Command line programming in linux

Linux Command Line Tutorial: the commands you need to know

The Linux command line, also called a terminal, shell, or console, is a text interface for your computer. With this computer program, you can interpret commands and write your own scripts.

It may look intimidating at first, but once you get used to it, it is easy to use. This article will walk you through some of the basics of the Linux command line, along with some real-world examples. We assume you have little or no prior knowledge.

Each Linux distribution comes with a command line. This article uses Ubuntu 20.04, the commands should work with most distributions.

Here’s what we will be covering today:

Learn advanced BASH commands and scripts

This course is for anyone interested in understanding the BASH shell. You’ll learn all the intricacies of advanced BASH scripting.

What is the Linux Command Line?

The Linux terminal gives you an interface where you can input commands and see the result printed as text. You can use the terminal to accomplish tasks like moving files or navigating a directory, without the use of a GUI. Terminals just transfer information. You need a shell for the operating system to understand this information.

With the Linux command line, you can execute commands manually by typing on the terminal. You can also automate commands using Shell Scripts.

Brief History of Linux Command Line

Let’s briefly go through the Linux command line history. Back in the early days of computing, there was an operating system called Unix, which was designed to run on mainframe computers as a multi-user system. Users connected to it remotely through terminals.

These terminals were very basic, as they could only send keystrokes and display data on the screen.

Since text is light on resources, users could interact quickly and efficiently. Every task the users performed was through this textual interface. To coordinate the execution of different programs, users would connect to single master programs.

The user commands were wrapped in “shell” programs. To make complex tasks easier, users could write shell scripts that automated a long series of shell commands.

Linux is a descendant of Unix. Its core part is designed to behave similarly to a Unix system. The Bourne Again Shell (BASH) was developed by Stephen Bourne and released in 1979 in the Version 7 Unix release. Over time, it has become the default shell for Linux systems.

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The Ultimate Linux Command Line Guide — Full Bash Tutorial

Welcome to our ultimate guide to the Linux Command Line. This tutorial will show you some of the key Linux command line technologies and introduce you to the Bash scripting language.

What is Bash?

Bash (short for Bourne Again SHell) is a Unix shell, and a command language interpreter. A shell is simply a macro processor that executes commands. It’s the most widely used shell packaged by default for most Linux distributions, and a successor for the Korn shell (ksh) and the C shell (csh).

Many things that can be done Linux operating system can be done via command line. Some examples are…

• Editing files
• Adjusting the volume of the operating system
• Fetching web pages from the internet
• Automating work you do every day

You can read more about bash here, via the GNU Documentation, and via the tldp guide.

Using bash on the command line (Linux, OS X)

You can start using bash on most Linux and OS X operating systems by opening up a terminal. Let’s consider a simple hello world example. Open up your terminal, and write the following line (everything after the $ sign):

zach@marigold:~$ echo "Hello world!" Hello world!

As you can see, we used the echo command to print the string “Hello world!” to the terminal.

Writing a bash script

You can also put all of your bash commands into a .sh file, and run them from the command line. Say you had a bash script with the following contents:

It’s worth noting that first line of the script starts with #! . It is a special directive which Unix treats differently.

Why did we use #!/bin/bash at the beginning of the script file?

That is because it is a convention to let the interactive shell know what kind of interpreter to run for the program that follows. The first line tells Unix that the file is to be executed by /bin/bash. This is the standard location of the Bourne shell on just about every Unix system. Adding #!/bin/bash as the first line of your script, tells the OS to invoke the specified shell to execute the commands that follow in the script. #! is often referred to as a “hash-bang”, “she-bang” or “sha-bang”. Though it is only executed if you run your script as an executable. For example, when you type ./scriptname.extension , it will look at the top line to find out the interpreter, whereas, running the script as bash scriptname.sh , first line is ignored.

Then you could run the script like so: For make file executable you should call this command under sudo chmod +x “filename”.

zach@marigold:~$ ./myBashScript.sh Hello world!

The script only has two lines. The first indicates what interpreter to use to run the file (in this case, bash). The second line is the command we want to use, echo, followed by what we want to print which is “Hello World”.

Sometimes the script won’t be executed, and the above command will return an error. It is due to the permissions set on the file. To avoid that use:

zach@marigold:~$ chmod u+x myBashScript.sh

And then execute the script.

Linux Command Line: Bash Cat

Cat is one of the most frequently used commands in Unix operating systems.

Cat is used to read a file sequentially and print it to the standard output. The name is derived from its function to concatenate files.

Usage

• -b , numer non-blank output lines
• -n , number all output lines
• -s , squeeze multiple adjacent blank lines
• -v , display nonprinting characters, except for tabs and the end of line character

Example

Print in terminal the content of file.txt:

Concatenate the content of the two files and display the result in terminal:

Linux Command Line: Bash cd

Change Directory to the path specified, for example cd projects .

There are a few really helpful arguments to aid this:

• . refers to the current directory, such as ./projects
• .. can be used to move up one folder, use cd .. , and can be combined to move up multiple levels ../../my_folder
• / is the root of your system to reach core folders, such as system , users , etc.
• ~ is the home directory, usually the path /users/username . Move back to folders referenced relative to this path by including it at the start of your path, for example ~/projects .

Linux Command Line: Bash head

Head is used to print the first ten lines (by default) or any other amount specified of a file or files. Cat is used to read a file sequentially and print it to the standard output.
ie prints out the entire contents of the entire file. — that is not always necessary, perhaps you just want to check the contents of a file to see if it is the correct one, or check that it is indeed not empty. The head command allows you to view the first N lines of a file.

if more than on file is called then the first ten lines of each file is displayed, unless specific number of lines are specified. Choosing to display the file header is optional using the option below

Usage

• -n N , prints out the first N lines of the file(s)
• -q , doesn’t print out the file headers
• -v , always prints out the file headers

Example

Prints in terminal the first ten lines of file.txt (default)

Prints in terminal the first seven lines of file.txt

head -q -n 5 file1.txt file2.txt

Print in terminal the first 5 lines of file1.txt, followed by the first 5 lines of file2.txt

Linux Command Line: Bash ls

ls is a command on Unix-like operating systems to list contents of a directory, for example folder and file names.

Usage

• -a , all files and folders, including ones that are hidden and start with a .
• -l , List in long format
• -G , enable colorized output.

Example:

List files in freeCodeCamp/guide/

ls ⚬ master CODE_OF_CONDUCT.md bin package.json utils CONTRIBUTING.md gatsby-browser.js plugins yarn.lock LICENSE.md gatsby-config.js src README.md gatsby-node.js static assets gatsby-ssr.js translations

Linux Command Line: Bash man

Man, the abbreviation of manual, is a bash command used to display on-line reference manuals of the given command.

Man displays the reletive man page (short for manual page) of the given command.

Usage

• -f , print a short description of the given command
• -a , display, in succession, all of the available intro manual pages contained within the manual

Example

Display the man page of ls:

Linux Command Line: Bash mv

Moves files and folders.

mv source target mv source . directory

The first argument is the file you want to move, and the second is the location to move it to.

• -f to force move them and overwrite files without checking with the user.
• -i to prompt confirmation before overwriting files.

That’s all. Go forth and use Linux.

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