servers.net.
com.			13482	IN	NS	b.gtld-servers.net.

;; ADDITIONAL SECTION:
a.gtld-servers.net.	81883	IN	A	192.5.6.30
b.gtld-servers.net.	3999	IN	A	192.33.14.30
c.gtld-servers.net.	14876	IN	A	192.26.92.30
d.gtld-servers.net.	85172	IN	A	192.31.80.30
e.gtld-servers.net.	95861	IN	A	192.12.94.30
f.gtld-servers.net.	78471	IN	A	192.35.51.30
g.gtld-servers.net.	5217	IN	A	192.42.93.30
h.gtld-servers.net.	111531	IN	A	192.54.112.30
i.gtld-servers.net.	93017	IN	A	192.43.172.30
j.gtld-servers.net.	93542	IN	A	192.48.79.30
k.gtld-servers.net.	107218	IN	A	192.52.178.30
l.gtld-servers.net.	6280	IN	A	192.41.162.30
m.gtld-servers.net.	2689	IN	A	192.55.83.30

;; Query time: 4 msec
;; SERVER: 192.168.0.1#53(192.168.0.1)
;; WHEN: Thu Jul 12 09:30:57 BST 2018
;; MSG SIZE  rcvd: 487

16. NSLookup Command

Nslookup is also a popular command line utility to query DNS servers both interactively and non-interactively. It is used to query DNS resource records (RR). You can find out “A” record (IP address) of a domain as shown.

$ nslookup google.com

Server:		192.168.0.1
Address:	192.168.0.1#53

Non-authoritative answer:
Name:	google.com
Address: 172.217.166.78

You can also perform a reverse domain lookup as shown.

$ nslookup 216.58.208.174

Server:		192.168.0.1
Address:	192.168.0.1#53

Non-authoritative answer:
174.208.58.216.in-addr.arpa	name = lhr25s09-in-f14.1e100.net.
174.208.58.216.in-addr.arpa	name = lhr25s09-in-f174.1e100.net.

Authoritative answers can be found from:
in-addr.arpa	nameserver = e.in-addr-servers.arpa.
in-addr.arpa	nameserver = f.in-addr-servers.arpa.
in-addr.arpa	nameserver = a.in-addr-servers.arpa.
in-addr.arpa	nameserver = b.in-addr-servers.arpa.
in-addr.arpa	nameserver = c.in-addr-servers.arpa.
in-addr.arpa	nameserver = d.in-addr-servers.arpa.
a.in-addr-servers.arpa	internet address = 199.180.182.53
b.in-addr-servers.arpa	internet address = 199.253.183.183
c.in-addr-servers.arpa	internet address = 196.216.169.10
d.in-addr-servers.arpa	internet address = 200.10.60.53
e.in-addr-servers.arpa	internet address = 203.119.86.101
f.in-addr-servers.arpa	internet address = 193.0.9.1

Linux Network Packet Analyzers

17. Tcpdump Command

Tcpdump is a very powerful and widely used command-line network sniffer. It is used to capture and analyze TCP/IP packets transmitted or received over a network on a specific interface.

To capture packets from a given interface, specify it using the -i option.

$ tcpdump -i eth1

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on enp0s3, link-type EN10MB (Ethernet), capture size 262144 bytes
09:35:40.287439 IP tecmint.com.ssh > 192.168.0.103.36398: Flags [P.], seq 4152360356:4152360552, ack 306922699, win 270, options [nop,nop,TS val 2211778668 ecr 2019055], length 196
09:35:40.287655 IP 192.168.0.103.36398 > tecmint.com.ssh: Flags [.], ack 196, win 5202, options [nop,nop,TS val 2019058 ecr 2211778668], length 0
09:35:40.288269 IP tecmint.com.54899 > gateway.domain: 43760+ PTR? 103.0.168.192.in-addr.arpa. (44)
09:35:40.333763 IP gateway.domain > tecmint.com.54899: 43760 NXDomain* 0/1/0 (94)
09:35:40.335311 IP tecmint.com.52036 > gateway.domain: 44289+ PTR? 1.0.168.192.in-addr.arpa. (42)

To capture a specific number of packets, use the -c option to enter the desired number.

$ tcpdump -c 5 -i eth1

You can also capture and save packets to a file for later analysis, use the -w flag to specify the output file.

$ tcpdump -w captured.pacs -i eth1

18. Wireshark Utility

Wireshark is a popular, powerful, versatile and easy to use tool for capturing and analyzing packets in a packet-switched network, in real-time.

