Hexadecimal basics using basic Linux utilities

This is short guide on how to convert strings and decimal values to hexadecimal and vice-versa using simple Linux utilities such as xxd and printf.

  1. To convert a string to hexadecimal, you can use xxd in Linux.

For example: To convert a string “kali” to hexadecimal, you can use the below format.

[email protected]:~$ echo -n kali | xxd
00000000: 6b61 6c69 kali

To group and display the output a single character at a time, use -c  flag with argument 1.

[email protected]:~$ echo -n kali | xxd -c1
00000000: 6b k
00000001: 61 a
00000002: 6c l
00000003: 69 i

This means the following:

00000000: 6b k <- k is a single character and its hexadecimal value is 6b
00000001: 61 a <- a is a single character and its hexadecimal value is 61
00000002: 6c l <- l is a single character and its hexadecimal value is 6c
00000003: 69 i <- i is a single character and its hexadecimal value is 69

2. Convert Hexadecimal value back to string:

[email protected]:~$ echo -n "6b61 6c69" | xxd -r -p
kali

Dont worry about the spaces, it should still work.

[email protected]:~$ echo -n "6b616c69" | xxd -r -p
kali[email protected]:~$

3. Convert decimal (number) to hexadecimal

-Lets say you want to convert number 12 to decimal value. To do so, you could use printf as shown below.

[email protected]:~$ printf "%x\n" 12
c

From above output, the hexadecimal value for number 12 (aka decimal 12) is character c.

To find hexadecimal values for values from 1 to 255, you could run through it in a loop

for num in {1..255}; do printf "%x\n" $num ; done | less

Hope these help.

PS: Knowing hexadecimal does come in handy while learning/debugging different protocols.

How to fix “Error opening terminal: xterm-termite” when trying to SSH to a remote host

I am using “termite” on my ArchLinux machine but when I try to SSH to a remote linux machine(Ubuntu) to run some commands, it throws me an error “Error opening terminal: xterm-termite“. This a post on how to fix it.

Quick Fix:

If you are already SSH’ed into the remote machine and you see this issue, you could update the TERM variable on the remote system: (Run on remote system)

export TERM=xterm-256color

Permanent Fix:

The permanent fix is to edit your local ~/.bashrc file to include the following:

if [ "$TERM" = xterm ]; then
TERM=xterm-256color;
fi
export TERM=xterm-256color

In my case, I had to add it to my Arch Linux’s ~/.bashrc file. (i.e not on the remote Ubuntu machine that I am connecting to)

Here is a screenshot from my box: (Right click image and open in new team to view in full screen)

arch linux termite i3-gaps

To verify its fixed, open a new terminal (or source ~/.bashrc again) and SSH into the remote machine. Ideally this should be fixed.

Hope this helps!

Note: If you terminal does not support 256-color, then you may need to change the TERM variable to “xterm-color” instead of “xterm-256color”.

Credits/References:

Github

Filter top using process name in Linux

To display statistics only for a process name that match a string using top, you could use the following:

top -c -p $(pgrep -d',' -f firefox)

Here is a screenshot which only shows stats for firefox:

top filtered output screenshot in Linux

Hope this helps! Cheers!

 

Source/Credits/Reference: StackOverflow

Mount TrueNAS Core Samba share on Linux

This is a guide that describes on how to mount a remote Samba share configured on TrueNAS on to a Linux machine.

 

Login in your Linux machine. (I am using a Linux Mint 19.3 in this demo. This should technically work on other Debian/Ubuntu based systems as well).

Use the following commands to get your current user’s user ID (UID) and group ID (GID) respectively.

id -u $USER
id -G $USER

Create a file /etc/.truenas_creds. This is where you would store the samba credentials.

Replace text in red with the username and password of the remote SMB share which was configured in TrueNas.

cat /etc/.truenas_creds
username=enter_username_here
password=enter_password_here

Modify the file permissions so that root is the owner and set the file permission to 600.

sudo chown root: /etc/.truenas_creds
sudo chmod 600 /etc/.truenas_creds

In your linux machine, create a folder to where you want the contents of the remote samba share to be mounts. For example: create a directory named /mnt/truenas/.

sudo mkdir /mnt/truenas/

-Below is a sample syntax that can be used for populating /etc/fstab.

//ip-of-nas-server/enter-remote-samba-share/location /enter-local-mount/location/here/ cifs credentials=/etc/.truenas_creds,iocharset=utf8,uid=enter_your_uid_here,gid=enter_your_gid_here,noperm 0 0

-Here is what that I added in /etc/fstab.

