Introduction to High Performance and High Throughput Computing


Introduction to the UNIX shell


  • Basic interaction with files and directories

    • command line execution
    • Unix directory and file basics
    • file properties
  • File manipulation

    • inspecting and editing files
    • searching and filtering
    • archiving and transfer over network
  • Scripting and workflows

    • scripting techniques for building workflows

Access to Legion


Connecting to a computer: 1970s

Connecting to a computer: working from home

Connecting to a computer: over the Internet

Communication unencrypted!

Connecting to a computer: over the Internet

Communication encrypted!

Access to Legion

  • To access one of the login nodes (remotely) from a Unix machine: ssh username@legion.rc.ucl.ac.uk

  • Log on using UCL username/password

  • On Windows (e.g. Desktop@UCL) you can use PuTTY.

Access to Legion

Access to Legion

Basic interaction with files and directories

Command Prompt

[ccaaxxx@login06 ~]$
  • Has the form [<username>@<host name> <present directory>]$
  • <user name> is the Unix user name (UCL user ID).
  • <host name> is the name of the computer that you are accessing.
  • <present directory> is the directory that the user is currently in.
  • Takes commands typed after the prompt and executed by pressing return.
  • Is usually controlled by a program called bash.

Command Line Execution

[ccaaxxx@login06 ~]$ ls
[ccaaxxx@login06 ~]$
  • ls lists the contents of the directory
  • ~ is an abbreviation for your "home directory"

Unix Directory Basics

[ccaaxxx@login06 ~]$ file ~
/home/ccaaxx: directory
[ccaaxxx@login06 ~]$
  • file identifies the file type
  • ~ = /home/ccaaxxx - your home directory

Unix Directory Basics

/home/ccaaxxx: directory

/home: directory

/: directory
  • /home/ccaaxxx - the full or absolute path to your home directory

Directory Navigation

[ccaaxxx@login06 ~]$ pwd

[ccaaxxx@login06 ~]$ cd /
[ccaaxxx@login06 /]$
  • cd - change directory
  • pwd - full path to present working directory
  • / - the "root" directory

Filesystem structure

Absolute path


Relative path


Hidden files and directories

[ccaaxxx@login06 ~]$ ls -a
.  ..  .bash_history  .bash_logout
.bash_profile  .bashrc  .cshrc  .emacs
Scratch  .ssh
[ccaaxxx@login06 ~]$ 
  • Files starting with "." are hidden

  • . - Present working directory (in this case ~)

  • .. - Directory above (in this case /home)

Everything is a file

[ccaaxxx@login06 ~]$ file .* *
.:             directory
..:            directory
.bash_history: ASCII text
.bash_logout:  ASCII English text
.bash_profile: ASCII English text
.bashrc:       Bourne-Again shell script text executable
.cshrc:        C shell script text executable
.emacs:        Lisp/Scheme program text (Emacs editor)
.ssh:          directory (for secure shell)
Scratch:       symbolic link to `/scratch/scratch/ccaaxxx'
  • * - All visible files
  • .* - All invisible files

Everything is a file

Just to make the point:

[ccaaxxx@login06 ~]$ which ls

[ccaaxxx@login06 ~]$ file /bin/ls
/bin/ls: ELF 64-bit LSB executable, AMD x86-64, version 1 (SYSV), for GNU/Linux 2.4.0,
dynamically linked (uses shared libs), stripped
  • /bin/ls is an executable file (!)
  • which shows the path to the argument, in this case ls

Everything is a file

Just to make the point:

[ccaaxxx@login06 ~]$ file /dev/sda1
/dev/sda1: block special (8/1)
  • Things in /dev are devices
  • /dev/sda1 is the first hard drive


Explore the / directory using ls, cd, ., .., pwd

  • If you feel lost, just execute cd with no arguments (or cd ~) and you will be returned to your home directory.

  • cd - - will return you to the previous directory you were in.

