Linux Tutorials

JOE Tutorial for Linux

Start joe:

joe ./textFile.txt

Show help:

Ctrl + K + H (to hide help, enter this command again)

Save file

Ctrl + K + D

Save and exit:

Ctrl + K + X

Exit without saving:

Ctrl + K + Q

Go to previous/next screen:

Ctrl + U/Ctrl + V

Move cursor to the start/end of file file:

Ctrl + K + U/Ctrl + K + V

UnDo recent change:

Ctrl + Shift + _

ReDo undone change:

Ctrl + Shift + ^

Insert or overwrite:

Ctrl + T

Selecting text:

Ctrl + K + B (block begin)
Ctrl + K + K (block end)

When you have block selected, to move block using:

Ctrl + K + M

Delete block:

Ctrl + K + Y

Copy block:

Ctrl + K + C

Search in the file:

Ctrl + K + F

Then choose between ignore (I), replace (R) options. To navigate to next result use:

Ctrl + L

If you want to change joe’s default setting use:

Ctrl + T

You can turn on/off auto-indent, word wrap, line numbers, highlighting and set tab width, left margin, etc.

Linux: File System Hierarchy

In this tutorial is described Filesystem Hierarchy Standard (FHS), which specifies required directories. The root directory is “/” and it should contain only the subdirectories.

/bin

• Contains binaries which can be executed from the command line (ls, grep, mkdir…)
• Programs that can be used by users (system, admin, normal users)
• It can be in single-user mode

/boot

• Contains everything required for the boot process
• Kernel
• Grant Unified Boot-loader
• LILO (LInux LOader)

/sbin

• Binaries
• Program used by system and admin
• Normal users can use programs in /bin if they are allowed
• Usually, normal users do not have this directory in $PATH variable

/dev

• Files of all devices
• Created during installation operating system.
• Create new devices: /dev/MAKEDEV

File	Description
Sda	First SCSI drive on the SCSI/SATA bus
md0	First group of meta discs (RAID)
ttyS0	First serial port
lp0	First parallel printer
null	bin for bits
random	Deterministic random bits
urandom	Non-deterministic random bits

/etc

File	Description
passwd	Users information
fstab	Partition and storage mounting information
rc or rc.d or rcX.d	Run commands – commands that runs when OS starts

/home

• The home directory for users
• All data and system settings of users
• Can be divided into groups (school, office, financial)

/root

• Home directory for user root
• Normal users don’t have permissions.

/lib

• Libraries for programs
• /lib/modules: kernel modules, network controls, file system control

/tmp

• Temporary files
• Used by running programs

/mnt

• Mounting temporary file systems
• File systems from /etc/fstab are mounted during start OS
• Network file systems
• /media: DVD, USB

/usr

• Programs, libraries installed from OS distribution
• Accessible for everyone
• /usr/

Directory	Description
local	Software installed by admin on local device
X11R6	Files of Windows OS
bin	Almost all commands for users
sbin	Usually server’s programs
include	Header files for C language
lib	Stable libraries

• /usr/share/

Directory	Description
X11	Files of Windows OS
dict	Glossary
man	Manual pages
doc	Documentation
info	Information files
src	source files

/var
Contains files that are changed when OS is running.

Subdirectory	Description
log	Logging files
run	Run-time variable data
spool	Program using queue (mails, printers)
mail	Mailbox
local	variable data from /usr/local
lib	Holds dynamic data libraries/files
lock	Lock files. Indicates that resource (database, file) is in use and should not be accessed by another process.

/opt
Third-party software

/proc

• Created by OS kernel
• Information about system
• Stored only in RAM
• Does not use any disc space
• Every process has a subdirectory (by PID)

Subdirectory	Description
/PID/status	Stats about process
/PID/cmdline	How was the process started and what input arguments
/PID/maps	Region of contiguous virtual memory in a process or thread
/PID/environ	Environment of process

• Interesting files

File	Description
cpuinfo	Information about CPU
meminfo	Usage of memory
version	Kernel version
cmdline	Kernel’s parameters from the boot loader
devices	List of drivers for the kernel
interrupts	Which interrupts are used and how many times
ioports	list of currently registered port regions used for input or output communication.
dma	ISA Direct Memory Access channel
kcore	Image of physical system memory
cmdline	Kernel’s parameters from the boot loader
cmdline	Kernel’s parameters from the boot loader

/lost+found

• Recovered or damaged data after a crash.
• Each partition has its own /last+found directory.

Generate SSL certificates using openssl with a Certificate Signing Request

The file ca.key and ca.crt are the Certificate Authority

We will be generating the .key and .csr (Certificate Signing Request) files from the below command.

[root@node01 ssl]# openssl req -new -sha256 -newkey rsa:2048 -nodes -keyout linuxcent.com.key -days 365 -out linuxcent.com.csr -sha256 -subj "/C=IN/ST=TG/L=My Location/O=Company Ltd./OU=IT/CN=linuxcent.com/subjectAltName=DNS.1=linuxcent.com"

The resultant files are a PEM certificate request .csr and a Private .key file. Now that we have successfully generated the .csr, we approach a Certificate Authority, Upload our CSR, and purchase the signer certificates along with Intermediate Chain keys for a given Number of days, typically done for 365 Days.