//192.168.1.12/mnt/truenas /mnt/truenas/ cifs credentials=/etc/.truenas_creds,iocharset=utf8,uid=1000,gid=1000,noperm 0 0

My TrueNAS server’s IP => 192.168.1.12

Remote samba share => /mnt/truenas

Local mount location => /mnt/truenas/

Credentials for samba share => /etc/.truenas_creds

-Once complete, run the following to mount all entries looking at /etc/fstab.

mount -a

-If there are no errors in the above command, check your local mount path to verify that the mount was successful.

ls -l /mnt/truenas/

References:

linuxize

Askubuntu

Bash script to loop through values in a file with space as a separator

Lets say we have a file with list of IPs that are space separated and you want to read each of the values to pass to a loop to perform an operation. Here is an example file with IP Addresses separated  by a space:

cat ips.txt
192.168.1.1 192.168.1.10

Now, lets say you want to loop through these IPs and run a ping command against each of them.

cat ping.sh
#!/bin/bash

# IFS is an internal bash variable. Here, we set its value as space.
IFS=$" "
# Read the file "ips.txt" and store the list of values to a variable "ips"
ips="$(cat ips.txt)"

# Run the following loop which will loop through each of the ips and run a ping test
for ip in $ips; do ping -c 1 $ip; done
# Unset the IFS variable so that it wont mess with the reset of the script
unset IFS

-Running this loop, will loop through the list of IP addresses and perform a ping.

./ping.sh
PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data.
64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=0.660 ms

--- 192.168.1.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.660/0.660/0.660/0.000 ms
PING 192.168.1.10 (192.168.1.10) 56(84) bytes of data.
64 bytes from 192.168.1.10: icmp_seq=1 ttl=64 time=0.108 ms

--- 192.168.1.10 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.108/0.108/0.108/0.000 ms

Hope this helps!

Happy scripting folks! 🙂

Source/References: Link

Run bash script from a Perl script

To run a bash script (Example: bash-script.sh) from inside a perl script, you could use the following syntax:

system("sh", "bash-script.sh")

Note: Here, once the bash script completes execution it will continue with the execution of the perl script.

Example:

Perl Script: perl-script.pl
Bash Script: bash-script.sh

Below is a perl script  “perl-script.pl” which calls an external bash script “bash-script.sh”.

#!/usr/bin/perl
use strict;
use warnings;

print "Running parent perl script. \n";
print "Starting to call external bash script\n";

# Sample Argument to be passed to the bash script
my $my_arg = "ARG1";

# With arguments - pass them inside quotes seperated by commas 
system("sh", "bash-script.sh","$my_arg");

print "Back to parent perl script\n";

Below is the sample “bash-script.sh” which prints the variable.

#!/bin/bash
echo "---Start of Bash script---"

a=$1
echo "Argument from Perl script is" $a

To test, execute the perl script:

./perl-script.pl

 

Credits/References
https://stackoverflow.com/questions/3200801/how-can-i-call-a-shell-command-in-my-perl-script

How to fix print_req_error: I/O error, dev fd0, sector 0 error

After a fresh install of Ubuntu, my terminal was being flooded with “print_req_error: I/O error, dev fd0, sector 0” error.

dev fd0

This is because, your kernel thinks you have a floppy disk fd0, but cant find one. To fix this issue, you can run the following in your terminal:

sudo rmmod floppy
echo "blacklist floppy" | tee /etc/modprobe.d/blacklist-floppy.conf
sudo dpkg-reconfigure initramfs-tools

Misc: If you are deploying a new virtual machine, you can avoid this issue by deleting the Floppy Disk drive.

Source: StackOverflow

View progress when using dd command

While using “dd”, by default it does not show the progress nor status of the transfer. However, you could use the flag “status=progress” to show the status/progress of the transfer. Here is a screenshot:

This flag is available in the newer version of dd. Here are the CLI outputs for the text ninjas:

$ sudo dd if=archlinux-2019.06.01-x86_64.iso bs=4M of=/dev/sdd status=progress oflag=sync
641728512 bytes (642 MB, 612 MiB) copied, 16 s, 39.1 MB/s
153+1 records in
153+1 records out
643825664 bytes (644 MB, 614 MiB) copied, 16.4601 s, 39.1 MB/s

PS: Use dd (data duplicator) with caution. ‘ddis also known as disk destroyer  in an alternate universe.

Regards,
ΞXΤЯ3МΞ

Check partition information in Linux

To show currently mounted partition in human readable format, use:

df -h

df -h sample output screenshot

Another way to check partition information using parted.

sudo parted /dev/sda print

parted sample output screenshot

Here, we can see that the disk size is 120GB along with the partition information.

To view the list of partitions using fdisk, use:

fdisk -l

To view the list of block devices:

lsblk

Hope this helps! Cheers 🙂

How to pass arguments to an alias in bash

Lets look at passing arguments to an alias by looking at an example.

The below command will open duckduckgo.com in a new tab in firefox.

firefox --new-window duckduckgo.com

Lets say, I want to create an alias but I want to pass the URL as a parameter to the alias.

For example, if I run the following command in the terminal, it should look at the first argument (which is the URL) and open it in a newtab in firefox.

ff duckduckgo.com

To achieve this, you can do the following:

Edit your ~/.bashrc or ~/.bash_profile using your favorite editor.

vim ~/.bashrc

Create a function inside the ~/.bashrc  with the following content. [Copy/paste the below inside your bashrc]

alias ff='function _ff()
{
firefox --new-window $1
};_ff'

Here, $1 is the first argument.

Once you save and close the bashrc file, you can source your bashrc for changes to take affect.

source ~/.bashrc

Now, if you enter the following command in the terminal, it will take “duckduckgo.com” which is the 1st parameter and open it in a new tab in firefox.

ff duckduckgo.com

Hope this helps. If you like my content, do share and subscribe for more content.

Source: StackOverFlow