1 Introduction

This short tutorial/workshop is meant to review and understand basic command-line as they are typed on a text terminal.

This specific workshop will focus on Macintosh Terminal, but most commands would also work on all Unix-style operating system (Linux, or Windows with added software.)

The goal is to review the most useful commands in order to operate the terminal for later workshops including those focused on Docker.

1.1 Learning objectives:

The main objective is to become at ease with the command-line to perform routine tasks:

  • Open a Terminal
  • Understand the computer organization (file structure and “path”)
  • Create, delete and navigate directories
  • Create, delete, explore and edit text files
  • Download Internet data files
  • Apply key concepts (standards and streams) to tasks

1.2 Terminal and Shell

1.2.1 Text terminal

In the early 1990’s “dumb terminals” were still used to access a remote, shared computer. The terminal was called “dumb” because it did not contain any operating system. The digital VT100 (“VT100” 2019) was a very popular terminal:

Text terminal VT100

Text terminal VT100

In contrast, a “personal computer” is an “all-in-one” that uses terminal emulation software within itself. On a Macintosh it is called Terminal and, on a Windows PC cmd or PowerShell.

These are emulation i.e. they mimic what the physical terminal used to do: let the user “talk” to the computer with typed commands. These commands are in the “shell language” that resembles English more than “machine language” made of binary or hexadecimal numbers.

1.2.2 Shell

The shell is both a language and a software that takes commands from the keyboard (user input) and transmits them to the “kernel,” the part of the operating system that can also “talk” to the hardware. For example, it is the kernel that will instruct the hard drive to make physical changes to record a file you are writing and saving.

This is perhaps why the shell is often represented surrounding both hardware and kernel.

The shell: an intermediate between user and OS.

Figure 1: The shell: an intermediate between user and OS

Figure 1.

1.2.3 shells

The most current shell is called bash (Bourne Again SHell, an enhanced version of the original sh by Steve Bourne.) This is the default shell on most Linux systems and on Macintosh until now.

Wikipedia Note1 https://en.wikipedia.org/wiki/Z_shell: The newest MacOS (Catalina) now uses the Z shell which is an extended Bourne shell with many improvements, including some features of bash, ksh, and tcsh.

Warning: shell commands are CaSe SenSiTive!

2 Set-up

We’ll use the iMacs form Biochemistry laboratories room 201.

These commands would also work on a Windows system with added software e.g. Ubuntu for Windows, see installation at https://tutorials.ubuntu.com/tutorial/tutorial-ubuntu-on-windows

To get started we first need to open a text terminal as detailed below.

Do one of the following:

  1. Find the Terminal icon in the /Applications/Utilities directory. Then double-click on the icon and Terminal will open.
  2. OR use the top-right icon that looks like a magnifying glass (Spotlight Search,) start typing the word Terminal and press return. Terminal will open.

3 Working in Terminal

Remember that Terminal is a software version of what used to be a physical hardware. Terminal itself is “dumb” and only allows you to “talk” to the computer operating system through the shell with the shell language somewhat similar to English.

3.1 Username

You will need to login the iMac with your NetID and a username will be created on this Mac is you have not logged in it before.

3.2 Home

When you start Terminal you are immediately connected internally with the operating system (OS,) here it is MacOS, a derivative of Unix, an OS developed in the 1960’s with very powerful features that we’ll discover below.

When the connection is established, you “land” in the “home directory” i.e. the area on the disk reserved for your username (these computers can have multiple -even concurrent- users.)

In most modern Macintosh, Windows and Linux systems the disk areas reserved to users are defined in a similar way as a “high level” directory. We’ll see a bit later how to know exactly where this disk space is located.

3.3 Prompt

When the terminal starts your username will appear together with the computer name followed by a $. This is called the prompt and simply signals that the terminal is ready to accept shell commands. For example, my prompt looks like this:

Last login: Wed Nov 27 08:57:02 on ttys000
BIOCNB-01014M:~ jsgro$

BIOCNB-01014M is the name of the computer I am using and jsgro is my username.

On this line we’ll type shell commands.

3.4 Preferences, $ and Variables

In most modern software you’d find a menu options called Preferences… where you can change predefined choices. In the same way, there exists preference setting for the shell. They are called “environment variables” and can sometimes play an important role.

