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An Introduction to Shell Scripting
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Transcript of An Introduction to Shell Scripting
Whatistheshell?
• AcommandlineuserinterfaceforUnix-like
operatingsystems.
• Interactiveandscriptingmodes
WhatistheBashShell?
• BourneAgainSHell,replacingtheolderBourneshellin1989
• DefaultshellonmostLinuxsystemsand
MacOS
Features
• Usercustomisable
• Wildcardmatching
• Re-direction
• Commandsubstitution
• Controlstructures
Whentousetheshell
• Asawrapperforaworkflow
• Whenlaunchingotherprocesses
• Whendoinglotsoffilesystemaccess
• Whenlowlevelaccesstohardwareisrequired
Whennottousetheshell
Shellscriptingisofmuchlessusewhenanyof
thefollowingarerequired
• Complexcalculations
• Agraphicaluserinterface
• Anykindofdebuggingbeyondverybasic
Startingup
• Oftenopenedviathegraphicaldesktop
• Startupfilesarereadtoprovideusercustomisations,eg.bash_profile,.bashrc
Someusefulcommandspaulbrosmacbook:varpaulbrown$cd$HOMEpaulbrosmacbook:~paulbrown$pwd/Users/paulbrownpaulbrosmacbook:~paulbrown$ls-ltotal104240drwxr-xr-x2paulbrownstaff6419Jun2017Anacondadrwxr-xr-x29paulbrownstaff92830Nov2017Androiddrwx------+93paulbrownstaff29761Nov12:34Documents-rw-r--r--1paulbrownstaff822Feb2018README.md-rw-r--r--1paulbrownstaff014Jul2015mcmc.csvlrwxr-xr-x1paulbrownstaff2520Sep2016meme->/Users/paulbrown/meme4.11paulbrosmacbook:~paulbrown$chmod755Documentspaulbrosmacbook:~paulbrown$cp-rAndroidAndroid.backuppaulbrosmacbook:~paulbrown$rm–rAndroid
EnvironmentVariablesNero:~paulbrown$echo$PATH/bin:/usr/bin:/usr/sbin:/sbin:/usr/local/bin:~/binNero:~paulbrown$meme-bash:meme:commandnotfoundNero:~paulbrown$exportPATH=$PATH:/usr/local/meme/binNero:~paulbrown$echo$PATH/bin:/usr/bin:/usr/sbin:/sbin:/usr/local/bin:~/bin:/usr/local/meme/binNero:~paulbrown$memeUSAGE:
meme <dataset>[optionalarguments]
Morevariables• Therearenovariabletypes
• VARNAMEisareference
• $VARNAMEisthevalueheldthereNero:~paulbrown$echoPATHPATH• Use${…}toaccesssubstrings
paul-browns-macbook:~paulbrown$STR="Helloworld"paul-browns-macbook:~paulbrown$echo${STR:6}Worldpaul-browns-macbook:~paulbrown$echo${STR/w/W}HelloWorld
Usingquotationmarks
• Importanttoknowthedifferencebetweensingleanddoublequotes
• Expressionsareevaluatedinside“…”,butnotinside‘…’paul-browns-macbook:~paulbrown$NAME="Paul"paul-browns-macbook:~paulbrown$echo"Hello$NAME"HelloPAULpaul-browns-macbook:~paulbrown$echo'Hello$NAME'Hello$NAME
Arrays
• Declaringanarrayfruits=('Apple''Banana''Orange’)• Accessingelementsecho${fruits[0]}
Readingfiles
cat,head,tail,morenero:~paulbrown$grep“paulbrown”/var/log/secure……Nov423:10:33nerosshd[44146]:pam_unix(sshd:session):sessionopenedforuserpaulbrownby(uid=0)
Writingfiles
• Anumberofinteractivetexteditors,egvi,
nano
• Alsousere-direction>,>>
• echo“somecontent”>>script.sh
Redirection
• Inputtoandoutputfromcommandcanbere-
directedawayfromstdinandstdout
• Re-directoutputtofilels–l>dircontent.txt
Re-directinputfromfile
sort–k5–n<dircontent.txt
Redirection
Pipesareusedtochaincommandstogetherso
theoutputofonebecomestheinputofthenext
tail–n1000logfile.log|sort|more
ls–l|sort–k5–n
Commandsubstitution
• Thisallowstheoutputofacommandtobecaptured
andusedpipedbacktobeusedasanargumentfor
somethingelse,ortobecapturedinavariable
• Preferredwayistouse$(…)
rm–f$(find.–name“*.txt”)
ArithmeticexpansionUsecommandsubstitutionpaul-browns-macbook:~paulbrown$echo2+32+3paul-browns-macbook:~paulbrown$echo$((2+3))5paul-browns-macbook:~paulbrown$echo$(2+3)-bash:2+3:commandnotfoundpaul-browns-macbook:~paulbrown$leta=$((2+3))paul-browns-macbook:~paulbrown$echo$a5Bashhandlesonlyintegertypes.Usebctoperformcalculationswithfloatingpointtypes
paul-browns-macbook:~paulbrown$echo'scale=3;4/3'|bc1.333
RemoteShells
• rsh(remoteshell).Donotuse,insecure
• ssh(secureshell,port22)
paul-browns-macbook:~paulbrown$sshnero.wsbc.warwick.ac.ukpaulbrown@nero.wsbc.warwick.ac.uk'spassword:Lastlogin:MonNov423:10:342019from95.149.133.253-sh-4.1$hostnamenero.wsbc.warwick.ac.uk• Alsosftpandscpscp/local/[email protected]:/home/paulbrown
Shellscripting
• Conventionally,fileshave.shextension
• Remembertosetexecutepermission
• Scriptbeginswith
#!/bin/bash
Inputarguments
• Referredtoas$1,$2etc..
• $#isthenumberofinputs
• Sameappliestofunctions
• Usereadtorequestuserinput
Conditionals
• Surroundanexpressionwith[[…]]
• Stringoperators:-z,-n,==,!=,<,>,=~
• Numericaloperators:-eq,-ne,-lt,-le,-gt,-ge
• Fileoperators:-e,-f,-d,-r,-w,-x
Conditionals
#!/bin/bashif[[$#-lt3]];then
echo“Notenoughinputarguments”exit0
elif[[$#-gt5]];thenecho“Toomanyinputarguments”exit0
elseecho“OK”
fi
Conditionals• Canbechainedtogetherusinglogicaloperators&&,||#!/bin/bashif[[$#-lt3]]||[[$#-gt5]];then
echo“Wrongnumberofinputarguments”exit0
elseecho“OK”
fi• Theseoperatorsallowconditionalexecutionmkdirnewdir||echo“Cannotcreatedirectory”mkdirnewdir&&touchnewdir/newfile
WhileLoops
whilereadline;dofields=(${line})#expandtoarray…
done<infilebreakandcontinuecanbeusedwithintheloopbody
ForloopsAforloopiteratesaseriesofwordsinastringforiin$(ls);do
echo$idoneAC-styleforloopcanbecreatedusingarithmeticexpressionsfor((i=0;i<100;i++));do
echo$1doneRangeexpressionforiin{1..10};do
echo$1done
Functions
myFunc(){
locallocalVar=“Hello”$1;
echolocalVar;
}
myFunc“Paul”
Returnvaluescanbecapturedbycommandsubstitution
Gettinghelp
• manpagesformostcommands
• Hugeamountononlineresources,egagood
cheatsheetathttps://devhints.io/bash
