01 The Unix Shell

This lesson introduces the shell, also known as the command line. There are many different shells: We are using the bash shell.

This lesson builds on An Introduction to the command line with Unix Basics and the Software Carpentry lesson on The Unix Shell.

1. Background and Motivation
2. Introductory Shell Tutorial on basic shell usage
   • Accessing the shell
   • Navigating Files and Directories
   • Creating directories and files
   • Copy, rename, delete

An optional lesson on more advanced intermediate shell features is also available for you if you are interested.

Background and Motivation

As a computational scientist you have one primary tool at your hand: a computer. And like an experimental scientist, you will have to be able to interact with it, adjust it, tweak it, fix it, and generally make it do things that it has never done before.

You are probably used to interacting with a computer via a graphical user interface ("GUI") — windows, browsers, a mouse or touch screen. Although convenient, this limits you to the interactions designed into the interface. To get more out of a machine you have to talk to it more directly.

We will interact with the computer using the text-based terminal through a so-called command line interface (CLI). The user types input commands; the commands are read, executed, and output is printed. The program responsible for doing this is called the shell (because it "encloses" the operating system to simplify user interaction with it).

The shell is a program like any other but it's primary purpose is to run other programs; most of the input "commands" are in fact other programs. The shell is very good at working with the file system (files and directories), combining multiple existing tools in powerful ways, and automating tasks. On a Unix-like operating systems (typically used for high-performance computing) using the shell is a very powerful (and essential) way to interact with the computer.

Introductory Shell Tutorial

Accessing the shell

• On a typical Linux system you open an application called terminal or xterm or kterm (or similar).

• On macOS you open Terminal.app (in the Utilities folder in Applications).

• On Windows we use Git Bash: Find it in the Program menu under All Programs/Git/Git Bash

Shell prompt

You should be greeted by the prompt, which can look like this

$

with the cursor as an underline or block showing that you can type. Often your prompt is more elaborate, e.g.

dvader@deathstar.empire.gov  ~$

Executing commands

Commands are executed by typing the name of the command (typically an abbreviated English word like cd for "change directory", ls for "list", but also python for the Python interpreter) and then finishing with return (or enter).

Type

whoami

and hit enter. You should see your username being printed to the screen.

Type

cd

and hit enter to begin the lesson. From now on, input will just be shown and you should enter it.

General syntax of a shell command

Commands generally take arguments (what to operate on, required) and options (modify the standard behavior, optional).

command_name -v -o optarg --long-opt  arg1 arg2

Often, omitting the argument also works and a default is assumed. You can learn more about commands with the help function (try running just with option -h or --help or on Linux/macOS, man command_name; the Git Bash installation does not come with man pages but you can use services on the web such as the Debian man pages).

Navigating Files and Directories

The operating system manages access to files and directories in the file system (typically, the stuff that you see on your disk).

• files = documents, data, pictures, …
• directory = "folder"; may contain any number of files or other directories

Instead of using your graphical File Explorer or Finder we want to use shell commands to

• move around the file system
• get information about files and directories
• specify the location of files and directories

Commands

• pwd (print working directory)
• ls (list directory)
• cd (change directory)

Paths

A path consists of directory names separated by forward-slashes "/" and possibly a final file name.

• / (the actual name of the "root" directory)
• /home/dvader or /Users/dvader
• /usr/bin/nano
• books.txt
• /home/dvader/Documents/deathstar/weaknesses.pdf
• Documents/deathstar/weaknesses.pdf

Paths starting with / are called absolute paths; anything else is a relative path, starting from the current working directory (check with pwd).

Special directory names:

• . is the current directory (e.g., ./Documents)
• .. is the parent directory (e.g., ../../home/dvader/..)
• ~ is the home directory (only when leading the path, e.g., ~/Documents)

Location in the files system: pwd

Assume user dvader has the following directory layout in his home directory /home/dvader ¹:

/home/dvader/
	     Documents/
		      deathstar/
			       weaknesses.pdf
			       electrical_bill.dat
		      work/
	     data/
		      planets.dat
		      bases

When Mr Vader logs in he starts in his home directory, /home/dvader. He can run pwd to "print the working directory":

pwd

which shows /home/dvader.

