CMPSC 311 - Introduction to Systems Programming Page 1

Institute for Networking and Security ResearchDepartment of Computer Science and EngineeringPennsylvania State University, University Park, PA

Systems and Internet Infrastructure Security

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Shell Programming(Part 1: Syntax and Scripting)

Devin J. Pohly<djpohly@cse.psu.edu>

Page 2CMPSC 311 - Introduction to Systems Programming

Shell programming

● aka “shell scripting,” “Bash scripting”

● What is it?● Series of commands● Programming with

programs

● What for?● Automating● System administration● Prototyping

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A sample script: shello

● First line: interpreter● The #! is important!

● Comment: # to EOL● Make it executable

● chmod +x shello● Not by file extension● Typical: .sh or none

● Run it● ./shello

#! /bin/bash

# Greetings!echo Shello world

# Use a variableecho Shello "$USER"

# Set a variablegreetz=Shellutationsecho "$greetz world"

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Shell variables

● Setting/unsetting● var=value● No spaces!● unset var

● Using the value● $var

● Untyped by default● Behave like strings● (See declare builtin)

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Special variables

● Getting info● $USER● $HOME

– (aside: ~ is similar)

● $PWD

● Changing behavior● $PATH

– Colon-separated● $PS1

● And more...

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Exercise

● Make a directory ~/bin● Move shello script

there● Prepend the directory

to your $PATH● PATH=~/bin:$PATH

● Change to home dir● Run by typing shello

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Shell initialization

● Set custom variables● ~/.bashrc

● Just a script● Runs at shell startup

● Anything you want

● (Note: set and export LD_LIBRARY_PATH=.)

Page 8CMPSC 311 - Introduction to Systems Programming

Fun with prompts

● Main prompt: $PS1● Special values

● \u: username● \h: hostname● \w: working dir● \e: escape (for colors)● many others

● Can set in .bashrc

Very detailed treatment: http://www.tldp.org/HOWTO/Bash-Prompt-HOWTO/

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Special characters

● echo Penn State is #1● echo Micro$oft Windows● echo Steins;Gate● What happened?

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Special characters

● echo Penn State is #1● echo Micro$oft Windows● echo Steins;Gate● What happened?● Many special characters

● Whitespace● #$*&^?!~'`"\{}[]<>()|;

● What if we want these characters?

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Quoting

● Removes specialness● Hard quotes: '…'

● Quote everything except closing '

● Soft quotes: "…"● Allow variables (and

some other things)● Good practice: "$var"

● Backslash (escaping)● Quotes next character

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Arguments

● In C: argc and argv[]● Split at whitespace

● How to override this?

● Arguments to a script● ./script foo bar● $# is the same as argc● "$@": all args● "$1" to "$9": individual

● "$_": last arg, prev cmd

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Debug mode

● Shows each command● Variables expanded● Arguments quoted

● Run with bash -x ● Temporary● bash -x shello

Page 14CMPSC 311 - Introduction to Systems Programming

Exercises

● Make a script that:● Prints its first argument

doubled

./script1 foofoofoo

● Prints its first four args in brackets, one per line

./script2 "foo bar" baz[foo bar][baz][][]