Variables have defined names, usually in uppercase, and the $ symbol is also used to printout the value of a variable.

For example, we can check the name of the shell that is running in Terminal with the shell command echo and the specific variable associated with it: SHELL. By default, echo will simply repeat (print back on screen) what is typed on the keyboard, but with a preceding $ to the typed word echo will print the value of a variable itself. This is best understood by practice as suggested below.

Note: without the $ the program echo simply repeats what it is given.

Run the following commands:

echo SHELL

Answer :_________________________________

echo $SHELL

Answer :_________________________________

In the same way what is the output of the following commands?

echo $USER

Answer :_________________________________

echo $HOSTNAME

Answer :_________________________________

echo $HOME

Answer :_________________________________

4 Home and username revisited

4.1 Who am I

Since we cannot see the directories in a graphical way, it is very useful to start with understanding where we “land” when we first launch Terminal and where we are “looking” inside the computer hard drive at any moment. The following commands will give us some clues about this:

Run the following commands:

whoami

Answer:_________________________________

This whoami command with an “existential feeling” used to be very useful when terminal stations were shared and someone forgot to log-off. In fact, in modern shells, this information is also found as part of the prompt as we have seen before.

4.2 Where am I looking

pwd

Answer:_________________________________

This is an important command that shows the present working directory.

In this case it is our “home” directory as we just arrived on the system.

4.3 Hard drive: where things are

Your home directory is just one area of the hard drive, that can be shared by the home directory of other users as illustrated below.

This type of organization is sometimes called an “inverted tree.” The top level is called root and it branches out into (contains) other directories, subdirectories, and files.

Hard drive organization.

Figure 2.

Important notes:

  • The home directory shows a path (green arrows and grayed folder) from the very top level of the computer system denoted / and called the “root.” (See more on path below.)

  • The separator between directory names is also the forward slash /

  • The complete home directory path can also be replaced by the single symbol ~ (more on this later.)

4.4 Summary so far:

Command Definition
$ shell prompt: awating for command
pwd print working directory
~ equivalent to home directory path
/ root and separator symbol

5 Directories

Directories as like file cabinets.

In this section we’ll learn to create, delete and navigate between multiple directories. We’ll also learn to list and delete files.

5.1 New directories

Creating separate directories for various projects is the best way to organize your work (and text files, data files, sub-directories etc.) On your laptop you could easily create a directory with the mouse. But while working in Terminal, it is sometimes simpler to just use the mkdir shell command to “make a directory” in the location we are looking, currently our home directory.

Important Notes:

  • Avoid blank spaces in directory and file names. (It is possible but makes them harder to handle.)
  • Names are case-sensitive: A is not the same as a. For example, Myfile is not the same as myFile.

Run the following commands:

First, we create a directory:

mkdir dir01

Did I hear OOPS ?

Perhaps the name should be something else, perhaps myproject01?

At this point there are 2 solutions:

  1. rename it with the shell mv (move) command:
    • mv dir01 myproject01
  2. delete it with shell command rmdir (remove directory) and create a new one again:
    • rmdir dir01
    • mkdir myproject01

Choose one method so that you now have myproject01 available to you.

OK - now we have a directory to work with.

To work within this myproject01 directory we have to move our focus into that directory. For this we use the shell command cd that means “change directory.”

cd myproject01

We can then verify that is where we have now landed with this useful shell command that we have already learned:

pwd

We should now be within the myproject01 directory which is empty. However, as part of the operating system, two invisible files are created that are an integral part of the directory. We can see them by adding -a to the ls command to list all file:

ls -a

What do you see?

Answer: ___ ____

These two items are the symbolic representation of the “current directory” (dot) and the “parent directory” (dot dot).

Notation Spoken Name Definition
. “dot” Current directory
.. “dot dot” Parent directory: directory “above” containing the “current” directory.

We will use this knowledge in the following section.

Current (dot) and parent (dot dot) directories.

Figure 3.

Note: If you use the command ls -aF instead you will see the following result:

./  ../

The trailing / signifies that these 2 items are in fact folders: the current one and the parent one.

5.2 Path

The existence of . and .. provides that we can specify the location of a file on the system with either an absolute or a relative path.

An absolute path is a description starting from / (root) which is therefore complete and unambiguous since there is only one root within the computer file organization.

