Introduction to Unix

Advanced Computing Center for Research and Education

http://www.accre.vanderbilt.edu

Why Unix?What comes to mind when you hear the word “Unix”?

Command line!

Unix is a programmer’s paradise Takes advantage of the keyboard and short commands (typically 2-4 letters in length) to efficiently manage files, folders, and so on Emphasizes performance Large user community (including most supercomputing centers) NOT designed to be a math editor

What is Unix?✦Unix is an operating system (un)like Windows ✦Originally created in the late 1960’s at AT&T Bell Labs ✦Designed to be a programmer’s OS ✦Turned out to be a portable, multi-tasking, multi-user OS (we take those things for granted now, but they were revolutionary at the time!)

✦There are many different “flavors” of Unix...

Different “flavors” of Unix have different niches

✦ IBM’s AIX and HP’s HP-UX are heavily used in business ✦Oracle’s (formerly Sun’s) Solaris once powered 70% of the web / e-mail servers on the Internet

✦Apple’s MacOS X is a user-friendly desktop ✦Linux is very high performance and free, which makes it ideal for HPC clusters

Unix ArchitectureShell

Kernel (OS)

Physical

Devices

✦Only the Kernel (the operating system) can manipulate physical devices (disks, the network, printers, etc.)

✦Users interact with the kernel via a shell (command interpreter)

✦Two primary shells: 1) bash, 2) tcsh

Unix Commands✦The format of Unix commands is command [ options ] [ arguments ]

✦ id is a command ✦ -f is an option to the ps command ✦example1 is an argument to the ls command (and -l is an option)

The most important command of all

✦The man command displays manual pages

✦Example at left is the output of man ls

✦Displays synopsis ✦Gives description of each option and, if applicable, argument to the command

Command History / Editing

✦The shell maintains a history of the commands you have previously executed (1000 commands by default)

✦Up and down arrow keys scroll thru your command history

✦Left and right arrow keys scroll thru the command ✦Edits can be made by adding, deleting, or replacing parts of the command

✦Pressing the enter / return key executes the command

Hierarchical Filesystem/

bin usr home scratchetc tmp

chmod

cp

date

grep

mv

rm

vi

nikkiannie codybin lib

bin docs src

libc.so

libgpfs.so

libjpeg.so

libstdc++.so

diff

find

gcc

id

make

perl

ssh

prog1.c

prog2.f77

prog3.cpp

myprog.sh

dothis.pl

dothat.py

Working With Directories✦pwd prints your present working directory

✦ cd changes directories (with no arguments always changes to your home directory)

✦mkdir makes a directory ✦ rmdir removes an empty directory; rm -r removes a non-empty directory

✦ ls lists directories (and files)

Working With Files✦file tells you what type of file a file is

✦more displays the contents of a file

✦ cp copies files ✦mv renames (moves) files

✦ rm removes files ✦The -i option makes cp, mv, and rm “interactive”

Other Handy Commandsecho

Used for displaying messages or variables (think of as a “print” command) Examples:

echo “Hello world!” echo $myvar

cat Used for displaying the entire contents of a file Example:

cat myfile.txt “more” and “less”

Used for scrolling through the contents of a file Examples:

more myfile.txt less myfile.txt

Command Substitution✦Any command enclosed in grave accents (not single quotes) is executed first and then its output is substituted in on the command line

✦Command substitution can be used with other commands (as the first example shows) or to assign a value to a variable (as the second example shows)

Input / Output Redirection

✦Any shell has 3 file handles open by default: ✦stdin - standard input, defaults to keyboard, file descriptor 0

✦stdout - standard output, defaults to screen, file descriptor 1

✦stderr - standard error, defaults to screen, file descriptor 2

Input / Output Redirection

✦ Input redirection - e-mail yourself a file: mailx Nikki.Husky@vanderbilt.edu < example1

✦Output redirection - myprogram > output.log ✦Error redirection - myprogram 2> error.log ✦Output and error redirection to different files - myprogram > output.log 2> error.log

✦Output and error redirection to the same file - myprogram > combined.log 2>&1

Editing Files✦There are three text editors available... ✦emacs - popular with programmers ✦ nano (pico) - basic, easy to use editor ✦ vi - the original Unix text editor; has steeper learning curve, but is the fastest of the three

✦Which should you use? ✦Whichever one you like!

Shell ScriptsSometimes rather than manually running a series of commands from the command line, it makes more sense to execute the commands from a script

For example, if you need to run the series of commands more than once

#!/bin/bash mkdir mydir cd mydir pwd echo “Hello world!” > myfile.txt ls -lh cd .. rm -r mydir

Create Your Own Bash ScriptCreate a bash script that creates 20 directories with names run1, run2, run3,…run20 Within each directory, create a file named foo.txt that contains a single line that reads “Hello world from runi”, where i is the directory number Hint:

Start with this:

#!/bin/bash for i in `seq 1 20` do echo $i done