CISC3130, Spring 2011Dr. Zhang

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Bash Programming Review

Outline

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Coding standard: how to get good grades in lab assignment

Review of standard input/output/errorHow to redirect them ? Pipeline

Review of bash scripting

Standard input/output/error

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By default, link to keyboard and terminal window respectively

Can be redirected to files Can be redirected to pipeline

input can be redirected to reading end of a pipeoutput and error can be redirected to writing end of a

pipeWhen a bash script’s input/output/error is

redirected: E.g., headtail 3 10 .bash_profile > outputls –l | headtail 10 24 | wc –l input/output/error for every command in the script

are redirected !

Save standard input if necessary

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cat > stdinput ## save standard input to a file

## so that we can use as many times as we want

wc –l stdinput grep PHONE stdinput

Redirection can be applied to loop

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for i in `ls`do echo $i cat $idone > all_files

Similar for <, |

Outline

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Coding standard: how to get good grades in lab assignment

Review of standard input/output/errorHow to redirect them ? Pipeline

Review of bash scripting

Bash scripting: general hints

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Use echo command to trace (like cout, printf in C/C++)

Sometimes there are alternatives ways to do things, choose one and remember it: $(( …)), and $[ … ] [[ ]] for test

Be careful about typo, shell wont complain variable not declared/assigned … The price of freedom

A walk-through of basic bash scripting

Bash Scripting

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VariablesEnvironment variable: affect behavior of shellUser defined variable: default type is string, can

declare it to be other typePositional variables: used to pass command line

arguments to bash scriptVariable assignment:

x=10 ## assign value 10 to variable x, no space around =

x=$x+1 ## add 1 to x’s value and assign to xPATH=$PATH:.:~/bin

To refer to a variable’s value, precede variable name with $

A script that display positional variable

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echo All arguments: $*echo Number of arguments: $#echo Script name: $0echo argument 1: $1echo argument 2: $2

for arg in $*do echo argument $argdone

demo

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[zhang@storm CISC3130]$ ./test 10 20 tmpfile

All arguments: 10 20 tmpfileNumber of arguments: 3Script name: ./testargument 1: 10argument 2: 20argument 10argument 20argument tmpfile

arithmetic operation

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As variable’s default type is string, to perform arithmetic operation, use the following syntax $[$x+1] or $(($x+1))

For simpler syntax: declare variable to be numerical

declare –i x x=$x*10+2Above are for integer arithmetic operations

only ..

Command bc

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An arbitrary precision calculator $ bc3.14159*10^2 314.15900130^216900sqrt(1000)31scale=4sqrt(1000)31.6277quit

An interactive calculator: * user input shown in normal font, * result shown in italics font

Internal variable scale: * control the number of decimal points after decimalpoint

bc in command line/script

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To evaluate an expression, simply send it using pipe to bcecho "56.8 + 77.7" | bc

Write a script that read a Fahrenheit degree from standard input, convert it to Celsius degree (up to 2 digits after decimal point):

C=(F-32)*5/9Base conversion, from base 10 (decimal) to

base 16 (hexadecimal)echo "obase=16; ibase=10; 56" | bc

Test/Conditions

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Any command or script, if it return 0, then test is successful

if rm tmp.txt then echo file tmp.txt has been deleted else echo fail to remove file tmp.txtFiUse ! to negate

Test Condition

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Numerical comparisons-lt, -ne, -ge, …

String comparisionPattern matching: using double brackets

To test if first argument is “–” followed by a number:

if [[ "$1" == -[0-9]* ]]then

….

AND/OR list construct

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Statement1 && statement2 && statement3 && …Each statement is executed; if it returns true, next

statement is executed…. Until finding first statement that returns false, or all statements return true

Note: the statement can be any command, such as echo (which always return true)

Statement1 || statement2 || statement3 || …Each statement is executed; if it returns false, next

statement is executed…. Until finding first statement that returns true (the construct then return true), or all statements return false (the construct then returns false).