Directory creation

[ccaaxxx@login06 ~]$ mkdir a_directory

[ccaaxxx@login06 ~]$ ls
a_directory Scratch
  • mkdir - create a directory

Directory creation

[ccaaxxx@login06 ~]$ mkdir -p a_directory/inside/the_other
[ccaaxxx@login06 ~]$ cd a_directory/inside/the_other
[ccaaxxx@login06 the_other]$ pwd
  • mkdir -p - creates consecutive sub-directories

Touching a file

[ccaaxxx@login06 ~]$ touch a_file
[ccaaxxx@login06 ~]$ ls
a_directory a_file Scratch
  • touch - create or update the date of a file
  • This is not usually how files are created

Copying files

[ccaaxxx@login06 ~]$ cp a_file copy_of_a_file
[ccaaxxx@login06 ~]$ ls
a_directory a_file copy_of_a_file Scratch
  • cp - copy a file (or a directory tree)


Create and copy a directory tree. Use the command man cp for more information

File manipulation

Editing files

[ccaaxxx@login06 ~]$ nano a_file

  • nano - simple file editor


  • nano - simple file editor
  • Commands are along the botton of editor screen
  • ^ - shorthand for control key


There are many other (better but more complex) text file editors on the system such as vim, emacs and nedit.

Use the one you feel most comfortable with (if you don't know any, use nano (or nedit if you have X-Windows)).

Moving/Renaming files and directories

[ccaaxxx@login06 ~]$ mv a_file control.in
[ccaaxxx@login06 ~]$ ls
a_directory control.in Scratch

[ccaaxxx@login06 ~]$ mv control.in a_directory

[ccaaxxx@login06 ~]$ ls a_directory
control.in inside
  • mv - rename or move a file/directory

Deleting files and directories

[ccaaxxx@login06 ~]$ rm a_directory/control.in

[ccaaxxx@login06 ~]$ rm a_directory
rm: cannot remove `a_directory/': Is a directory

[ccaaxxx@login06 ~]$ rm -r a_directory
[ccaaxxx@login06 ~]$
  • rm - remove a file
  • rm -r - remove a directory and all its contents
  • remove operations are irreversible!
  • The -i argument is highly recommended


In ~/Scratch...

  • Create the following directory tree:
  • Create the file input.txt with some random lines in it.

  • Move the file to input_data and rename it in the same command to control01.txt

  • Create the directory tree in one line only: work/experiment/results/report

  • Delete all directory trees created in one single command without explicit reference to any of the directory and file names except work.

File properties

[ccaaxxx@login06 ~]$ ls -l
total 8
drwxr-xr-x 2 ccaaxxx rcops 4096 2009-12-08 07:31 a_directory
-rw-r--r-- 1 ccaaxxx rcops    0 2009-12-08 07:31 a_file
drwxr-xr-x 2 ccaaxxx rcops 4096 2009-12-08 06:50 Scratch

  • ls -l - list file properties (details)

File properties

drwxr-xr-x 2 ccaaxxx rcops 4096 2009-12-08 07:31 a_directory

        drwxr-xr-x      - File type and permissions
        2               - Number of links to the file
        ccaaxxx         - User name of file owner
        rcops           - Group to which the file belongs
        4096            - size of file in bytes
        2009-12-08      - last change data
        07:31           - last change time
        a_directory     - file name
  • ls -l - list file properties (details)

File permissions

  • There are three user security classifications that apply to the ownership of a file:

    • user: the individual user that has ownership of the file
    • group: a group of users to which that user belongs
    • others: all other users (not owner or in that group)
  • Each of these has three file access classifications:

    • read: permission to read the file
    • write: permission to write the file
    • execute: permission to execute (run) the file

File permissions

drwxr-xr-x has four fields:

code meaning
d - the file is a directory
rwx - the user has read, write and execute access
r-x - the group has read and execute access
r-x - others have read and execute access
Permissions read write execute
rwx yes yes yes
rw- yes yes no
r-- yes no no
r-x yes no yes
--- no no no

