session 2: shell scriptingcreating a shell script creating a shell script consists of three basic...

Introduction Variables Comments Command Line Parameters Conditional Execution Loops File Redirection Accessing Scripts from Anywhere Case Studies

Session 2: Shell Scripting

Craig A. Struble, Ph.D.

July 14, 2010

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Creating a Shell Script

Creating a shell script consists of three basic steps:

1 Create a text file with a text editor like pico, vi or emacscontaining the sequence of commands to execute.

2 Make the file executable with the command

$ chmod a+x scriptname

where scriptname is the name of your script.

3 Execute the script from the command line by typing

$ ./scriptname

assuming your script is in your working directory.

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Exercise

1 Using a text editor, create a file named hello with thecontents

#!/bin/bash

read name

echo Hello $name

2 Make it executable with the command

$ chmod a+x hello

3 Run the script

$ ./hello

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Variables

Variables are named locations storing values.

General format for assigning variables

variable=value

A simple example

year=2010

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Retrieving Values

Retrieve values preceding name with a dollar sign ($).

So if we have the script

year=2010

echo The year is $year

The output of the script will be

The year is 2010

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Without Variables

#!/bin/bash

echo The calendar for 2010 is:

cal 2010

echo Last "year’s" calendar was:

cal 2009

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With Variables

#!/bin/bash

year=2010

echo The calendar for $year is:

cal $year


cal $(( year - 1 ))

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Exercises

1 Modify the calendars script so that it has a variable namedp year for previous year. Assign it the value of the yearprevious to the one stored in year.

2 Continue to modify the calendars script so that it prints out

The calendar for the previous year 2009 was:

instead of

Last year’s calendar was:

using the p year variable.

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With Comments

A comment starts with a # and continues until the end of the line.

#!/bin/bash

#

# Print out this year’s and last year’s calendars.

#

year=2010


cal $year


cal $(( year - 1 ))

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Command Line Parameters

Consider the command

./myscript one two three

The command has three command line parameters, which areprovided to numbered variables $1, $2, and $3 as illustratedbelow. Note that the command itself is assigned to $0.

./myscript one two three

$0 $1 $2 $3

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With Command Line Parameters

#!/bin/bash

#


#

year=$1


cal $year


cal $(( year - 1 ))

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Number of Command Line Parameters

The number of command line parameters provided to a script isstored in the special variable $#.

#!/bin/bash

#

# Script name: nparams

# Print the number of command line parameters.

#

echo You passed $# parameters.

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Exercise

1 Use the nparams script to find out how many command lineparameters are passed in the following cases:

$ ./nparams

$ ./nparams one two three

$ ./nparams "one two" three

$ ./nparams "one two three"

$ ./nparams one ’two three’

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Exercise

2 Write a script named params to print out the first 5command line parameters. Then, use that script to see whatthe parameters are for the following runs.

$ ./params

$ ./params one two three

$ ./params "one two" three

$ ./params "one two three"

$ ./params one ’two three’

To get started, the script should look like

#!/bin/bash

echo The first parameter is $1

echo The second parameter is $2

to print the first two parameters.

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Conditional Execution

The general form of a conditional statement is

if CONDITION

then

COMMAND1

COMMAND2

...

fi

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With Conditional Statements

#!/bin/bash

#


#

# Check for one parameter. If the number of parameters

# is not equal (-ne) to 1, print usage and exit.

if [ $# -ne 1 ]

then

echo "Usage: $0 year"

exit 1

fi

# Everything is OK

year=$1


cal $year


cal $(( year - 1 ))

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For Loop

The general form of a for loop is

for VARIABLE in WORDS

do

COMMAND1

COMMAND2

...

done

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Countdown

#!/bin/bash

#

# Countdown

#

for i in 10 9 8 7 6 5 4 3 2 1

do

echo T minus $i

done

echo "Liftoff!"

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While Loop

The general form of a while loop is

while CONDITION

do

COMMAND1

COMMAND2

...

done

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Another Countdown

#!/bin/bash

#

# Another Countdown

#

i=10

while [ $i -gt 0 ]

do

echo T minus $i

i=$(( i - 1 ))

done

echo "Liftoff!"

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Redirect Output

To make the output of a command go to a file instead of thescreen, use the > operator. The following will create a file namedhello.out containing the text Hello World.

echo Hello World > hello.out

If the file already exists, it will be overwritten, so be careful.

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Redirect Output, Appending

To append instead of overwrite the file, use the >> operator. Thefile will be created if it does not already exist, but will add to theend of an existing file. The following will add the text GoodbyeWorld to the file hello.out.

echo Goodbye World >> hello.out

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Redirect Input

Use the < operator to read input from a file instead of typing in onthe keyboard. The following will copy the contents of somefileand output them to the file duplicatefile.

cat < somefile > duplicatefile

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Here Documents

#!/bin/bash

name=$1

cat <<EOF > afile

Hello World

Goodbye World

My name is $name

EOF

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Condor Submission Generation

#!/bin/bash

#

# Create a simple Condor submission file.

# This doesn’t handle complicated quoting

# or executables outside of the working directory.

#

if [ $# -lt 1 ]

then

echo "Usage: $0 program argument1 argument2 ..."

exit 1

fi

program=$1

shift # Remove the first argument

# Generate the submission file

cat <<EOF > $program.submit

Universe=vanilla

Executable=$program

Arguments=$@

Output=$program.out

Error=$program.err

Log=$program.log

Queue

EOF

echo $program.submit created.

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Accessing Scripts from Anywhere

If a script you write is generally useful, you’ll want to access itfrom any directory, not just your working directory. Almost allshells use a special variable $PATH to list the directories wherecommands our found. On pere.marquette.edu, the bin

directory in $HOME (i.e. $HOME/bin) is in the path. So to use yourscript from anywhere, move it with

$ mv scriptname $HOME/bin

and then execute with

$ scriptname

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Case Study 1: Preparing Data

In this case study, we will prepare a script to generate directoriesand input files for analysis on a Cartesian grid. We specify themaximum x and y coordinates of our study on the command lineand generate a simple input file containing x and y. Each input filewill be stored in a directory named runN, where N is defined by

N(x, y) = maxx ∗ y + x

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Case Study 2: Make Project Structures

In this case study, we will prepare a script to create a simpleproject directory structure and a template README.txt file. Thedirectory structure will be

$ProjectName

bin data doc workspace

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