The -days flag is optional, and can be skipped as we are only generating a Signing Request.

Here we can use the openssl command to verify the .csr file that is generated as shown below:

[root@node01 ssl]# openssl req -in linuxcent.com.csr -noout -text
Certificate Request:
Data:
Version: 0 (0x0)
Subject: C=IN, ST=TG, L=MY Location, O=Company Ltd., OU=IT, CN=linuxcent.com/subjectAltName=DNS.1=linuxcent.com
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
Public-Key: (2048 bit)
Modulus:00:e4:b4:24:d7:22:ec:5d:c1:37:8c:d1:a0:62:17:
96:24:77:8d:75:4e:d5:74:15:4d:61:e0:8b:66:d6:
                Exponent: 65537 (0x10001)
        Attributes:
            a0:00
    Signature Algorithm: sha256WithRSAEncryption
         87:ef:83:b2:a6:f5:3a:f3:6f:1c:e4:02:ec:bf:5d:75:64:1d:

- OUTPUT TRUNCATED --

In the next section we shall see How the .csr can be signed by a CA to generate a .crt PEM certificate

Signing a .csr with a Certificate Authority [Demo Purpose] – Sample CA files

Here is the process of Generating a Selfsigned certificate(Not to be used on public facing sites)

Now we will using the root ca.key and ca.crt to digitally sign this .csr and generate a .crt PEM certificate

x509 is a Certificate Data Management and Certificate Signing Utility

This generally takes the private key as input, and signs the certificate requests and Converting the certificate to various formats

[root@node01 ssl]# openssl x509 -req -in linuxcent.com.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out linuxcent.com.crt -days 365 -sha256

-subj "/C=IN/ST=TG/L=My Location/O=Company Ltd./OU=IT/CN=linuxcent.com/subjectAltName=DNS.1=linuxcent.com"

We have generated the .crt file from the .csr

[root@node01 ssl]# ls linuxcent.com.crt linuxcent.com.key 
linuxcent.com.crt linuxcent.com.key

We have successfully generated the linuxcent.com.key file and linuxcent.com.crt, and digitally self signed with the root CA key and certificates.

Generating Self Signed SSL certificates using openssl x509

The x509 is the certificate signing utility we will be using here.\ to generate a PEM certificate

Below is the complete command to generate the ssl self signed certificate.

openssl req -x509 -days 365 -sha1 -newkey rsa:2048 -nodes -keyout linuxcent.com.key -out linuxcent.com.crt -sha256 -subj "/C=IN/ST=State/L=My Location/O=Company Ltd./OU=IT/CN=linuxcent.com/subjectAltName=DNS.1=linuxcent.com"

The Days parameter can be specified to any number of days depending on your requirement

The Self signed certificates are mostly commonly used within the internal network or among small group of familiar individuals like an office for specific purposes and not advised to be used out in the public domain as the browser does not identify the certificate authenticity or the ingenuity of the concerned website. The Self-signed certificates are not validated with any third party until and unless you import them to the browsers previously.

Generating a Wildcard certificate Request.

[root@node01 ssl]# openssl req -new -sha256 -newkey rsa:2048 -nodes -keyout linuxcent.com.key -out linuxcent.com.csr -sha256 -subj "/C=IN/ST=TG/L=My Location/O=Company Ltd./OU=IT/CN=linuxcent.com/subjectAltName=DNS.1=*.linuxcent.com, linuxcent.com"

Mentioning the Alternate Domain as *.linuxcent.com will create a wildcard .CSR

The SANs often seen in shortform to SubjectAltName allows us to secure multiple subdomains using a SSL certificate

The CA signing process of the wildcard is identical and obviously costs more.

The Bash being a scripting language does tend offer the conditional if else, We shall look at them in the following sections.

Firstly there needs to be a conditional check that has to be performed in order for the corresponding Block of code to be executed.

To break down the semantics of conditional control structures in BASH we need to understand The conditional keyword that performs the validation, the It is represented most commonly as “[“ and very rarely represented as “test” keyword.

It can be better understood by the following demonstration:

vamshi@linux-pc:~/Linux> [ 1 -gt 2 ]
vamshi@linux-pc:~/Linux> echo $?
1
vamshi@linux-pc:~/Linux>
vamshi@linux-pc:~/Linux> [ 1 -lt 2 ]
vamshi@linux-pc:~/Linux> echo $?
0

The [ is synonymous to the command test on the linux kernel.

vamshi.santhapuri@linux-pc:~/Linux> test 1 -gt 2

vamshi.santhapuri@linux-pc:~/Linux> echo $?
1
vamshi.santhapuri@linux-pc:~/Linux> test 1 -lt 2
vamshi.santhapuri@linux-pc:~/Linux> echo $?
0

We Shall now look at the different variations of Conditional controls structures.