You can also save data it has captured to a file for later inspection. It is used by system administrators and network engineers to monitor and inspect the packets for security and troubleshooting purposes.

Read our article “10 Tips On How to Use Wireshark to Analyze Network Packets to learn more about Wireshark”.

19. Bmon Tool

bmon is a powerful, command line based network monitoring and debugging utility for Unix-like systems, it captures networking related statistics and prints them visually in a human friendly format. It is a reliable and effective real-time bandwidth monitor and rate estimator.

Read our article “bmon – A Powerful Network Bandwidth Monitoring and Debugging Tool to learn more about bmon”.

Linux Firewall Management Tools

20. Iptables Firewall

iptables is a command line tool for configuring, maintaining, and inspecting the tables IP packet filtering and NAT ruleset. It it used to set up and manage the Linux firewall (Netfilter). It allows you to list existing packet filter rules; add or delete or modify packet filter rules; list per-rule counters of the packet filter rules.

You can learn how to use Iptables for various purposes from our simple yet comprehensive guides.

1. Basic Guide on IPTables (Linux Firewall) Tips / Commands
2. 25 Useful IPtable Firewall Rules Every Linux Administrator Should Know
3. How To Setup an Iptables Firewall to Enable Remote Access to Services
4. How to Block Ping ICMP Requests to Linux Systems

21. Firewalld

Firewalld is a powerful and dynamic daemon to manage the Linux firewall (Netfilter), just like iptables. It uses “networks zones” instead of INPUT, OUTPUT and FORWARD CHAINS in iptables. On current Linux distributions such as RHEL/CentOS 7 and Fedora 21+, iptables is actively being replaced by firewalld.

To get started with firewalld, consult these guides listed below:

1. Useful ‘FirewallD’ Rules to Configure and Manage Firewall in Linux
2. How to Configure ‘FirewallD’ in RHEL/CentOS 7 and Fedora 21
3. How to Start/Stop and Enable/Disable FirewallD and Iptables Firewall in Linux
4. Setting Up Samba and Configure FirewallD and SELinux to Allow File Sharing on Linux/Windows

Important: Iptables is still supported and can be installed with YUM package manager. However, you can’t use Firewalld and iptables at the same time on same server – you must choose one.

22. UFW (Uncomplicated Firewall)

UFW is a well known and default firewall configuration tool on Debian and Ubuntu Linux distributions. It is used top enable/disable system firewall, add/delete/modify/reset packet filtering rules and much more.

To check UFW firewall status, type.

$ sudo ufw status

If UFW firewall is not active, you can activate or enable it using the following command.

$ sudo ufw enable

To disable UFW firewall, use the following command.

$ sudo ufw disable 

Read our article “How to Setup UFW Firewall on Ubuntu and Debian” to learn more UFW).

If you want to find more information about a particular program, you can consult its man pages as shown.

$ man programs_name

That’s all for now! In this comprehensive guide, we reviewed some of the most used command-line tools and utilities for network management in Linux, under different categories, for system administrators, and equally useful to full-time network administrators/engineers.

You can share your thoughts about this guide via the comment form below. If we have missed any frequently used and important Linux networking tools/utilities or any useful related information, also let us know.

Source

In programming, a library is an assortment of pre-compiled pieces of code that can be reused in a program. Libraries simplify life for programmers, in that they provide reusable functions, routines, classes, data structures and so on (written by a another programmer), which they can use in their programs.

For instance, if you are building an application that needs to perform math operations, you don’t have to create a new math function for that, you can simply use existing functions in libraries for that programming language.

Examples of libraries in Linux include libc (the standard C library) or glibc (GNU version of the standard C library), libcurl (multiprotocol file transfer library), libcrypt (library used for encryption, hashing, and encoding in C) and many more.

Linux supports two classes of libraries, namely:

• Static libraries – are bound to a program statically at compile time.
• Dynamic or shared libraries – are loaded when a program is launched and loaded into memory and binding occurs at run time.

Dynamic or shared libraries can further be categorized into:

• Dynamically linked libraries – here a program is linked with the shared library and the kernel loads the library (in case it’s not in memory) upon execution.
• Dynamically loaded libraries – the program takes full control by calling functions with the library.

Shared Library Naming Conventions

Shared libraries are named in two ways: the library name (a.k.a soname) and a “filename” (absolute path to file which stores library code).

For example, the soname for libc is libc.so.6: where lib is the prefix, c is a descriptive name, so means shared object, and 6 is the version. And its filename is: /lib64/libc.so.6. Note that the soname is actually a symbolic link to the filename.

