Command line & Shells

Introduction

This is an introduction to navigating your computer via a command line interface like Windows Powershell or Bash. While useful for all programmers, it is essential for installing and using many computational tools including Git, Python and their management facilities like.

A useful 35-min. video tutorial (dated Oct 2020) on terminals, shells, and the command line is: How do you use the Command line? PowerShell, cmd, bash? This simultaneously demonstrates commands and behaviors using Windows command line, Powershell and Bash. Concepts addressed include: >”shells vs terminals”; >”commands & command aliases”; >command “switches” or “parameters” (and use of “help” or “man”); >using “tab” and “history”; >”home directory”, “prompts”, “redirects, appends & pipes”; >the concept of “*administrator””, and more - including differences / similarities in these three shells.

Shells

A “shell” is just a program that has access to your computer’s operating system. Shells can run other programs, open and save files, or install programs on your machine. The start menu/file explorer (windows) or dashboard/finder (mac) may be more familiar, but as a programmer or data analyst you will discover that using a shell will be more efficient for many tasks.

Mac and Linux computers typically use BASH as the default shell, and Windows computers typically use Windows Powershell, however many alternative shell programs exist for those interested in exploring options. Windows recently introduced the Windows Terminal which is not in fact a shell, but a versatile tool for managing one or more shells. See Microsoft’s Windows Terminal introduction for more details.

The command line

Start your preferred shell and you will see a very sparse window - likely black, with just a few characters, and a Prompt such as $ or > or C:\folder1\yourfolder. Commands are typed at the cursor that probably blinks just after this prompt. There are useful key strokes (such as the up-arrow which recalls previous commands) and many (many!) commands, but listing all these capabilities for specific shells is beyond the scope of this page.

Basic navigation

Most computer users can use their graphical user interface to find files, navigate to the folders where files can be found, make a text file, open it for editing, save it, move it from one folder to the next and so on. MacOS users do this using the “Finder” program and Windows users do this using “File Explorer”. All this can also be done from the command line. Here is is a conversational introduction, including comparison between the cmdl and gui approach on Windows. The following is a summary of this concept:

If your shell has just started, it likely assumes you are “at” your “home” directory (or folder). Find out by typing pwd to determine your “present working directory”.

Now let’s make a new folder (i.e. directory) and start a new text file in that space.

• Make a new directory under this “home” directory by typing mkdir my_folder.

• Check that it is there by “listing” all files and folders: type ls.

• Change to this new directory by typing cd my_folder. Check this has happened by typing pwd again.

  • Note that cd is the “change directory” command. It was given the name of the directory you want to got to, but that assumes this folder is directly “below” your current place.

  • You can provide a complete path to change to some other directory. (What’s a “path”? Google what is a file’s path, or see wikipedia’s article on path_(computing))

  • You can also give it the symbol ~ to return to your “home” directory, or .. to change to the “parent” folder, that is one level up from where you are now.

• to make a new text file, type notepad newfile.txt. You will likely be asked whether to create this new file - say yes, then type a few lines, save and close notepad.

  • For Mac, type vim newfile.txt to edit a text file in the terminal (but this requires knowledge of some vim commands). Or to edit in TextEdit (Mac’s equivalent of Window’s Notepad), type touch newfile.txt to create the file, then open newfile.txt to open the file in TextEdit.

• Is this new file there? Type ls. What’s in it? Type type newfile.txt.

  • For Mac, type cat newfile.txt.

• To remove this file type del newfile.txt and check it’s gone with ls.

  • For Mac, type rm newfile.txt (not sure if this line is needed since the bash equivalent is in the table below).

Bash and powershell command reference

Here is a short list of common command-line commands, from the Numerical Methods course page. Any command in the first three columns should work. Other “short lists”, or documentation for each task, can easily be Googled.

In the following table, a Cmdlet is microsoft’s term for a PowerShell command, the Alias is an alterative spelling, and the Bash equiv is the equivalent action in Bash (Mac and Linux) and also an alternative alias in PowerShell.

Cmdlet	Alias	Bash Equiv.	Description
Get-ChildItem	gci or dir	ls	List the directories and files in the current location.
Set-Location	sl	cd	Change to the directory at the given path. Typing .. rather than a path will move up one directory.
Push-Location	pushd	pushd	Changes to the directory.
Pop-Location	popd	popd	Changes back to the previous directory after using pushd
New-Item	ni	(touch)	Creates a new item. Used with no parameter, the item is by default a file. Using mkdir is a shortcut for including the parameter -ItemType dir.
mkdir	none	mkdir	Creates a new directory. (See New-Item.)
Explorer	start .	open .)	Open something using File Explorer (the GUI)
Remove-Item	rm	rm	Deletes something. Permanently!
Move-Item	mv	mv	Moves something. Takes two arguments - first a filename (i.e. its present path), then a path for its new location (including the name it should have there). By not changing the path, it can be used to rename files.
Copy-Item	cp	cp	Copies a file to a new location. Takes same arguments as move, but keeps the original file in its location.
Write-Output	write	echo	Outputs whatever you type. Use redirection to output to a file. Redirection with >> will add to the file, rather than overwriting contents.
Get-Content	gc	cat	Gets the contents of a file and prints it to the screen. Adding the parameter -TotalCount followed by a number x prints only the first x lines. Adding the parameter -Tail followed by a number x prints only the final x lines.
Select-String	sls	(grep)	Searches for specific content.
Measure-Object	measure	(wc)	Gets statistical information about an object. Use Get-Content and pipe the output to Measure-Object with the parameters -line, -word, and -character to get word count information.
>	none	>	Redirection. Puts the output of the command to the left of > into a file to the right of >.
|	none	|	Piping. Takes the output of the command to the left and uses it as the input for the command to the right.
Get-Help	none	man	Gets the help file for a cmdlet. Adding the parameter -online opens the help page on TechNet.
exit	none	exit	Exits PowerShell

Remember the keyboard shortcuts of tab for auto-completion and the up and down arrows to scroll through recent commands. These shortcuts can save a lot of typing!

This link has one example of a fairly good single page (although fairly long) introduction to Power shell. It includes a PDF cheat sheet of commands and aliases, but that contains way more than you will need unless you start managing systems and implementing shell scripts (i.e. automating repetative sequences of complex commandline activity).

• Mac terminal commands cheat sheet