File permissions

[ccaaxxx@login06 ~]$ ls -l 
drwxr-xr-x 2 ccaaxxx rcops 4096 2009-12-08 07:31 a_directory

[ccaaxxx@login06 ~]$ chmod go-rx a_directory

[ccaaxxx@login06 ~]$ ls -l 
drwx------ 2 ccaaxxx rcops 4096 2009-12-08 07:31 a_directory
  • chmod - change the permissions of a file

File permissions

How did the permissions definition go-rx work?

chmod [ugoa][+/-][rwx] file
  • ugoa - user, group, other, all

  • +/- - add/remove

  • rwx - read, write, execute

If you don't specify u,g,o or a, default is ALL (so chmod +x makes file executable for everyone).

  • chmod - change the permissions of a file


  • Change the permission of a full directory tree with one single chmod command (look in the man pages for more information).

  • When typing the command ls /sh, press the tab key after typing /sh. What happens?

Inspecting files

[ccaaxxx@login06 ~]$ cd /shared/ucl/apps/examples/openmp_pi_dir
[ccaaxxx@login06 examples]$ less openmp_pi.f90
  • less - visualise a text file:
    • use arrow keys
    • page up/page down
    • search by typing "/"
    • quit by typing "q"

Other file inspection tools

command use
head - visualise the first 10 lines of a file
tail - visualise the last 10 lines of a file
cat - concatenate files provided as input and dump the result to stdout
sdiff - visualise and compare two files side-by-side

(Use e.g. man head to see more information on the head command)

Filtering directory listings

[cccaaxxx@login06 ~]$ ls /shared/ucl/apps/BLAST/install | grep blast
  • grep - prints lines containing a string. Also searches for strings in text files.

Searching for strings in files

[cccaaxxx@login06 ~]$ grep SWAP /shared/ucl/apps/BLAST/install/test.faa
  • grep - prints lines containing a string. Also searches for strings in text files.


  • Created with "ln"

  • Two types:

    • "Hard" - indistinguishable from files
    • "Soft" (or "Symbolic") - like a shortcut

Hard links

  • Inode table keeps track of hard links

  • Deleting a file = "unlinking" it

  • Can only be used inside a single file system

Soft links

  • Shortcut, e.g.
[cccaaxxx@login06 ~]$ ln -s ~/some_project/2012/part531 ~/current_project
[cccaaxxx@login06 ~]$ ls -l ~ 
lrwxr-xr-x  1 ccaaxxx  staff  11 10 Oct 17:56
               current_project -> /home/ccaaxxx/some_project_2012/part531

Soft links

  • Can use relative or absolute paths!

  • Create using absolute paths to make sure they go where you want

  • "Scratch" in your home directory is a soft link

    • Makes it easier to access your scratch area

Archiving and compression

[ccaaxxx@login06 Scratch]$ tar -zcvf work.tgz work

[ccaaxxx@login06 Scratch]$ ls
work.tgz work
  • tar -zcvf - archives and compresses directory trees and files
    • c - create archive
    • z - compress
    • v - verbose
    • f - in the following file

Extracting files from a compressed archive

[ccaaxxx@login06 Scratch]$ ls
[ccaaxxx@login06 Scratch]$ tar -zxvf work.tgz 
[ccaaxxx@login06 Scratch]$ ls
work.tgz work
  • tar -zxvf - extracts and uncompresses directory trees and files
    • x - extract archive
    • z - uncompress
    • v - verbose
    • f - from the following file

Transferring files across a network

From Legion:

[ccaaxxx@login06 Scratch]$ scp work.tgz ccaaxxx@socrates.ucl.ac.uk:~/
work.tgz         100%  340     0.3KB/s   00:00

To Legion:

[ccaaxxx@login06 Scratch]$ scp ccaaxxx@socrates.ucl.ac.uk:~/work.tgz .
work.tgz         100%  340     0.3KB/s   00:00