1. if then..fi

   if [ Condition ] ; then
   
   statement1...statementN
   
   fi

2. if then..else..fi

   if [ Condition ] ; then
   
       If Block statements
   
   ...
   
   else
       else-Block statement
   
   fi

3. if..then..elif then..elifN then..fi

   if [ Condition ] ; then
   
       If Block statement1
   
   ...
   
   elif [ elif Condition ]; then   # 1st elif Condition
   
       elif Block statement1
   
   
   elif [ elif Condition ]; then    # 2nd elif Condition
   
       elif Block statements
   
   elif [ elif Condition ]; then    # nth elif Condition
   
       elif Block statements
   
   fi

   An else can also be appended accordingly when all the if and elif conditions fail, which we will see in this section .

    

4. if..then..elif then..elifN then..else..fi

   The “if elif elif else fi” control structure is like multiple test checking control diversion strategy in bash, gives the user the power to write as many test conditions as possible until a test condition is matched leading in the resultant block of code being executed. Writing this multiple elif can be tedious task and the switch case is mostly preferred

   if [ Condition ] ; then
   
       If Block statement
   
   elif [ elif Condition ]; then   # 1st elif Condition
   
       elif Block statement1
   
   elif [ elif Condition ]; then    # nth elif Condition
   
       elif Block statement
   
   ...
   
   else Block statementN # else block while gets control when none of if or elif are true.
   
       else Block statements
   
   fi

   Atleast one of the block statements are executed in this control flow similar to a switch case. The else block here takes the default case when none of the if nor the elif conditions matches up.

5. Nested if then..fi Control structure Blocks

   Adding to the if..elif..else there is also the nested if block wherein the nested conditions are validated which can be Demonstrated as follows:

   if [ condition ]; then
   
       Main If Block Statements
   
       if [ condition ]; then # 1st inner if condition
   
           1st Inner If-Block statements
   
           if [ condition ]; then # 2nd inner if condition
   
               2nd Inner If-Block statements
             
               if [ condition ]; then 
                   Nth Inner If Block statements 
   
               fi
   
           fi
   
       fi
   
   fi

   This logic of nested ifs are used while dealing with scenarios where the outermost block of statements must be validated before, if the test succeeds then the control flow is passed to the innermost if test statement execution. Thus the name Nested if.

 

Here is the switch case bash script with practical explanation.
We will look at the Exit codes within the BASH in the next sections.

In the systemd Environment there has been an architectural change and the systemd daemon controlling all the essential processes

root 1 0 0 Apr15 ? 00:00:12 /usr/lib/systemd/systemd --switched-root --system --deserialize 32

We can make use of the hostnamectl command to immediately reflect the system hostname change.

Command to get the current hostname on Linux:

[vamshi@server02 ~]$ sudo hostnamectl 
   Static hostname: server02.linuxcent.com
   Pretty hostname: SERVER02
         Icon name: computer-vm
           Chassis: vm
        Machine ID: 2338f55840d640689fc5ac4b356b160c
           Boot ID: 418256281d2f4e11822809dde7c1b09e
    Virtualization: kvm
  Operating System: CentOS Linux 7 (Core)
       CPE OS Name: cpe:/o:centos:centos:7
            Kernel: Linux 3.10.0-1062.18.1.el7.x86_64
      Architecture: x86-64

As you can see the current hostname is set to SERVER02

The file /etc/hostname also used to have the same effect but on the cloud systems this is dynamically generated and doesn’t hold true on cloud and tends to get overwritten after reboot.
On the general DataCenter or PC environment we can use the static files to populate the hostnames and make them permanent:

$ cat /etc/sysconfig/network
HOSTNAME=node2.linuxcent.com

But only /etc/hostname file is given importance since the systemd version and updating this ensures persistence.

$ cat /etc/hostname 
node02.linuxcent.com

 

Process to set or change the hostname in Linux Commandline:

The systemd provides a sophisticated command hostnamectl to set the hostname, It will also update the static file /etc/hostname and ensure the changes are permanent across reboots.

$ sudo hostnamectl set-hostname node02.Linuxcent

To get the hostname along with FQDN

[vamshi@server02 ~]$ hostname --fqdn
node02.linuxcent.com

[vamshi@node02 ~]$ sudo hostnamectl status 
   Static hostname: node02.linuxcent.com
         Icon name: computer-vm
           Chassis: vm
        Machine ID: 2338f55840d640689fc5ac4b356b160c
           Boot ID: 33619e39ea4c4900bd848e13d2a6239e
    Virtualization: kvm
  Operating System: CentOS Linux 7 (Core)
       CPE OS Name: cpe:/o:centos:centos:7
            Kernel: Linux 3.10.0-1062.18.1.el7.x86_64
      Architecture: x86-64

Changing hostname is ubuntu and Debian systems can be done through hostnamectl command

vamshi@debian:~$ hostnamectl 
Static hostname: debian
Icon name: computer-vm
Chassis: vm
Machine ID: b4adcdb84c724856b577524ebbfa0003
Boot ID: 67e1bf27946a4770b8e939f2420d06fd
Virtualization: oracle
Operating System: Debian GNU/Linux 10 (buster)
Kernel: Linux 4.19.0-5-amd64
Architecture: x86-64