Check password: maximum 5 tries

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tries=0; limit=5while [ "$input" != "secret“ ] && [ $tries –lt $limit ]do echo "Enter your password" read input; tries=$(($tries+1))doneif [ $input= “secret” ]then

echo "welcome!“else

echo “retry limit (5) reached”fi

Infinite loop

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while [ 1 ]do echo -n "Enter your password" read input if [ $input = "secret" ] then break else echo -n "Try again... " fidone

Useful command : select loop

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#!/bin/bash

OPTIONS="Hello Quit“

select opt in $OPTIONS; do

if [ "$opt" = "Quit" ]

then

echo done

exit

elif [ "$opt" = "Hello" ]

then

echo Hello World

else

echo bad option

fi

done Recall pick command ?

Next:

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More advanced bash scripting ArrayFunctionHere document

Array

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Bash provides one-dimensional array variables. There is no maximum limit on the size of an

array, nor any requirement that members be indexed or assigned contiguously.

Arrays are indexed using integers and are zero-based.

Assign values to array: array=( one two three )files=( "/etc/passwd" "/etc/group" "/etc/hosts" )limits=( 10, 20, 26, 39, 48)

To Iterate Through Array Values

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for i in "${arrayName[@]}“ do # do whatever on $i done -----------------------------------------------------------------------#!/bin/bash # declare an array called array and define 3 vales array=( one two three ) for i in "${array[@]}" do echo $i done

Functions

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One can define functions to increase modularity and readability of shell scripts

More efficient than breaking large scripts into many smaller ones… Why ? foo() {

Echo “in function foo”

}echo “start script…”fooecho “end script …”

About functions

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Need to be defined first, and then can be called

Parameter passing#! /bin/bashcalsum(){ echo `expr $1 + $2`}x=1;y=2;calsum $x $y

About functions

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Result returningThrough setting a variableUse return commandUse echo command#! /bin/bashcalsum(){ echo `expr $1 + $2`}x=1;y=2;calsum $x $yz=`calsum $x $y`

calsum(){ z=`expr $1 + $2`}x=1;y=2;calsum $x $yecho z=$z

About functions

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Local variable: its scope is within the function#! /bin/bashcalsumsqr(){

local sum=`expr $1 + $2`; echo `expr $sum * $sum`}x=1;y=2;calsum $x $yz=`calsum $x $y`

Here document

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A special way to pass input to a command: here document, i.e., from the shell script itself#!/bin/bashcat <<!FUNKY!HelloThis is a hereDocument!FUNKY!

Here document starts with <<,followed by a special string which is repeated at the end of the document.

Note: the special string should be chosen to be rare one.

Here document:2

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Benefits: store codes and data together, easier to maintain

Example: 411 scriptgrep “$*” << EndDial-a-joke 212-976-3838Dial-a-prayer 212-246-4200Dial santa 212-976-141End

~/Demo/Examples/411

A case study: bundle program (P 97)

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Suppose a friend asks for copies of shell files in your bin directory

$ cd /user/you/bin$ for i in *.sh> do> echo ===== This is file $i

===============> cat $i> done | mail yourfriend@hotmail.com

Pipeline & input/output redirection can be applied to for, while, until loop.

Make it better ?

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Construct a mail message that could automatically unpack itself, i.e., to generate the original files packed inside

A shell script contains instructions for unpacking, and files themselvesUse here document mechanism

Generate this shell script using another script

Bundle script

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#!/bin/bashecho ‘# To unbundle, bash this file’for ido echo “echo $i 1>&2” echo “cat >$i <<‘End of $i’” cat $i echo “End of $i”done

~/Examples/writescript bundle.sh

An example bundle file

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Try it out: ./bundle.sh bundle.sh 411 > junk

Inside junk:

Inside junk file

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#To unbundle, bash this fileecho bundle.sh 1>&2cat >bundle.sh <<'End of

bundle.sh'#!/bin/bashecho '#To unbundle, bash this

file'for i in $@ ## or for ido echo "echo $i 1>&2" echo "cat >$i <<'End of $i'" cat $i echo "End of $i"done

end of bundle.shecho 411 1>&2cat >411 <<'End of 411'grep "$*" <<Enddial-a-joke 212-976-3838dial-a-prayer 000000000dial santa 8900999today is `date`endend of 411