First Step in Command Line

Some Familiar Terms

A Shell is a computer interface to an operating system which expose the OS's services to the human user or other programs.

The shell takes our command and gives them to the operating system to perform.

It's named a "shell" because it is the outer layer around the OS, like the shell around an oyster.

A Terminal is a program that runs a shell. Originally, terminals were physical devices but these days, we work with software terminals.

On most Linux based systems, the default shell program is Bash. There are many other shells but Bash is currently the most popular.

The name "Bash" is an acronym for "Bourne Again SHell", a reference to Stephen Bourne, the creator of Bash's direct ancestor shell, sh.

Bash runs on pretty much all Unix and Unix-like systems.

The Prompt

When we open up our terminal, we'll see our prompt which will likely include your username@machinename, followed by a ~ symbol and a dollar sign $.

The prompt is what we'll see whenever the shell is ready to accept new input.

All we need to do is type some commands and then hit enter.

if we try typing some gibberish, the shell attempts to find a command with that name before telling us "command not found".

Our very first commands

Let's begin with the most basic and easy command, the date command.

It may not be the most useful command of all time, but it is a great place to start.

Try typing date in your terminal and then hit enter. You should see the current date printed out.

Case Matters

Commands are case sensitive, so Date is not the same thing as date.

- If you're using OS X, some commands are not case sensitive but others are. So it is safe to assume that all commands are case sensitive.

ncal Command

Try typing ncal in your prompt and hit enter. You should see the current month's calendar printed out.

ncal stands for "new cal". There is also a "cal" command that does the exact same thing but ncal adds some fancier functionality.

If the ncal command is not available in your Linux terminal, you can install it with this command,

sudo apt install ncal        

The Arrow Keys

In the terminal, we can use the left and right arrow keys to move through a line of text, one character at a time.

The up arrow key can be used to access the previously entered commands, saving you tons of typing.

Command Structure

Most commands support multiple options that modify their behavior. We can decide which options to include, if any, when we execute a command.

Similarly, many commands accept arguments (the things that the command acts upon or uses).

command -options arguments        

Arguments

The terms "argument" and "parameter" are often used interchangeably to refer to values we provide to commands.

The echo command is extremely simple example. It takes the arguments we provide to it and and prints them out. It echoes them back at us.

Arguments in ncal

Similarly, the ncal command accepts values to control the specific month(s) and year it displays.

If we specify only an year, ncal will print out the calendar for that entire year.

If we specify a month and an year, ncal will print only that specific month's calendar.

Options

Each command typically supports a host of options that we can choose to use when executing a command. These options modify the behavior of the command in predefined ways.

Options are prefixed by a dash, as in -h or -3.

command -option        

Example

By default, the ncal command highlights today's date in the output. We can provide the -h option to turn off the highlighting of today's date.

More options

The -j option tells ncal to display a calendar using Julian days (days are numbered starting from Jan 1st).

We can provide the -M option to tell ncal to use Monday as the first day of the week instead of Sunday.

The -3 option tells ncal to display the previous, current and next month.

Combining Options

We can provide multiple options at once. This example uses the -3 option to display the previous, current and next month AND the -h option to turn off the highlighting of the current date.

Another syntax

When we provide multiple options to a single command, we can use a shorter syntax where we only need a single dash (-) character.

Long Form Options

All these short one-character options can get confusing! Some options also support equivalent long format options that are usually full words and are prefixed with two dashes instead of just one.

For example, the date -u option is used to print the date in Coordinated Universal Time (UTC). We can instead use date --universal to accomplish the same end result.

Options with Parameters

Some options require us to pass in an additional value. For example, ncals's -A option is used to display a certain number of months AFTER a specific date. We need to tell it how many months to display.

In this example, ncal -A 1 prints out the current month with one month afterwards.

Note: this can also be written as ncal -A1 (no space between A and 1).

There is also a -B option to print a number of months BEFORE the specific date. We need to pass it a number of months.

In this example, ncal -B2 prints out the current month with two previous months.

This example prints out the calendar for July 1969, with one month before (June) and two months later (August and September).