Locating Shared Libraries in Linux

Shared libraries are loaded by ld.so (or ld.so.x) and ld-linux.so (or ld-linux.so.x) programs, where x is the version. In Linux, /lib/ld-linux.so.x searches and loads all shared libraries used by a program.

A program can call a library using its library name or filename, and a library path stores directories where libraries can be found in the filesystem. By default, libraries are located in /usr/local/lib, /usr/local/lib64, /usr/lib and /usr/lib64; system startup libraries are in /lib and /lib64. Programmers can, however, install libraries in custom locations.

The library path can be defined in /etc/ld.so.conf file which you can edit with a command line editor.

# vi /etc/ld.so.conf 

The line(s) in this file instruct the kernel to load file in /etc/ld.so.conf.d. This way, package maintainers or programmers can add their custom library directories to the search list.

If you look into the /etc/ld.so.conf.d directory, you’ll see .conf files for some common packages (kernel, mysql and postgresql in this case):

# ls /etc/ld.so.conf.d

kernel-2.6.32-358.18.1.el6.x86_64.conf  kernel-2.6.32-696.1.1.el6.x86_64.conf  mariadb-x86_64.conf
kernel-2.6.32-642.6.2.el6.x86_64.conf   kernel-2.6.32-696.6.3.el6.x86_64.conf  postgresql-pgdg-libs.conf

If you take a look at the mariadb-x86_64.conf, you will see an absolute path to package’s libraries.

# cat mariadb-x86_64.conf

/usr/lib64/mysql

The method above sets the library path permanently. To set it temporarily, use the LD_LIBRARY_PATHenvironment variable on the command line. If you want to keep the changes permanent, then add this line in the shell initialization file /etc/profile (global) or ~/.profile (user specific).

# export LD_LIBRARY_PATH=/path/to/library/file

Managing Shared Libraries in Linux

Let us now look at how to deal with shared libraries. To get a list of all shared library dependencies for a binary file, you can use the ldd utility. The output of ldd is in the form:

library name =>  filename (some hexadecimal value)
OR
filename (some hexadecimal value)  #this is shown when library name can’t be read

This command shows all shared library dependencies for the ls command.

# ldd /usr/bin/ls
OR
# ldd /bin/ls

Sample Output

	linux-vdso.so.1 =>  (0x00007ffebf9c2000)
	libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003b71e00000)
	librt.so.1 => /lib64/librt.so.1 (0x0000003b71600000)
	libcap.so.2 => /lib64/libcap.so.2 (0x0000003b76a00000)
	libacl.so.1 => /lib64/libacl.so.1 (0x0000003b75e00000)
	libc.so.6 => /lib64/libc.so.6 (0x0000003b70600000)
	libdl.so.2 => /lib64/libdl.so.2 (0x0000003b70a00000)
	/lib64/ld-linux-x86-64.so.2 (0x0000561abfc09000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003b70e00000)
	libattr.so.1 => /lib64/libattr.so.1 (0x0000003b75600000)

Because shared libraries can exist in many different directories, searching through all of these directories when a program is launched would be greatly inefficient: which is one of the likely disadvantages of dynamic libraries. Therefore a mechanism of caching employed, performed by a the program ldconfig.

By default, ldconfig reads the content of /etc/ld.so.conf, creates the appropriate symbolic links in the dynamic link directories, and then writes a cache to /etc/ld.so.cache which is then easily used by other programs.

This is very important especially when you have just installed new shared libraries or created your own, or created new library directories. You need to run ldconfig command to effect the changes.

# ldconfig
OR
# ldconfig -v 	#shows files and directories it works with

After creating your shared library, you need to install it. You can either move it into any of the standard directories mentioned above, and run the ldconfig command.

Alternatively, run the following command to create symbolic links from the soname to the filename:

# ldconfig -n /path/to/your/shared/libraries

To get started with creating your own libraries, check out this guide from The Linux Documentation Project(TLDP).

Thats all for now! In this article, we gave you an introduction to libraries, explained shared libraries and how to manage them in Linux. If you have any queries or additional ideas to share, use the comment form below.

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Very recently I have written a post on fdupes utility which is used to find and replace duplicate files in Linux. This post was very much liked by our readers. If you have not gone through the fdupes utility post, you may like to go through it here:

1. fdupes Tool to Find and Delete Duplicate Files

This post aims at throwing light on what is fslint, its features, installation and usages.