Note: remote machine (in this case Socrates) must be running SSH server. So, from your destop:

[you@desktop ~]$ scp work.tgz ccaaxxx@login05.external.legion.ucl.ac.uk:Scratch
[you@desktop ~]$ scp ccaaxxx@login05.external.legion.ucl.ac.uk:Scratch/work.tgz .
  • scp - securely copy files across a network

Scripting and workflows

Output redirection and piping

  • Two very important concepts:

    • Standard Output (stdout) - default destination of a program's output. It is generally the terminal screen.
    • Standard Input (stdin) - default source of a program's input. It is generally the command line.

Output redirection and piping

[ccaaxxx@login06 ~]$ ls /home > ls_home.txt
  • > - redirect stdout to a file
[ccaaxxx@login06 ~]$ ls /home | less
  • | "pipe" stdout to stdin of a command

Output redirection and piping

Output redirection and piping

Output redirection and piping

ls -l

Output redirection and piping

ls -l > list

Output redirection and piping

ls -l | grep name > list

Output redirection and piping

You can chain any number of programs together to achieve your goal:

This allows you to build up fairly complex workflows within one command-line.

Shell scripting

# This is a very simple hello world script.
echo "Hello, world!"
  • #! - tells the shell that it should use /bin/bash as the interpreter
  • # - a comment (ignored by /bin/bash)
  • echo - a command that prints arguments to stdout


  • Create a "Hello world"-like script using command line tools and execute it.

  • Copy and alter your script to redirect output to a file using >.

  • Alter your script to use >> instead of >. What effect does this have on its behaviour?

Exercise Solutions:

. not present in the environment variable $PATH:

[ccaaxxx@login06 ~]$ hello_world.sh
bash: hello_world.sh: command not found

File permissions not set to executable by user:

[ccaaxxx@login06 ~]$ ./hello_world.sh
bash: ./hello_world.sh: Permission denied

using ./ and chmod u+x:

[ccaaxxx@login06 ~]$ ./hello_world.sh
Hello World!


  • Use seq 1 75 > numbers.txt to generate a file containing a list of numbers. Use the head, tail, and less commands to look at it, then use grep to search it for a number.

Environment Variables

[ccaaxxx@login06 ~]$ variable='string'
[ccaaxxx@login06 ~]$ other_variable=$variable
  • Defined and used within a shell script only.

  • No spaces around the assignment symbol "=".

  • $ means "the value assigned to the following variable".

Global Environment Variables

  • defined within a shell and also in scripts called from the shell
[ccaaxxx@login06 ~]$ export ENV_VARIABLE='value'


  • Using two nested scripts, show that the value of an exported variable in the environment where you launch the scripts, propagates all the way down to the second script.

Updated process diagram

Special environment variables

What is the output of this command?

[ccaaxxx@login06 ~]$ echo $PATH

Note the structure: <path1>:<path2>:<path3>

PATH is an environmental variable which Bash uses to search for commands typed on the command line without a full path.

Exercise: Use the command env to discover more.

Variables in shell scripts

# This is a very simple hello world script
message='Hello World!'
echo $message
echo ${message}
symbol purpose
#! tells the shell that this is the interpreter (/bin/bash)
# followed by a blank character is a comment line
message variable to which the string 'Hello World!' is assigned
echo prints to screen the contents of the variable "$message"

Command line arguments

Variables can also be defined through the command line

[ccaaxxx@login06 ~]$ ./script.sh arg another

Within the script: $1 contains "arg" $2 contains "another"

Exercise: Using $1 and $2, write a script that print both variables to the screen.

