The UNIX shell

You can find the Software Carpentry lesson at:

On this page, you will find:



  • A family of Operating Systems
  • BSD, System V, Linux, MacOS

The UNIX philosophy

"the idea that the power of a system comes more
from the relationships among programs than from
the programs themselves"

..., which means:

  • write programs that do one thing and do it well
  • write programs to work together
  • write programs to handle text streams, because that is a universal interface


  • a program that allows to talk to the computer
  • powerful, fast, text-based
  • not really beautiful

Instructor notes

These notes are mainly written for the instructors. Yet, they contain most (if not all) commands and important points taught during a workshop, which may be interesting if you want to remember how much you learnt during a workshop.

We tend to follow the Software Carpentry schedule, but if time is lacking, we skip the loops (especially if the second day is dedicated to Python).

First, talk about the prompt ($ / PS1)

$ whoami
$ # show `which` to say the shell runs X
$ pwd # print working directory

Action: draw the filesytem organization (/ for root dir, inverted tree, different on Windows, file extensions are a convention)

Action: open Finder to see the files.

$ ls
Applications Documents    Library      Music        Public
Desktop      Downloads    Movies       Pictures
$ ls -F
Applications/ Documents/    Library/      Music/        Public/
Desktop/      Downloads/    Movies/       Pictures/

Question: how to get help?

$ ls --help
Usage: ls [OPTION]... [FILE]...
$ man ls

The --help option does not always work depending on the Operating System. It may work with -h or man ls.

$ ls Desktop

Action: ls with and without argument/flag (most commands have flags)

$ ls Desktop/data-shell
creatures/          molecules/          notes.txt           solar.pdf
data/               north-pacific-gyre/ pizza.cfg           writing/

Navigating with cd:

$ cd Desktop
$ cd data-shell
$ cd data
$ pwd
$ ls

Error when not found:

$ cd data-shell
-bash: cd: data-shell: No such file or directory

Explain special directories like .. and .

$ cd ..
$ pwd

Option -a is for "show all":

$ ls -F -a
amino-acids.txt   elements/     pdb/            salmon.txt
animals.txt       morse.txt     planets.txt     sunspot.txt

Talk about hidden files

Action: recap commands, options, and help.

$ ls
$ cd
$ pwd

Go directly where you want:

$ cd Desktop/data-shell/data

Talk about relative/absolute paths:

$ pwd
$ cd /Users/bebatut/Desktop/data-shell

Explain the following shortcuts: ~ and -.

Action: give link to zip file from a scientist called Nelle:

Organizing files

Explore the data.

Knowing just this much about files and directories, Nelle is ready to organize the files that the protein assay machine will create.

  1. She creates a directory called north-pacific-gyre
  2. She creates a directory called 2012-07-03, which is the date she started processing the samples

Each of her physical samples is labelled according to her lab’s convention with a unique ten-character ID, such as “NENE01729A”. This is what she used in her collection log to record the location, time, depth, and other characteristics of the sample, so she decides to use it as part of each data file’s name. Since the assay machine’s output is plain text, she will call her files NENE01729A.txt, NENE01812A.txt, and so on. All 1520 files will go into the same directory.

Question: is it a relative/absolute path? Think about other challenge questions.

Talk about tab completion.

Working with files and directories

How to create files and directories?

$ pwd # ensure we are in data-shell/
$ ls -F

Question: how to create a directory? we want to "make a directory".

$ mkdir thesis
$ ls -F

Action: show in the Finder

Good names for files and directories

Common rules:

  • Don't use whitespaces
  • Don't begin the name with - (dash)
  • Stick with letters, numbers, . (period), - (dash) and _ (underscore)
$ ls -F thesis # empty, let's create a file!
$ cd thesis

Present the nano text only editor.

The Git Bash users will not have nano installed but rather vi, which is complicated to use since it does not give any information on how to use it.

Question: what do you use to write a paper? or thesis?

$ nano draft.txt

Explain that Control = Ctrl = ^ key (hence the info in nano).

Action: write some content, then save and quit.

Question: where is your file? how to check?

$ ls

Removing a file (be careful, no trash bin):

$ rm draft.txt
$ ls

Action: try to remove a directory not empty by recreating the file first, then go to parent and rm.

$ pwd
$ nano draft.txt
$ ls
$ cd ..
$ rm thesis

Talk about recursively removing files, but tell the audience that it is dangerous. They should use interactive mode -i.

$ rm -r thesis

Create draft.txt:

$ pwd
$ mkdir thesis
$ nano thesis/draft.txt
$ ls thesis

Renaming a file by moving it:

$ mv thesis/draft.txt thesis/quotes.txt
$ ls thesis

mv is for files and directories (no mvdir for instance).

We can move files to directories too, but it does not transform a file into a directory, it moves the file somewhere else (physically):

$ mv thesis/quotes.txt .
$ ls thesis

See if the file is present:

$ ls quotes.txt

Copy files with cp:

$ cp quotes.txt thesis/quotations.txt
$ ls quotes.txt thesis/quotations.txt

Error expected if any of the files given to ls does not exist:

$ rm quotes.txt
$ ls quotes.txt thesis/quotations.txt

Create a file with touch:

$ touch new-file
$ ls
$ nano new-file

Pipes and Filters

wc stands for word count.

wc -l for counting lines.