What is fslint?

fslint is a Linux utility to remove unwanted and problematic cruft in files and file names and thus keeps the computer clean. A large volume of unnecessary and unwanted files are called lint. fslint remove such unwanted lint from files and file names. Fslint help fight against unwanted files by coping with duplicate files, empty directories and improper names.

Features of fslint

1. It is a combination of different tools that look after duplicate files, empty directories and improper name.
2. Simple GTK+ Graphic front-end as well as command-line.
3. Fslint cope with lint that relates to Duplicate files, Problematic filenames, Temporary files, Bad Symlinks, Empty directories and Non-stripped binaries.
4. Help you in reclaiming disk space that were used by unnecessary and unwanted files.

Install fslint on a Linux

Installation of latest version of fslint package can be installed as easy as executing following command on Debian based systems such as Ubuntu and Linux Mint.

$ sudo apt-get install fslint

On CentOS/RHEL based distributions, you need to active epel repository to install fslint package.

# yum install  fslint
# dnf install  fslint    [On Fedora 22 onwards]

How do I use fslint Command?

Hope you know one of the basic rule of computation and understand the risk – have backup. Before you start testing this application make sure you have backup of everything on your system, so that even if an important file gets deleted you may restore almost immediately.

Now as you know that fslint is one such application that has a command-line interface as well as a front-end GUI at the same time. You may use either.

For developers and administrators, CLI version is preferred as it gives you immense power. GUI front-end is best suited to newbies and those who prefer GUI over CLI.

fslint Command Line Usage

The command line version of fslint is not on the path of most of the Linux users. You may access it at the location /usr/share/fslint/.

$ ./usr/share/fslint/fslint/fslint

Sample Output

-----------------------------------file name lint
./.config/google-chrome/Default/Pepper\ Data/Shockwave\ Flash/WritableRoot/#SharedObjects/NNPAG57S/videos.bhaskar.com/[[IMPORT]]
./Documents/.~lock.fslint\ -\ Remove\ duplicate\ files\ with\ fslint\ (230).odt#
./Documents/7\ Best\ Audio\ Player\ Plugins\ for\ WordPress\ (220).odt
./Documents/7\ Best\ WordPress\ Help\ Desk\ Plugins\ for\ Customer\ Support\ (219).odt
./Documents/A\ Linux\ User\ using\ Windows\ (Windows\ 10)\ after\ more\ than\ 8\ years(229).odt
./Documents/Add\ PayPal\ to\ WordPress(211).odt
./Documents/Atom\ Text\ Editor\ (202).odt
./Documents/Create\ Mailchimp\ account\ and\ Integrate\ it\ with\ WordPress(227).odt
./Documents/Export\ Feedburner\ feed\ and\ Import\ it\ to\ Mailchimp\ &\ setup\ RSS\ Feed\ Newsletter\ in\ Mailchimp(228).odt

----------------------------------DUPlicate files
Job 7, “/usr/share/fslint/fslint/fslint” has stopped

Important: Two things you should be kept in mind at this point. First fslint don’t delete any file on its own, It just shows you the lint files, their location and their name. You have to decide what to do with them. Second is fslint by default start searching from your ‘/home’ directory.

To search a different other than your /home directory, you must pass the directory name with the command, as:

$ /usr/share/fslint/fslint/fslint /home/avi/Pictures

To search recursively to all the sub-folders, you should use flag ‘-r’, simply as:

$ /usr/share/fslint/fslint/fslint -r /home/avi/Music/

fslint GUI Usage

You may fire the GUI Application built on top of fslint by typing fslint from Linux terminal or from the Application Menu.

$ fslint-gui

Everything in GUI is simple to understand. All you need to do is:

1. Add/remove the directories to scan.
2. Select to scan recursively or not by checking/unchecking checkbox on the top-right.
3. Click on ‘Find’. And all done!

Again you should remember, this utility do not delete the lint files but provide you with the information only and leaves everything on you.

Conclusion

fslint is a perfect tool that remove lint of various types from a file system. Though it needs improvement in certain gray areas: –

1. A bit slow for duplicate photo detection.
2. Requires some improvement in User Interface.
3. No Progress meter.

Hope you liked the post. If yes! Be audible. Post your valuable feedback in the comments below. Stay tuned and connected to Tecmint while I am working on another post you will love to read. Like and share us and help us get spread.

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One of the Linux’s beauties is that you can control almost everything about it. This gives a system administrator a great control over his system and better utilization of the system resources.

