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
3. Intermediate level (optional)
   • Pipes and Filters
   • Using git to get data for the class
   • Shell scripts

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 Mac OS X 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

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  ~$

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.

Navigating Files and Directories

Commands

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

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/Mac OS X, 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).

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:

/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.

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.

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.

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

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). List the files there. Did you find "nano" and "grep"? If not, look into /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 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.

Creating directories and files

Commands

• mkdir (make directory)
• your editor of choice (atom by default for the class, nano is an alternative choice ¹)

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:

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. ²
• Plain mkdir cannot create multiple directories deep in one go unless you have the -p option (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:

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:

atom TODO

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 (optional)

This is a more extended exercise in using various shell commands. For today, this is optional, but you are encouraged to do it.

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 !)

Intermediate Level

The following is very useful but for right now not essential and is optional. You can work through the tutorial in your own time.

Pipes and Filters

Commands

• cat
• head
• tail
• less
• wc
• sort
• uniq
• cut and paste
• grep

and special shell characters

• | ("pipe", joins commands together)
• > (redirects output to a file)
• < (redirects file to input)

Working with redirection and command pipelines

Download planets.dat³ and put it in the directory data/. You can do this with your web browser or if you have the curl program installed, try

curl https://asu-compmethodsphysics-phy494.github.io/ASU-PHY494-2019/public/data/planets.dat -O

(all on one line).

Output the whole file to the screen:

cat planets.dat

Look at the first three lines of the file

head -3 planets.dat

should give

Alderaan             12500  grasslands/mountains
Yavin_IV             10200  jungle/rainforests
Hoth                  7200  tundra/icecaves/mountainranges

Count the number of planets

wc planets.dat

      60     180    2944 planets.dat

Make a file in which all planets are duplicated:

cat planets.dat planets.dat > planets_2.dat

(cat concatenates files and then you redirect it to a new file.)

Activity

1. Test that wc -l gives just the number of lines ("60").
2. What does wc planets.dat planets_2.dat do?
3. What happens when you do wc < planets.dat?
4. Run wc then type any number of lines of text (pressing enter to terminate each line) and when you get bored, press ^D (control+D). What happened?

Pipes

Sort by name and look at the first five using a pipes and filters:

sort planets.dat | head -5

Sort by diameter (-k2,2 is column 2 and numeric sort -n), biggest first (-r reverse sort), and write the top 3 to a file biggest_planets:

sort -k2,2 -n -r planets.dat | head -3 > biggest_planets

Count the number of planets with unknown diameter:

grep "unknown" planets.dat | wc -l

Get the first letter of each planet name and sort alphabetically:

cut -b 1,1 planets.dat | sort

Get the terrain types

cut -b 29-  planets.dat

Activity

1. Count the number of planets in planets_2.dat.
2. Find planets where the rebel might have a base (hint: you know it's cold there… use grep). How many planets will you investigate more closely? Write the list to the file bases.
3. How many unique terrain types are there? (Hint: uniq needs a sorted list as input)
4. What is the most frequent and the least frequent first letter amongst these planets? (Hint: uniq -c)

Shell glob patterns

Use the character * to match "any part of a file name", e.g.

ls *.dat

will list all files ending in .dat.

The ? character matches a single character. Neither of them matches a leading . in a file name or a space.

Using git to get data for the class

git is a version control software and we will come back to explain its main functionality later. Right now we use it as a convenient tool to get additional data.

There is a "repository" at https://github.com/ASU-CompMethodsPhysics-PHY494/PHY494-resources that contains data and code to be used during the class. It will be updated as we go along.

Get the data for today by cloning the repository:

cd ~
git clone https://github.com/ASU-CompMethodsPhysics-PHY494/PHY494-resources.git

(You only need to do this once.)

At any later time, pull in the latest updates from inside the repository::

cd PHY494-resources
git pull

(This can be done as often as you like.)

Go into the PHY494-resources/01_shell/data directory.

Activity

1. Read the README file (e.g. using atom or cat or less (for the latter, use h to get help and q to quit)).

2. Count the number of entries in all files ending in "csv". (Hint: use a glob pattern)

3. Use cut -f 1 -d ',' people.csv to extract each name to a file names and a similar command to extract weight to a file weights.

4. Use the paste command to generate a new list that contains "weight name" (reordered and separated by space):

    paste weights names
   

   Use this approach to sort the people in order of decreasing weight.

Shell scripts

You can save commands in a file. This is called a script. A script allows you to reuse commands (laziness is a programmer's virtue!) without having to retype them over and over again. It also allows you to solve a task once and then forget about how you did it in detail because it is written in the script.

Make directory ~/bin for your scripts in your home directory.

mkdir ~/bin

(I strongly suggest you do this really in your home directory because in the following I will assume it; if you changed the path to e.g. ~/classes/2016/PHY494/bin then you will need to use that path in all the following examples.)

Usingatom, create the following script ~/bin/update_resources.sh:

# PHY 494 script to update the resources repository

GIT_REPOSITORY="${HOME}/PHY494-resources"

cd "${GIT_REPOSITORY}"
git pull

echo "Updated resources in ${GIT_REPOSITORY}"

(You create the script by (1) atom ~/bin/update_resources.sh (opens empty file if it does not exist), (2) type all the lines into the editor (or copy & paste), (3) save the file and exit the editor.)

Notes:

• All the lines above should be in your file (first line will start with # PHY 494 and the last line will begin with echo).

• The line starting with # is a comment: it is not a shell command and is ignored by the shell. However, adding comments to scripts is a really, really good idea!

• The shell has variables: Some like HOME are pre-defined, others you can define yourself (GIT_REPOSITORY=...). Using all-caps is a convention that you should follow.

  The contents (value) of variables is accessed with the dollar $ sign in front of the variable name.

• echo prints to the standard output (typically, the screen)

Execute the script with

bash ~/bin/update_resources.sh

It should show output similar to

Already up-to-date.
Updated resources in /Users/oliver/PHY494-resources

However, during the course of the year more data will be added to the repository and then you can just run your update command to get the data and you might see output like the following:

remote: Counting objects: 15, done.
remote: Compressing objects: 100% (10/10), done.
remote: Total 15 (delta 2), reused 15 (delta 2), pack-reused 0
Unpacking objects: 100% (15/15), done.
From https://github.com/ASU-CompMethodsPhysics-PHY494/PHY494-resources
   c3b5c04..23a4083  master     -> origin/master
Updating c3b5c04..23a4083
Fast-forward
 01_shell/bin/update_resources.sh | 8 ++++++++
 02_python/gutentag.py            | 4 ++++
 2 files changed, 12 insertions(+)
 create mode 100644 01_shell/bin/update_resources.sh
 create mode 100644 02_python/gutentag.py
Updated resources in /Users/oliver/PHY494-resources

Footnotes