GNU/Linux


CentOS 6: install / start and stop / enable and disable ssh server

Install

To install the openssh-server, you need to install the openssh-server package:

sudo yum install -y openssh-server;

Start

To start the sshd daemon (openssh-server) in the current session:

sudo service sshd start;

Stop

To stop the active (if any) sshd daemon in the current session:

sudo service sshd stop;

Enable

To configure the sshd daemon to start automatically at boot time:

sudo chkconfig sshd --add;
sudo chkconfig sshd on --level 2,3,4,5;

Disable

To configure the sshd daemon to stop automatic initialization at boot time:

sudo chkconfig sshd off;
sudo chkconfig sshd --del;

Fedora 26: C++: static linking cannot find -lstdc++ -lm and -lc

Recently, we were trying to compile a C++ application with the following compilation command on a Fedora 26 64bit :

g++ -static -O2 -lm -Wall -Wno-unused-result -std=c++14 -DCS_ACADEMY -DONLINE_JUDGE 510152025.cpp -o 510152025;

unfortunately, we got the following errors:

 /usr/bin/ld: cannot find -lstdc++
 /usr/bin/ld: cannot find -lm
 /usr/bin/ld: cannot find -lc
 collect2: error: ld returned 1 exit status

To resolve the issues, we performed the following installations to install the static versions of the glibc and libstdc libraries:

sudo dnf install glibc-static libstdc++-static -y;

 


CentOS 7: Setup a DHCP server and provide specific IP based on MAC address

Step 1: Install DHCP service

We installed the Dynamic host configuration protocol software (DHCP service) using the command:

yum install dhcp;

The dhcp package provides the ISC DHCP service and relay agent.

Step 2: Configure the DHCP service

Afterwards, we created the file /etc/dhcp/dhcpd.conf using the following content:

subnet 192.168.0.0 netmask 255.255.255.0 {
 option routers                  192.168.0.254;
 option subnet-mask              255.255.255.0;
 option domain-name              "bytefreaks.net";
 option domain-name-servers       192.168.0.1;
 option time-offset              -18000;     # Eastern Standard Time
 range 192.168.0.90 192.168.0.99;
}

host coolServer {
 hardware ethernet 0e:e0:4b:b4:28:82;
 fixed-address 192.168.0.80;
}

This configuration allowed us to provide a DHCP service to the network for the subdomain 192.168.0.x with the range [90,99].
Also, we statically defined the IP for our coolServer using a filter based on the MAC address of the machine.
If you do not want to provide any range, only static IPs, then comment out (#) the line that starts with the word range .

Step 3: Start DHCP service

systemctl start dhcpd.service;

Step 4: Check the status of DHCP service

systemctl status dhcpd.service;

It is a good idea to verify that there are no errors, so be sure to check the status of the service.
You can ignore the error that says “you did not define a subnet declaration for all devices” if you do not really need to do it.

Step 5: Permanently enable the DHCP service

systemctl enable dhcpd.service;

Additional:

Disable the DHCP service

systemctl disable dhcpd.service;

Stop the DHCP service

systemctl stop dhcpd.service;


Bash: After redirected input file is done, allow user to control application via STDIN

Recently, we needed to start an application using a script, which application had its own CLI.
After starting it, we had to feed it with some input to configure it before handing it over to the end user.

The application we used was named dog. We wrote into a plain text file (named food) the commands that we needed to feed the application and then we started the execution using a plain input redirect like so dog < food;.
Doing so, resulted into properly feeding the dog application the food data  but it would cause the application to terminate immediately after the food was done as it would also feed the EOF (End Of File) directive to dog.
Apparently, the application after receiving the EOF, it would then terminate without allowing the end user to take control.

To mitigate the problem, we used the cat command to concatenate the input file along with the stdin stream, permitting us to first feed all the data in the food file and then allow the user to input data using the standard input stream.
Our final command resulted in the following solution as below

cat <(cat food) - | dog;

Where - is the special sign for standard input stdin.
cat food can be of course replaced with other commands that produce output on the standard output (stdout).

A bad side-effect of this solution, is that we lost some functionality of the terminal including, but not limited to, using the backspace and navigation.


Find all git repositories and perform a pull operation on them.

The following command will find all git projects in your home folder and perform a pull operation on them.

find ~ -name ".git" -type d -exec bash -c "echo '{}' && cd '{}'/.. && git pull" \;

The above command is based on finding the .git folders that exist in any clone of a git repository. Once a .git folder is found, it will navigate to its parent folder where it will perform the pull request.

Bonus – Automate the procedure using a cron job

The following entry in crontab allows us to periodically perform a pull request on all the cloned repositories that we have in a specific folder. Specifically, it will perform this operation once every five minutes.