While some might have never thought about doing this, it is important to know that in Linux you can limit how much resource a single user may use and for how long.

Read Also: How to Increase Number of Open Files Limit in Linux

In this short topic, we will show you how to limit the number of processes started by user and how to check the current limits and modify them.

Before we go any further there are two things we need to point:

1. You need root access to your system to modify the user limits
2. You must be extremely careful if you plan to modify these limits

To setup user limits, we will need to edit the following file:

/etc/security/limits.conf

This file is used to apply ulimit created by the pam_module.

The file has the following syntax:

<domain> <type> <item> <value>

Here we will stop to discuss each of the options:

• Domain – this includes usernames, groups, guid ranges etc
• Type – soft and hard limits
• Item – the item that will be limited – core size, file size,  nproc etc
• Value – this is the value for the given limit

A good sample for a limit is:

@student          hard           nproc                20

The above line sets a hard limit of maximum 20 processes on the "student" group.

If you want to see the limits of a certain process has you can simply “cat” the limits file like this:

# cat /proc/PID/limits

Where PID is the actual process ID, you can find out process id by using ps command. For more detailed explanation, read our article that says – Find Running Linux Processes and Set Process Limits Per-User Level

So here is an example:

# cat /proc/2497/limits

Sample Output

Limit                     Soft Limit           Hard Limit           Units     
Max cpu time              unlimited            unlimited            seconds   
Max file size             unlimited            unlimited            bytes     
Max data size             unlimited            unlimited            bytes     
Max stack size            8388608              unlimited            bytes     
Max core file size        0                    unlimited            bytes     
Max resident set          unlimited            unlimited            bytes     
Max processes             32042                32042                processes 
Max open files            1024                 4096                 files     
Max locked memory         65536                65536                bytes     
Max address space         unlimited            unlimited            bytes     
Max file locks            unlimited            unlimited            locks     
Max pending signals       32042                32042                signals   
Max msgqueue size         819200               819200               bytes     
Max nice priority         0                    0                    
Max realtime priority     0                    0                    
Max realtime timeout      unlimited            unlimited            us   

All of the lines are pretty much self explanatory. However if you want to find more the settings you can input in limits.conf file, you can have a look at the manual provided here.

If you have any questions or comments, please do not hesitate to submit them in the comment section below.

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One of the various commands we mentioned was the who command which displays users who are currently logged on to a Linux system, including the terminals they are connecting from.

This article will explain some useful examples of who command for Linux newbies.

The basic syntax for using who command is as follows.

$ who who [OPTION]... [ FILE | ARG1 ARG2 ]

1. If you run who command without any arguments, it will display account information (user login name, user’s terminal, time of login as well as the host the user is logged in from) on your system similar to the one shown in the following output.

$ who

ravi		tty1	        2018-03-16	19:27
tecmint	        pts/0		2018-03-16	19:26	(192.168.56.1)
root		pts/1		2018-03-16	19:27	(192.168.56.1)

2. To print the heading of the columns displayed, use the -H flag as shown.

$ who -H

NAME            LINE                   TIME             COMMENT
ravi		tty1	        2018-03-16   19:27
tecmint	        pts/0		2018-03-16   19:26	(192.168.56.1)
root		pts/1		2018-03-16   19:27	(192.168.56.1) 

3. To print the login names and total number of logged on users, use the -q flag.

$ who -q

ravi   tecmint    root
# users=3

4. In case you want to show only hostname and user associated with stdin, use the -m switch.

$ who -m

tecmint	        pts/0		2018-03-16	19:26	(192.168.56.1)

5. Next, to add user’s message status as +, - or ?, use the -T option.

$ who -T

ravi	      +  tty1	        2018-03-16	19:27
tecmint	      +  pts/0		2018-03-16	19:26	(192.168.56.1)
root	      +  pts/1		2018-03-16	19:27	(192.168.56.1)

The who command also helps you to view some useful system information such as last boot time, current runlevel (target under systemd), print dead processes as well as processes spawned by init.

6. To view the time of last system boot, use the -b flag and adding the -u option allows for listing of logged on users in the same output.

$ who -b

system boot  2018-01-19 02:39

$ who -bu

                system boot  2018-03-16 19:25
ravi		tty1		2018-03-16		19:27  00:33		2366
tecmint	        pts/0	        2018-03-16	        19:26	 .              2332     (192.168.56.1)
root		pts/1		2018-03-16		19:27	00:32           2423     (192.168.56.1)

7. You can check the current runlevel with the -r option.

$ who -r

run-level 3  2018-03-16 02:39

8. The following command will print dead processes.

$ who -d

pts/1        2018-03-16 11:10              9986 id=ts/1  term=0 exit=0

9. Furthermore, to see active processes spawned by init, use the -p option.

$ who -p

10. Last but not least, the -a flag allows for printing of default output combined with information from some of the options we have covered.