Activity: Find Your Home

• Open Terminal
• Type

  pwd
  

and hit enter. Note the directory name: this is your home directory.

• share your home directory name (copy and paste into the chat)

Listing file system contents: ls

If he executes ls ("list") he will see something like

Documents
data

Activity: List Your Home directory

• open a terminal (or use the open terminal from the last exercise)
• Type

  ls
  

• Compare the files that you see with those of your neighbor: copy and paste into the chat

Moving around the file system: cd

If he executes the cd command

cd /home/dvader/Documents/deathstar

then he will have moved to the deathstar directory; confirm with pwd!:

pwd

will print

/home/dvader/Documents/deathstar

If he runs

ls

he will see

electrical_bill.dat  weaknesses.pdf

The cd command took an argument, the directory to go to.

Activity: Going up

• Go to the parent directory of your home dir:

  cd ..
  

• List the contents

  ls
  

Activity: Going Home

• Type

  cd
  

• Where are you now? (What command can you use to print the directory that you're in?)

More on cd

The command cd on its own always returns you to your home directory. (It is equivalent to cd ~.)

In order to get to the data directory he could use the command cd /home/dvader/data but instead he uses the special directory .. (which means "the directory above this one"):

cd ../../data

A second special directory is . ("this directory"). . and .. are understood by all commands.

Autocompletion

• Use the TAB key while typing: autocompletion is one of the best features of the shell!
• The other great interactive feature is the history: try using the cursor-up and -down keys.

Wildcards

The shell contains a simple pattern matching syntax (wildcards or "glob patterns") to select all files that match the pattern. Commonly used patterns are

• *: matches zero or more characters in a name, e.g., *.dat matches all files ending in .dat, and report_2021-*.txt matches report-2021-11.txt, report-2021-12.txt, but not report-2020-2.txt
• ? matches a single character in a name, e.g., ?.jpg matches 1.jpg, a.jpg, but not ab.jpg

Neither of them matches a leading . in a file name or a space or the directory separator /.

Example: List all dat files

List all "dat" files in the current directory: Use the character * to match "any part of a file name"

ls *.dat

Example: List all commands with "oo" in their name

List all files containing "oo" in the /usr/bin directory (will only work on Linux or macOS):

ls /usr/bin/*oo*

Example: Delete jpg files in directories 19xx

Warning: DO NOT COPY AND PASTE ANY COMMAND LINE CONTAINING rm WITHOUT THINKING TWICE ABOUT IT. You have been warned.

Delete all jpg images in directories from the last century

rm Pictures/19??/*.jpg

(Note that 19?? matches anything that looks like a year from the last century such as 1999, 1969, … although it would also match 19xy; the *.jpg pattern matches all files with suffix jpg.)

Example: Show all two-letter Unix commands

Show commands with exactly two letters (assuming that you have a Unix-like OS such as Linux or macOS where the commands are located in /bin and /usr/bin):

ls /bin/?? /usr/bin/??

(Two letter commands are important — learn more about these commands, e.g., with the three-letter command man.)

Example: Copy Jupyter notebooks to another directory

As part of working with code in this class, we will have to copy Jupyter notebooks from one directory to another directory

cp ~/PHY494-resources/10_ODEs/*.ipynb ~/PHY494/10_ODEs/

Error messages

Unix commands are terse: If everything works, they say nothing. If they fail, you get a short (sometimes cryptic) error message.

Try

cd bogus

This gives

bash: cd: bogus: No such file or directory

Always read error messages!

Activity: Explore your file system

1. What does the following sequence of commands show?

    cd
    ls -a
   

2. Go to the bin directory that is located in the root directory (at least on Linux and macOS) or to the bin directory in ~/Anaconda3/bin (on Windows). List the files there. Did you find "cp" and "grep"? If not, see if you have /usr/bin.
3. What does ls -R / show? (Try ^C, i.e., press CONTROL and C at the same time…)
4. Try other options of ls such as -sh or -sha. — ask ls for help!
5. Is there a difference between ls -sha, ls -ash, and ls -a -s -h?