*/5    *    *    *    *    cd /home/bytefreaks/Projects; find . -name ".git" -type d -exec bash -c "echo '{}' && cd '{}'/.. && git pull" \; &> /tmp/bf.git.log

Please note that it would be easier to use an ssh key that does not have a password for this automation.
If you do not, the you will need to either pass the password via this configuration line (not recommended) or have a key agent running to provide the password for the key.

Redirecting standard error (stderr)

The following command will redirect stderr to a different file than the one stdout is redirected to:

command >log.txt 2>errors.txt;

In case you want to redirect stderr to stdout (&1), and then redirect stdout to a file you can use the following setup:

command >mixed-log.txt 2>&1;

The following command will have the same effect as the previous one, the difference between them is the way they are implemented. This time we will redirect both the stdout and stderr to a file:

command &> mixed-log.txt;


Regular expression to match any ASCII character

The following regular expression will match any ASCII character (character values [0-127]).

[\x00-\x7F]

The next regular expression makes the exact opposite match, it will match any character that is NOT ASCII (character values greater than 127).

[^\x00-\x7F]

 

gEdit - regular expression to match any ASCII character

gEdit – regular expression to match any ASCII character

gEdit - regular expression to match any Non-ASCII character

gEdit – regular expression to match any Non-ASCII character


NTFS Support on CentOS 7

Solution

sudo yum --enablerepo=extras install epel-release;
sudo yum install ntfs-3g -y;

Background – Explanation of commands

By default, CentOS does not have installed the necessary drivers to mount ntfs drives.

sudo yum --enablerepo=extras install epel-release;

To install them, you need to enable the Extra Packages for Enterprise Linux (EPEL).

Extra Packages for Enterprise Linux (or EPEL) is a Fedora Special Interest Group that creates, maintains, and manages a high quality set of additional packages for Enterprise Linux, including, but not limited to, Red Hat Enterprise Linux (RHEL), CentOS and Scientific Linux (SL), Oracle Linux (OL).

EPEL packages are usually based on their Fedora counterparts and will never conflict with or replace packages in the base Enterprise Linux distributions. EPEL uses much of the same infrastructure as Fedora, including buildsystem, bugzilla instance, updates manager, mirror manager and more.

From: https://fedoraproject.org/wiki/EPEL

You can install EPEL by running yum --enablerepo=extras install epel-release. The epel-release package is included in the CentOS Extras repository that is enabled by default. The package includes gpg keys for package signing and repository information. Installing this package for your Enterprise Linux version should allow you to use normal tools such as yum to install packages and their dependencies.

sudo yum install ntfs-3g -y;

After you’ve enabled the repository, you should be able to install the Linux NTFS userspace driver packaged in ntfs-3g. ntfs-3g is a stable, open source, GPL licensed, POSIX, read/write NTFS driver for Linux and many other operating systems. It provides safe handling of the Windows XP, Windows Server 2003, Windows 2000, Windows Vista, Windows Server 2008 and Windows 7 NTFS file systems. NTFS-3G can create, remove, rename, move files, directories, hard links, and streams; it can read and write normal and transparently compressed files, including streams and sparse files; it can handle special files like symbolic links, devices, and FIFOs, ACL, extended attributes; moreover it provides full file access right and ownership support.

To install it, we used the following command: yum install ntfs-3g -y.


Ubuntu: Headless wireshark (or wireshark from terminal)

Recently, we wanted to use wireshark on an Ubuntu through ssh and no X-Server forwarding enabled.
After a quick search we found tshark.

TShark is a network protocol analyzer. It lets you capture packet data from a live network, or read packets from a previously saved capture file, either printing a decoded form of those packets to the standard output or writing the packets to a file. TShark‘s native capture file format is pcap format, which is also the format used by tcpdump and various other tools.
Without any options set, TShark will work much like tcpdump. It will use the pcap library to capture traffic from the first available network interface and displays a summary line on stdout for each received packet.
TShark is able to detect, read and write the same capture files that are supported by Wireshark.

From: man tshark

Install tshark on Ubuntu

sudo apt-get install tshark -y;

Using tshark to capture all traffic on eth0 to a pcap file

sudo tshark -i eth0 -w something.pcap;

Note: If you just want to capture network traffic on a network interface and not use the additional features wireshark has to offer, you can also use tcpdumpas follows

#The following command will create a files that has in its name the current date and time using the date function.
sudo tcpdump -i eth0 -w "data.`date +%Y-%m-%d\ %H.%M`.pcap";


Bash: Remove the last character from each line

The following script, uses rev and cut to remove the last character from each line in a pipe.
rev utility reverses lines character-wise.
cut removes sections  from each of line.
It is a very simple script where we reverse the line once, remove the first character (which was the last one in the original form of the line) and finally we reverse the line back with the last character missing.

echo -e "hi\nHI" | rev | cut -c 2- | rev;

# Will produce:
h
H