$ who -a
 
system boot  2018-06-16 02:39
           run-level 3  2018-01-19 02:39
LOGIN      tty1         2018-01-19 02:39              3258 id=1
LOGIN      ttyS0        2018-01-19 02:39              3259 id=S0
tecmnt   + pts/0        2018-03-16 05:33   .          20678 (208.snat-111-91-115.hns.net.in)
           pts/1        2018-03-14 11:10              9986 id=ts/1  term=0 exit=0

You can find more options by consulting the who man page.

$ man who 

In this article, we have explained 10 who command examples for Linux newbies. Use the comment section below to ask any questions or give us your feedback.

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The timedatectl command is a new utility for RHEL/CentOS 7 and Fedora 21+ based distributions, which comes as a part of systemd system and service manager, a replacement for old traditional date command used in sysvinit daemon based Linux distributions.

The timedatectl command allows you to query and change the configuration of the system clock and its settings, you can use this command to set or change the current date, time and timezone or enable automatic system clock synchronization with a remote NTP server.

In this tutorial, am going to take you through the ways you can manage time on your Linux system by setting the date, time, timezone and synchronize time with NTP from the terminal using new timedatectl command.

It is always a good practice to maintain the correct time on your Linux server or system and it can have the following advantages:

1. maintain a timely operation of system tasks since most tasks in Linux are controlled by time.
2. correct time for logging events and other information on the system and many more.

How to Find and Set Local Timezone in Linux

1. To display the current time and date on your system, use the timedatectl command from the commandline as follows:

# timedatectl  status

In the scrrentcast above, RTC time: is the hardware clock time.

2. The time on your Linux system is always managed through the timezone set on the system, to view your current timezone, do it as follows:

# timedatectl 
OR
# timedatectl | grep Time

3. To view all available timezones, run the command below:

# timedatectl list-timezones

4. To find the local timezone according to your location, run the following command:

# timedatectl list-timezones |  egrep  -o “Asia/B.*”
# timedatectl list-timezones |  egrep  -o “Europe/L.*”
# timedatectl list-timezones |  egrep  -o “America/N.*”

5. To set your local timezone in Linux, we will use set-timezone switch as shown below.

# timedatectl set-timezone “Asia/Kolkata”

It is always recommended to use and set the coordinated universal time, UTC.

# timedatectl set-timezone UTC

You need to type the correct name timezone other wise you may get errors when changing the timezone, in the following example, the timezone “Asia/Kalkata” is not correct therefore causing the error.

How to Set Time and Date in Linux

6. You can set the date and time on your system, using the timedatectl command as follows:

Set Time in Linux

To set time only, we can use set-time switch along the format of time in HH:MM:SS (Hour, Minute and Seconds).

# timedatectl set-time 15:58:30

Set Date in Linux

7. To set date only, we can use set-time switch along the format of date in YY:MM:DD (Year, Month, Day).

# timedatectl set-time 20151120

8. To set both date and time:

# timedatectl set-time '2015-11-20 16:14:50'

How to Find and Set Hardware Clock in Linux

9. To set your hardware clock to coordinated universal time, UTC, use the set-local-rtc boolean-value option as follows:

First Find out if your hardware clock is set to local timezone:

# timedatectl | grep local

Set your hardware clock to local timezone:

# timedatectl set-local-rtc 1

Set your hardware clock to coordinated universal time (UTC):

# timedatectl set-local-rtc 0

Synchronizing Linux System Clock with a Remote NTP Server

NTP stands for Network Time Protocol is a internet protocol, which is used to synchronize system clock between computers. The timedatectl utility enables you to automatically sync your Linux system clock with a remote group of servers using NTP.

Please note that, you must have NTP installed on the system to enable automatic time synchronization with NTP servers.

To start automatic time synchronization with remote NTP server, type the following command at the terminal.

# timedatectl set-ntp true

To disable NTP time synchronization, type the following command at the terminal.

# timedatectl set-ntp false

Summary

These are very easy examples described in this tutorial and I hope you will find them helpful for setting various Linux system clocks and timezones. To learn more about this tool, head over to timedatectl man page.

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