For example, in the hard drive organization figure above the absolute path to File is: /Users/jsgro/File.

A relative path is a description starting from afolder other than root.

Examples:

  • pwd provides an absolute path starting from the very beginning of the root with /.

  • ls .. is a command that will list files from the parent directory relative to our current directory location.

5.3 Home again

If you are “lost” on the system of course pwd can help, but just typing cd will bring you back into your home directory.

6 Files

A file is a “container” of information. Some files contain plain text and are easy to handle and explore on a text-only terminal. More complex files contain binary information that would display as “gibberish” on a text-only terminal.

It is useful to name files with a filename extension, for example .txt for a plain text file as a reminder of the type of content.

(In addition, on most current operating systems, the filename extension will lead the OS to show the file graphically with a specific icon.)

6.1 Exploring file contents

In this section we’ll learn to download a file directly from the Internet, view top and bottom portions of a text-only file, and finally to read its content one screen at a time.

On MacOS the shell command to copy a web address curl can be used to download files directly into the current directory. We’ll download a plain text file listing the known chemical elements named chemical_elements.txt. At the same time we’ll change the name of the file into a new, simpler name: elem.txt for easier typing.

Run the following commands: (type on a single line)

curl -o elem.txt https://static-bcrf.biochem.wisc.edu/tutorials/unix/survival_command/chemical_elements.txt

Note: if you need to actually type you can use this short link instead: tiny.cc/chemelem

Note: you can type ls to verify that the file has been transferred and that its name is what it should be.

Now we are going to explore the content of the file.

  • list the first few lines at the top of the file. The default for head is 10 lines, but we can modify that number simply. For example to see the first 5 lines:
head -5 elem.txt
  • In the same way the command tail can print the lines from the end of the file. To see the last 3 lines of the file issue the command:
tail -3 elem.txt

Two other commands are useful:

  • cat will print the whole content of the file at once, therefore making it impossible to see the top if the file is too long. (In Windows DOS the command would be Type.)
  • more (or its newest incarnation less) will show one screen at a time. Pressing Enter will show one more line at a time, while pressing Space bar will show one screenfull at a time. Pressing q will quit and return to the prompt.

Note: to remove a file use the command rm followed by the name of the file.

Note: Earlier we learned rmdir to delete an empty directory. However, to remove a directory that is not empty, the (dangerous) command is using a recursive method (adding -r) for example: rm -r somedirectory (-Warning!- there is NO UNDO and this command will remove everything thus make sure you are in the right place with e.g. pwd before issuing it!)

6.2 File editing

The ability to create your own text files is essential and the full-screen text editor nano can be very helpful. This software can be used to open and modify existing files, or to create new text files.

nano can open an existing file to modify its content or create a new file. Let’s create a simple file called simple.txt containing just a few lines.

cd ~/myproject01

nano simple.txt

This will open a full screen editor. Ctrl command options are shown at the bottom of the screen:

  GNU nano 2.0.6             File: simple.txt                                
- - - - THIS IS THE AREA WHERE YOU TYPE TEXT - - - -
- - - - Use up, down, left, and right arrows - - - -
- - - - to navigate, NOT the mouse!          - - - -
- - - - When done, type Ctrl X to exit       - - - -

^G Get Help ^O WriteOut ^R Read File^Y Prev Page^K Cut Text ^C Cur Pos
^X Exit     ^J Justify  ^W Where Is ^V Next Page^U UnCut Tex^T To Spell

Write some simple text, then press Control and X keys at the same time to exit the program and write the new file to the current directory.

On exiting you may have to answer Yes or Y to the questions:

Save modified buffer (ANSWERING "No" WILL DESTROY CHANGES) ?
 Y Yes
 N No           ^C Cancel

Note: The command shown as ^O for Control + O (capital letter Oh) would write the current changes but would not close the program but stay within the editing mode for further text editing.

7 Compressed web files

We learned the command curl above.

It is often necessary to download files from the Internet. These files can also be compressed. The following section is a short exploration on how to handle such matters.