$ ls molecules
$ cd molecules
$ wc *.pdb

Wildcards * (zero or more characters) and ? (a single character, not really used).

Exercise ideas: use wilcards

$ wc -l *.pdb
  20  cubane.pdb
  12  ethane.pdb
   9  methane.pdb
  30  octane.pdb
  21  pentane.pdb
  15  propane.pdb
 107  total

Which of these files is shortest? It’s an easy question to answer when there are only six files, but what if there were 6000?

Redirection with > (which also creates the file if it does not already exist).

It overwrites the file if it exists and may lead to altered data.

$ wc -l *.pdb > lengths.txt
$ ls lengths.txt

cat for displaying the content of the file (concatenate):

$ cat lengths.txt

sort for sorting things (alphabetically).

sort -n means the sort is numerical instead of alphabetical:

$ sort -n lengths.txt

Sorting without -n gives different results depending on the Operating System, the locale, and maybe other things too. On some system, running sort without -n can give the expected result for sorting things numerically, but you should always specify the -n option if you intend to sort things numerically.

It does not change the file:

$ sort -n lengths.txt > sorted-lengths.txt

head for displaying the beginning (head) of the file (-n 1 = first line):

$ head -n 1 sorted-lengths.txt

Redirecting to the same file is a very bad idea!

Too many intermediate files, solution is pipe:

$ sort -n lengths.txt | head -n 1
$ wc -l *.pdb | sort -n

$ wc -l *.pdb | sort -n | head -n 1
# head of sort of line count of *.pdb

Things worth explaining:

  • Process
  • Standard input
  • Standard output

Sucess of Unix: creation of lots of simple tools that each do one job well and that work well with each other --> pipe and filter.

Exercise ideas:

  • Nelle has run her samples through the assay machines and created 1520 files in the north-pacific-gyre/2012-07-03 directory described earlier. She ask you to check that the number of lines is the same in all files as there should be 300 lines per file.
$ ls *Z.txt


Loops: key productivity improvements through automation --> repetition of commands.

$ cd ../creatures

Only 2 files here, but the principe can be applied to many more files at once:

$ cp *.dat original-*.dat # can not be used
$ cp basilisk.dat unicorn.dat original-*.dat # expanding version
$ for filename in basilisk.dat unicorn.dat
> do
>   head -n 3 $filename
> done

Follow the prompt!

Same symbols, different meanings: > and $:

  • shell printing: typing expecting
  • you typing, shell redirection or variable
$ for x in basilisk.dat unicorn.dat # name of variable
> do
>  head -n 3 $x
> done

This is a Bash for loop. It will not work as is on ZSH for instance (which is another shell).

$ for filename in *.dat
> do
>   echo $filename # explaining what is echo, importance of echo
>   head -100 $filename | tail -n 20
> done

Avoid whitespaces:

$ for filename in *.dat
> do
>   cp $filename original-$filename
> done

Exercise ideas:

  • Processing files
    1. Check she select the right files: the onles whose names end in 'A' or 'B', rather than 'Z'
    2. Check what happen if you prefix each input file's name with "stats" (for the output of goostats program)
    3. Apply bash goostats $input $output to each file
    4. Apply bash goostats after adding an echo to check on which file it is running

Shell scripts

Shell scripts are small programs.

$ cd ../molecules
$ nano
    head -n 15 octane.pdb | tail -n 5
$ bash
$ nano
    head -n 15 "$1" | tail -n 5
$ bash octane.pdb
$ bash pentane.pdb
$ nano
    head -n "$2" "$1" | tail -n "$3"
$ bash pentane.pdb 15 5
$ bash pentane.pdb 20 5
$ nano
    # Select lines from the middle of a file
    # Usage: bash filename end_line num_line
    head -n "$2" "$1" | tail -n "$3"
$ wc -l *.pdf | sort -n
$ nano
    # Sort filenames by their length
    # Usage: bash one_or_more_filenames
    wc -l "$@" | sort -n
$ bash *.pdb ../creatures/*.dat
$ history | tail -n 5 >

Finding things

grep stands for Global/Regular Expression/Print" --> find lines in files.

$ cd writing
$ cat haiku.txt
$ grep not haiku.txt
$ grep The haiku.txt # in larger words: Thesis
$ grep -w The haiku.txt
$ grep -w "is not" haiku.txt
$ grep -n "it" haiku.txt
$ grep -n -w "the" haiku.txt
$ grep -n -w -i "the" haiku.txt #case insensitive
$ grep -n -w -v "the" haiku.txt #inverted search
$ grep --help

find is for finding files:

$ find .
$ find . -type d
$ find . -type f
$ find . -name *.txt # run: find . -name haiku.txt
$ find . -name '*.txt'
$ wc -l $(find . -name '*.txt')
$ grep "FE" $(find .. -name '*.pdb')

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