Outline. Unix shells Bourne-again Shell (bash) Interacting with bash Basic scripting References

Outline

• Unix shells

• Bourne-again Shell (bash) • Interacting with bash

• Basic scripting • References

Unix shells

• This lets users issue commands to the Unix

operating system

• Users can interact with various parts of the Unix

operating system such as

– the input/output system

– the scheduler

– memory management

• The shell is simply a program whose job is to

execute other programs

• Different shells exist such as the C shell (csh),

the Korn shell (ksh), the Bourne shell (sh) and the Bourne-again shell (bash)

Bourne-again shell (bash)

• bash is the default shell on modern Linux

distributions as well as Mac OS X

• bash is both a command interpreter and a

high-level programming language

• When used as a programming language, bash

processes commands stored in files called shell scripts

• Most system shell scripts are written for bash • Like other programming languages, bash has

variables and control flow commands (i.e. for loops and if statements)

Active participation

• It is highly encouraged to try all the examples as

we go along

• You may use your own shell from a Linux/Mac

terminal

• You may also use the free linux shell provided

Shell Variables

• Variables in a bash script are typically written

with all-uppercase letters

– This is not a strict requirement

• Assignment is done using the equals sign

without spaces

VAR1=“test string”

• Referencing the value of a variable is done using

the dollar sign

echo “My assigned variable contains $VAR1”

– An exception to this rule is evaluating arithmetic expressions, covered later

Documenting bash scripts with

comments

• It is always a good idea to document bash

scripts with comments

• Any text immediately following the # character

will be considered a comment and ignored by the shell

# This is a comment

• The exception to this the shebang #! found at

the top of most scripts

• The shebang specifies which shell to use to

execute the script

Command substitution

• To assign the value of another bash command/

script to a variable, command substitution is used

TODAYS_DATE=$(date “+%m/%d/%y”) TODAYS_DATE=`date “+%m/%d/%y”` echo $TODAYS_DATE

echo $(TODAYS_DATE)

• Newer scripts will use $(), while older scripts

will use backticks ` `

• The $() convention makes it easier to read and

•  Visit _{http://www.simpleshell.com}

•  Click ‘Start my session’

•  nano hello_world.sh •  Ctrl-X •  Y to save modified buffer •  _{Hit return/} enter to accept filename

Running our first bash script

• We can run our bash script several ways

• We can pass the script to the shell of our choice

/bin/bash ./hello_world.sh /bin/sh ./hello_world.sh

• We can specify which shell to use with the

shebang, and then make the script executable nano hello_world.sh

Insert _#!/bin/bash at the top Ctrl x, y, enter to save chmod u+x ./hello_world.sh ./hello_world

Embedding variable values in text

• Suppose you have the following variable defined

PREFIX=sun

• If you wanted to echo the following words using

this prefix: sunflower, sunshine, sunset you would use

echo ${PREFIX}flower echo ${PREFIX}shine echo ${PREFIX}set

• Note the use of the braces

Without the braces, _{echo $PREFIXflower} would return nothing since the variable _PREFIXflower is undefined

Using parameters in a bash script

• scripts are often called using input parameters •  To reference these parameters use ${position #}

where _{position #} is the specific parameter to use that starts at 1

• Example:

echo “The first input parameter is ${1}” echo “The second input parameter is ${2}”

• You can also get the number of input

parameters passed using _${#}

echo “There were ${#} input parameters passed”

Setting default values for parameters

value if none is given

•  Suppose you have a script that is expecting a directory as an input parameter, but needs to default to the _$HOME

directory if none is specified

•  default_param.sh:

#!/bin/bash

# List the contents of the 1st _argument

# Use the $HOME directory if none is specified TARGET=${1:-$HOME}

Setting default values for variables

default values assigned to them

• default_variable.sh:

#!/bin/bash

# Show the first 5 lines of $TARGET file TARGET=/proc/cpuinfo

head -5 ${TARGET:=./default_variable.sh} echo “TARGET file is $TARGET”

unset TARGET

echo “---”

head -5 ${TARGET:=./default_variable.sh} echo “TARGET file is $TARGET”

The for control structure

do

commands

done

• The loop-index takes on the values of each of

the command line arguments

• Example:

# Display all of the command line arguments for INPUT_ARG

do

echo $INPUT_ARG done

The for…in control structure

do

commands

done

• The loop-index takes on the values of each

argument in the specified argument-list

• Example:

# Display all of the animals in a given list for ANIMAL in lions tigers bears

do

echo $ANIMAL done

The for control structure with C like

syntax

• bash also allows a syntax for for loops much

like the C programming language

• # count from 1 to 10 on a single line

for (( count=1; count<10; count+=1 )) do

echo –n “$count ” done

References

•  A Practical Guide to Linux® Commands, Editors, and

Shell Programming by Mark G. Sobell

•  Bash Cookbook: Solutions and Examples for Bash

Users by Carl Albing, JP Vossen, and Cameron Newham

•  http://en.wikibooks.org/wiki/Bash_Shell_Scripting

•  These slides will be posted on

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