The following example will download a random DNA sequence file from the University of California at Santa Cruz (UCSC) from chromosome 4. The web page is at http://hgdownload.cse.ucsc.edu/goldenpath/hg19/chromosomes/

Here is the command to download file chr4_gl000194_random.fa.gz with curl:

Run the following commands:

curl -o chr4_gl000194_random.fa.gz  http://hgdownload.cse.ucsc.edu/goldenpath/hg19/chromosomes/chr4_gl000194_random.fa.gz

Note that on Linux system there exists also the command wget (“web get”) that can accomplish the same task. In this case the -o to specify output is not required. wget is not standard on Macs and curl is used instead.

wget http://hgdownload.cse.ucsc.edu/goldenpath/hg19/chromosomes/chr4_gl000194_random.fa.gz

Note: You can also go to the short URL equivalent of the web page https://bit.ly/2kJpA0K and then “right-click” on file chr4_gl000194_random.fa.gz and select “Copy link”

The file is a Fasta sequence format (.fa) and is also compressed with the gnu-zip program gzip and can be decompress by gunzip as shown with the following command:

gunzip chr4_gl000194_random.fa.gz

Note that the suffix .gz will automatically be removed. Now the file is a simple text file and can be explored as seen previously, e.g. with head, tail and more.

Note: The files ending with .zip should be uncompressed with the unzip command as these are different formats and software.

7.1 Section summary:

Command Definition
mkdir create directory
rmdir delete empty directory
rm remove file
rm -r recursively remove everything within a non empty directory
ls list content of directory. -a for all, -d for directory etc.
mv move and/or rename file or directory
cd change directory
. current directory
.. parent directory
path Absolute start with /. Relative uses . and/or ..
TAB computer finishes tying
head show first 10 lines by default
tail show last 10 lines by default
cat type all content of file on screen
more type file content one screen at a time. Newer: less
nano text editor
wget obtain file from web address

8 Streams and pipes: key concepts

One of the reasons that Unix-like systems and its shells have withstood Time is the inherent power of a few key concepts detailed below.

8.1 Standard input and output

Unix-like systems comprise Unix, Linux and MacOS operating systems.

  • The default standard input is your keyboard.
  • The default standard output is the screen

Standard output (which is printed on the screen by default) is split between the normal output (stdout) and an error output (stderr) in case errors are to be reported.

There are therefore three “streaming” channels for Input/Output (I/O) labeled from zero to 2:


Understanding I/O stream numbers
Handle Name Description
0 stdin Standard input
1 stdout Standard output
2 stderr Standard error

Standard Input/Output channels..

Figure 4

The above figure illustrates the standard input and output concepts.

On the top image, the keyboard (stdin) is used to enter a shell command (e.g. list the content of a directory with ls) which creates an output. The ouput is sent to the standard output (stdout) which is the computer screen by default.

The bottom image further shows that the output represents 2 channels, one for the normal output and another for error output.

8.2 Data stream and pipes

We can redefine the 3 standards above with the added notion of “stream” as preconnected input and output communication channels Ritchie (1984), (“Standard Streams” 2019):

  • Standard input is stream data (often text) going into a program.
  • Standard output is the stream where a program writes its output data.
  • Standard error is another output stream typically used by programs to output error messages or diagnostics.

One can imagine data flowing from input to process to output.

More importantly the data could be at any point redirected (hijacked) to a different destination.

8.2.1 Redirection

The first example is redirecting the standard output, normally destined to appear onto the screen, into a file that will be saved onto the computer. This is accomplished with the redirect > symbol.

As example we can create a new file called 5lines.txt based on a previous head example above. In this case nothing will appear on the screen and a new file will be created instead, containing what would have been shown on the screen:

Run the following commands:

head -5 elem.txt > 5lines.txt

We can verify that this is the case:

ls
cat 5lines.txt

Important Note: using the same command with > would overwrite any existing file. To instead add to the end of the file (append) one should use the double redirect >> instead.

One useful use of this method is the ease of creating text files without a text editor. We can use the command cat that normally takes the content of a file and sends it to standard output. Instead of a file we can use the keyboard input and redirect the data into a file.

cat >  example.txt

Note that there is no $ prompt visible.

Here we write some text that will be saved 
to a file named example.txt
 
Everything we write here is redirected
into the file (redirected standard output.)

We can add as many lines as we want.

BUT WHEN DONE WE NEED TO PRESS: CTRL-D 
as a signal that we are done!

CTRL-D means "end of file" and is like
"closing the file", ending the redirection
and returning us to the $ prompt.

