Author: GET

SSH server is not installed by default on a Ubuntu 18.04 system. Watch this video to learn to install the openssh-server to remotely access your Ubuntu machine. Learn to install openssh-server.

You will learn to install ubuntu 18.04 in VirtualBox environment. Learn to install Ubuntu 18.04 Desktop in a VirtualBox environment.

This article guides you with Oracle JRE and JDK installation on Ubuntu 18.04 operating system.

Check if Java is already on the system

 $  java -v

Update Repositories

Update the repositories using the command below.

 $ sudo apt update

Install Oracle JRE (Java Run-time Environment)

Run the following command to install the default version of JRE.

 $ sudo apt install -y default-jre

Verify JRE installation

Verify the version of JRE installed using the following command.

 $ java -version

Install Oracle JDK

Run the following command to install the default version of JDK..

 $ sudo apt install default-jdk

Verify JDK installation

Verify the version of JRE installed using the following command. Please note that “javac” is the java compiler, and it is not part of the JRE package. The javac comes with the JDK package.

 $ javac -version

“less” command displays the text one screen-full at a time similar to the “more” command. You can use the “less” command on a file or pass the output from a command to the “more” command using pipe.

The “less” command more powerful than the “more” command. This is because, the “more” command will have to read the entire file before it can show you the initial screen text. On the other hand, the “less” command does not have to read the entire file before it can show you the initial set of lines on the terminal.

Viewing a file

Viewing a file with just a few lines will display the contents and the less command exists. Viewing a large file with “less” command would display the text one screen-full at a time.

   $ less us-states.txt 

Navigating

“less” Option	Usage
q	Quit more command.
spacebar	Move text one screen forward.
b	Move text one screen backward.
ENTER	Move text one line forward.
:	Last line at the button of the terminal is activate to enter commands.
:f	Displays the file name, lines displayed, total lines, bytes etc.
:n	Jump “n” lines forward on the screen.
:ng	Make nth line the top line on the screen
/[string]	Search for text. Replace [string] with the desired text.

Display line numbers when viewing a file

Use option -N to display the line numbers

 $ less -N us-states.txt 

Force “less” to display line numbers every time

Squeeze multiple blank lines into one (-s option)

 $  less -s file.txt 

Make less scroll 20 lines at a time

 $  less -20 file.txt 

Open file from a specific line number ( +number)

 $  less +10 file.txt 

Using “less” with “ps -ef”

 $  ps -ef | less

Using “less” with “dmesg”

 $  dmesg| less

Display help information

 $ less --help 

Display version information

 $ less --version 

“split” command is used to split a large file into manageable small pieces.

Syntax: split [OPTION]… [FILE [PREFIX]]

Here are the command options:

Split a file into pieces

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When you split a file with no options, the file chunks created start with x followed by two alphabets in the following manner.

Customize the suffix length (-a option)

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You can customize the length of the suffix of the generated chunks using –a option.

Split a file by number of lines (-l option)

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You can create the chunks with desired number of lines. The example below creates files with 500 lines in each.

Split a file by file size (-b option)

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You can split a file based on the desired size of the chunk.

• -b nK    splits into nKB files
• -b nM    splits into nMB files
• -b nG    splits into nGB files

In the example below, we split a 559k file into 100k files.

 $ split -b 100k addressbook.db 

Split a file with numeric suffix (-d option)

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You can have the suffix with numbers using the -d option.

$ split -d addressbook.db

Split a file with custom suffix

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You can have the suffix with numbers using the -d option.

In the example below, we will add myfiles- as the suffix.

 $ split addressbook.db  myfiles-

Split a file into n number of files (-n option)

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You can split a file into fixed number of chunks using the -n option.

The example below split the file into three files

  $ split -n3 addressbook.db

Do not generate empty output files (-e option)

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With certain scenarios, the split command ends up creating empty files. Using the -e option, you can force the split command not to create empty files.

  $ split -e addressbook.db

Print additional information (–verbose option)

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use –verbose potion

Display help

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  $ split --help

Display version information

  $ split --version 

“nohup” stands for no hangups. “nohup” is not a regular command. But, you will run other commands or scripts with nohup command.

When you execute a command or script, it may run for a second or few and it terminates (finishes). However, when you need to run a long-running jobs (commands or scripts), it might take over your terminal until it finishes. This is where we usually run the process in the background using the “&”. However, if you logout or close the session, the process will most likely die. This is where nohup helps.

When a command or script is executed with nohup, the process continues to run even if the user logs out or closes the session. In other words, the nohup ignores signals 01 (hangup) and 03 (quit). This is what makes the process continue to run even after the user logs out or closes the session.

Syntax:

 nohup [ Command or Script ]

However, nohup commands are generally executed in the background by using “&”.

Here is an example of running “nighly_backups.sh” script using nohup:

 linuxsignal@ubuntua:~$nohup nightly_backups.sh &

nohup.out

By default, the nohup command writes the output to nohup.out file because there is no terminal associated to it.

nohup.out is created in the directly where nohup command is executed. 

Both standard out and standard error are written to this file.

You can override it by redirecting to a desired file, as below.

$ nohup nightly_backups.sh > /home/bkmanager/nbackup.log &

Linux users run into scenarios where large files are needed for testing. Fortunately, Linux is already equipped with tools & commands needed for this purpose.

Using fallocate command

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Linux distros come with the fallocate command.

Here is an example of creating a 5GB file in a second. This is the fastest way to create large files. Faster than the dd command.

$ fallocate -l 5G example

Using the dd command

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“dd” command can also be used to create large files quickly.

You need to pass the buffer size and count. The file size created is [buffer size ] * [count]

The example below creates a 10GB file and it is very fast as it puts all zeros.

$ dd if=/dev/zero of=sample1.dummy count=1 bs=1 seek=$((10 * 1024 * 1024 * 1024 -1))

The example below creates a 10GB file with random content (using buffer size of 1mb).

$ dd if=/dev/urandom of=sample2.dummy bs=1M count=10240

The example below creates a 10GB file with random content (using buffer size of 10mb).

$ dd if=/dev/urandom of=sample3.dummy bs=10M count=1024