Tips to make your (shell) life easy

• Use the shell's convenience features:
  • TAB completion
  • up/down arrow to get commands that you already typed.
  • history shows all the commands you typed so far
  • Try out Control+R (^R) to search through your shell history.
  • ^A and ^E will likely take you to the beginning and end of a line
• Use cd .. to go up, cd ../.. to go up twice etc.
• Use cd - to go to the previous directory I was in (only works for cd).
• Use the tilde character ~ for your home directory, e.g. cd ~/Documents is equivalent to cd /home/dvader/Documents.
• Use wildcards (*) but test them with ls before using them with rm!

Creating directories and files

We want to

• create, copy, rename, and delete files and directories
• edit text files

Commands

• mkdir (make directory)
• your editor of choice (atom by default for the class, nano is an alternative choice ²)

Activity: Directory structure for the class

Make a directory PHY494 in your home directory for the class and inside it, one called 01_shell for today's lesson, using the following sequence of shell commands:

cd ~
mkdir PHY494
cd PHY494
mkdir 01_shell

Note:

• Avoid spaces and most special characters in file names. All letters and numbers together with underscore _, minus sign -, and period . are ok. ³
• Plain mkdir cannot create multiple directories deep in one go unless you have the -p option available (check!).

Activity

1. Inside your 01_shell directory, create three directories, data and Documents/work.
2. Go to Documents/work

Creating text files with a text editor

Run atom by either launching it from a menu or from the commandline ⁴

atom

You should see something like this:

We will now create a TODO list in a file with name TODO and content

Plan for today:

1. find rebel base
2. destroy!

1. Open a new file via the menu option

   File → New File

   You should have a tab named untitled.

   Close all the other tabs. It should look like this:

   

2. Type the todo list into the editor window.

   

3. Save it to file with name 'TODO' with menu option

   File → Save

   Make sure to navigate to your the PHY494/01_shell/Documents/work directory.

   

   You can hide the "project" view with the directory structure.

4. Quit atom (details depend on your operating system; find the Quit menu option either under File or Atom.

5. Check that the file is in the directory with ls.

You can re-open the file from the command line ⁴:

atom TODO

(Alternatively, open atom from your GUI and find the file TODO through the File → Open dialog.)

Activity: create a file with your editor

• Create a file ~/PHY494/Documents/work/lesson.txt with three lessons from today:

  1. computers are tools
  2. the shell is powerfull
  3. basic shell commands: pwd, cd, ls
  

• save the file and check that it is there with

  ls -l ~/PHY494/Documents/work/lesson.txt
  

• Open the file again and add a 4th lesson regarding file editing.

Copy, rename, delete

• cp (copy file, cp -r copy recursively, including directories)
• mv (move, i.e. rename, also to another directory)
• rmdir (remove empty directory)

• rm (remove file, rm -r remove recursively (dangerous!))

  WARNING: There is no "Trashcan" or built in backup. Once you rm something, it is gone. Be especially careful with rm -r.

  WARNING: Do not execute rm -rf *. It will erase everything permanently.

Note: all these commands can also work on multiple filenames.

Activity (homework)

This is a more extended exercise in using various shell commands.

1. Make a backup (call it TODO.bak) of the TODO list with the cp command.
2. Rename TODO to TODO.txt with the mv command.

3. Make a directory notes under the data directory: You should now have a directory tree similar to

    ~/PHY494/01_shell/
                      Documents/
                                work/
                                     TODO.txt
                                     TODO.bak
                      data/
                          notes/
   

   Check with ls -R ~/PHY494.

4. Put a copy of TODO.txt into the notes directory (using cp).
5. Create a new text file data/notes/hints.txt and write any hints for possible rebel bases into this file.
6. Open TODO.txt in atom and add a note to item 1 too look in the hints.txt file. Save and exit.

7. Make a copy of your notes directory in your work directory:

    ~/PHY494/01_shell/
                      Documents/
                                work/
                                     TODO.txt
                                     TODO.bak
                                     notes/
                                          TODO.txt
                                          hints.txt
                      data/
                          notes/
                                TODO.txt
                                hints.txt
   

8. Remove data/notes/hints.txt with rm.
9. Remove data/notes with rmdir. (Hint: Read the error message!)
10. Move work/notes/hints.txt into the work directory.
11. Remove the useless work/notes directory with rm -r (careful !)

Footnotes