Again: CTRL + D will ends this process and return us to the $ prompt.

8.2.2 Pipes

The data stream issued from the process of a program can also be redirected in a different way with the pipe symbol |. In this case the data stream from one program coming out through standard output will be used a standard input by the next program. Multiple processes can therefore be connected in a single pipeline, as illustrated below:

process1 | process2 | process3 | process4

The process could be any running program that has proper standard input and standard output compliance. As an example we can use the utility wc (word count) to count the number of words of a data stream:

Run the following commands:

head -5 elem.txt | wc

Answers: ___ ___ ___

The three numbers represent the number of:

  • lines,
  • words and
  • characters (including the return character.)

This other example uses the program grep to recognize a simple text pattern, here the pattern is ron, chosen to select only the lines that contain this pattern within any of the words on each line. The program cat sends the content of file elem.txt into the data stream as standard input which is then “piped” into program grep:

cat elem.txt | grep ron

Answers: B______
Answers: I______
Answers: S______

We could even string these processes in a longer pipeline where we accomplish 3 tasks:

  • send the data from elem.txt into the data stream. From cat the data arrives as part of the standard output, which serves as standard input to the next program grep.
  • grep processes the data to recognize the pattern ron.
  • finally, wc does the lines, word and character counting. The final standard output arrives onto the screen.
cat elem.txt | grep ron | wc

Here is the result you should obtain:

      3       3      21

Of course, we could have also recuperated the final output into a file instead!

cat elem.txt | head -5 | grep ron | wc > count_5lines.txt

9 Remote connection

In some cases, you may want (or need) to connect to a remote computer. In this case the remote computer is most likely to be a Linux cluster. For this we would use the command ssh which means “Secure SHell” as all connections and transmissions are encripted (hence secure.)

In the Biochemistry department the connection to the Linux cluster required a NetID as well as prior authorization.

If you are authorized to connect, the command would have this form, where myname is your NetID.

ssh myname@submit.biochem.wisc.edu

You would be then prompted to accept the connection and prompted for your password. For further details refer to the Biochem web site.2 https://bcrf.biochem.wisc.edu/bcc/

10 Summary

10.1 Concepts

Concept Definition
Standard input Default: the keyboard. Input piped data
Standard output Default: the screen display. Redirect to file or pipe
Standard error Default: the screen display.
Redirect Take standard input and send to file with > or >>
Pipe Take standard output and pass to next command as standard input with |

10.2 Symbols


Symbols and filters
Symbol Meaning
$ Shell prompt
$ Add to variables to extract value: e.g. echo $SHELL
~ Shortcut for home directory
/ Root directory. Separator on path names
> Single redirect: sends standard output into a named file.
>> Double redirect: appends standard output to named file.
| Pipe: transfers standard output to next command/software.

File descriptors
File Meaning
. Current directory. Can be written as ./
.. Parent directory. Can be written as ../
/dev/stdin Standard input
/dev/stdout Standard output
/dev/stderr Standard error

10.3 Commands learned or mentioned:


Commands learned
Command man page definition and/or example
echo write arguments to the standard output. echo $SHELL
whoami display effective user id.
pwd return working directory name.
cd change directory
ls list directory contents. ls -F, ls -a
wget grabs a file from Internet with provided web address
unzip list, test and extract compressed files in a ZIP archive.
mkdir make directories.
mv move files. (Can rename file/directory in the process.)
cat types file onto screen (or sends to standard output.)
head display first lines of a file. Default 10.
tail display the last part of a file. Default 10
nano (Text editor) Nano’s ANOther editor, an enhanced free Pico clone.
wc word, line, character, and byte count.
rm remove directory entries i.e. remove files. Remove non-empty dir with rm -r
rmdir remove directories (empty dirs)
cp copy files.

REFERENCES

Ritchie, D. M. 1984. “A Stream Input-Output System.” AT&T Bell Laboratories Technical Journal 68 (8). https://cseweb.ucsd.edu/classes/fa01/cse221/papers/ritchie-stream-io-belllabs84.pdf.

“Standard Streams.” 2019. Wikipedia. Wikimedia Foundation. https://en.wikipedia.org/wiki/Standard_streams.

“VT100.” 2019. Wikipedia. Wikimedia Foundation. https://en.wikipedia.org/wiki/VT100.