Storing output of commands in variables

Run commands inside $( ) and assign the wrapped command to a variable

[ccaaxxx@login06 ~]$ ls 
a_directory  a_file  Scratch 
[ccaaxxx@login06 ~]$ dir_contents=$( ls ) 
[ccaaxxx@login06 ~]$ echo $dir_contents 
a_directory a_file Scratch

Basic arithmetic

[ccaaxxx@login06 ~]$ two=2 
[ccaaxxx@login06 ~]$ result=$(( $two + 2 )) 
[ccaaxxx@login06 ~]$ echo $result  
[ccaaxxx@login06 ~]$
  • Integer arithmetic can be done inside $(( ))
    • + addition
    • - subtraction
    • / integer division
    • * multiplication

The for loop

for i in first second third
  echo $i iteration
echo finished!
  1. Interpreter definition line.
  2. Definition of the loop in which the variable i will take the values "first", "second" and "third" in that order.
  3. Start the loop iteration definition.
  4. Print the string contained by i and the string iteration, separated by a space.
  5. End the loop definition.
  6. Print some useful information to the screen.

The for loop using an iterator

for (( i=1 ; i<=5 ; i++ )) 
    echo iteration$i
echo finished! 

(Note how it is possible to create number labels)


Write a script that creates five directories named calculation_?, where ? is a number.

Formatted sequences based on numbers

[ccaaxxx@login06 ~]$ seq 1 5
  • seq - generates sequences based on numbers

Formatted sequences based on numbers

[ccaaxxx@login06 ~]$ seq 1 2 9
  • seq - generates sequences based on numbers

Formatted sequences based on numbers

[ccaaxxx@login06 ~]$ seq -f %03g 1 2 9
  • g - the number of significant digits is 3
  • 0 - in front of 3 indicates that the number is padded with zeros if smaller than 100.
  • % - start the format definition for the number (place the number)

The for loop revisited

for i in $(seq -f %03g 1 4) 
   echo creating directory calculation$i
   mkdir calculation$i 
echo finished!

You can now create directory trees and files automatically!

Generating scripts with scripts

cat <<EOF > child_script.sh 
echo The child script says Hi! 
  • <<EOF - concatenates the text until the string "EOF", redirecting it to cat
  • > child_script.sh - redirects the output of cat to child_script.sh.
  • Indenting the top line will break the #!.


  • write a parent_script.sh that creates and executes the child_script.sh

  • write a parent_script.sh that creates and executes 10 different child_script.sh that print out their individual number

Process control

  • A process is:

    • foreground when it is interacting with the user via an interface (usually the shell).
    • background if it is running without interacting with the user.
    • suspended if it is neither interacting nor running.
  • To run a process in the background: add the symbol & at the end of the command line.

  • To send a foreground process to the backgound: press Ctrl+z and then execute the command bg

  • To bring a background process to the foreground: execute the command fg

Which processes are running?

Use the commands top and ps

[ccaaxxx@login06 ~]$ ps aux
Field contents
USER - the user name
PID - the process ID
%CPU - the percentage of CPU time being used
%MEM - the percentage of Memory being used
VSZ - the size of Virtual memory
RSS - the size of real memory
TTY - the virtual teletype terminal this proces is attached to
STAT - the state of the process (see "man ps")
START - the time when the process was started
TIME - the time the process has been running
COMMAND - the command that was called


[ccaaxxx@login06 ~]$ ps xjf

Why have we been showing you this?

  • Tomorrow, we will be talking about how to submit your workloads to Legion

  • The scripts you submit are bash shell scripts with some special comments read by the scheduler at the top.

#!/bin/bash -l
#$ -S /bin/bash
#$ -l h_rt=0:30:0
#$ -l mem=1G
#$ -N Analysis
#$ -P <your_project_id> 
#$ -wd /home/<your_UCL_id>/Scratch/output 

echo "Copying input files."
cp -R ~/inputdata .
echo "Analysing data."
~/bin/analyse -i inputdata/exp.in > exp.out.$JOB_ID
echo "Copying data back."
cp exp.out.$JOB_ID ~/Scratch/output
echo "Done."

More information

  • Many topics not covered here:

    • if statements
    • case switches
    • defining functions
    • and many, many more...
  • Google and the man pages are your friends!