If you've done a little bit of research into how websites and databases are ran, you might have encountered something called the command line. In this guide, you will learn what it is and how to use it.
Generally, the command line is just a prompt that you type commands into, and the computer runs those commands. These commands can vary from simply echoing something to the screen to backing up full production databases.
However, the syntax of these commands vary from operating system to operating system. In this guide we will be covering the two most-used ones: Windows and Bash (Unix/Linux).
Generally, dollar signs are used to represent an interactive prompt. You'll see these in command examples just about everywhere. However, do note that these are not part of the actual commands.
For example:
$ test-command
$ python test.py
$ node who-cares.js
There are a couple similarities between the different shells.
Generally, to change the current directory, use the cd command.
For example:
$ cd cool-directory
A single period . generally represents the current directory. Two periods .. generally represent the directory above the current directory.
Generally, to echo something to the screen, use the echo command.
For example:
$ echo programming is cool
programming is cool
$ echo i know, right?
i know, right?
The Windows command line is not really used all that often for programming, but some servers do use it, and it's probably the easiest way to get experience with a command line.
In Windows command line, variables are created with the set command and are called with %percent signs% surrounding them.
Strings do not have to be contained within any sort of quotes or delimiters.
For example:
$ set TEST=i love programming club
$ echo %TEST%
i love programming club
In Windows command line, to list the contents of the current directory, use the dir command. However, this prints a lot of (usually unnecessary) information, so just add the /b flag to truncate it to just the files/folders.
For example:
$ dir
09/16/2016 02:02 PM <DIR> .
09/16/2016 02:02 PM <DIR> ..
09/16/2016 02:02 PM <DIR> Programming-Club
04/20/2016 06:09 AM 4,332 windows-cmd-line-is-cool.txt
$ dir /b
Programming-Club
windows-cmd-line-is-cool.txt
The Bash command line is the most commonly used command line within programming and computers in general. Most servers run a Linux distribution of some sort, usually Ubuntu, which all use Bash.
In Bash, variables have no explicit definition command. They are called with a $dollar sign in front of them.
However, as opposed to Windows, strings must be contained within 'quotes'. (Double quotes have a special meaning in Bash. We'll get to that!)
For example:
$ TEST='i love programming club'
$ echo $TEST
i love programming club
In Bash, the type of quotes you use when creating strings matters. 'Single quotes' do nothing special, but "double quotes" do variable interpolation (putting the variable contents in the string).
For example:
$ VARIABLE="extra content"
$ echo 'test string with $VARIABLE'
test string with $VARIABLE
$ echo "test string with $VARIABLE"
test string with extra content
In Bash, to list the contents of the current directory, use the ls command. However, this does not contain hidden files/folders (which are prefixed with a .period). To show those, add the -a flag to the command.
For example:
$ ls
Programming-Club bash-cmd-line-is-cool.txt
$ ls -a
. .. Programming-Club .hiddenfile bash-cmd-line-is-cool.txt