UNIX Shell Scripting

1
UNIX Shell Scripting

• gt Echo A quick how-to

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What is shell scripting?

• Interpreted (non-compiled) language
• Slower compared to compiled languages
• Much more portable than compiled language though
• This presentation uses the bash scripting language

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Configuring for execution

• In order to execute a shell script in a UNIX
  environment, you have to configure the files
  permissions with the chmod command
• gt Chmod 700 file1.csh
• The script must be executed from the folder which
  it resides in.

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Shortcut alias

• You can set a script to be run from any folder by
  setting an alias for the file.
• This can be done by editing the user file which
  contains aliases, for example, .bashrc
• alias run/home/user/file1.csh

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Hello World
gt hello.csh gt Hello World gt
!/bin/bash echo Hello World

• The !/bin/bash line must be on the first line of
  all bash scripts - it indicates where the
  compiler for the script is located.
• echo is the shell command to print out a line

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All for one, and quotes around them all

• In order to include whitespace in variables, and
  in several other cases, statements must be
  surrounded by quotation marks. There are three
  different types

(double quotes) - basic type, enclosed
variables are substituted with their values
(single quotes) - same as doubles, except no
variable substitution (back ticks) - used for
evaluating enclosed commands
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Variability

• Variables are defined as follows
• variable_namevalue
• There must be no whitespace between the operands
  and the operator (xy)
• Unlike most compiled languages, bash scripts do
  not have different variable types - so no
  declaring variables of type int, char, etc.

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Echoing variables

• Except at execution, when referring to variables,
  they must be prefixed with a
• Depending on which type of quotation mark you use
  to enclose an echo commands statement, enclosed
  variables may be substituted, as in the following
  example, where x 12.

INPUT echo variable x x echo variable x
x
OUTPUT gt variable x 12 gt variable x x
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Control Structures

• if - if (statement) is true, then do one set of
  instructions, otherwise, do a different one
• while - while (statement) is true, do a set of
  instructions.
• Also, there are the for, case, and until
  structures, but we will not go over these two.

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If (wishes fishes)

• This structure basically goes
• if (x)then (y)else (z)fi
• There neednt necessarily be an else, but if you
  use an if statement, you must conclude with a
  fi statement.

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Ifthenelsefi

• In the following code segment, from_address is a
  variable which contains the name of the sender of
  an email.

if from_address Wendy then echo
You have mail from Wendy else echo Wendy
has not sent you any mail fi
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While - the loopiest of the loops

• The while control structure basically goes
• While (x)then (y)done
• While indicates the beginning of the loop and
  tests the condition.
• Then is executed as long as While is true.
• Done indicates the end of the loop segment.

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Whiledodone

• The following is a short while loop that counts
  to 3

!/bin/bash number1 echo -n "Testing..." while
"number" -lt 4 do echo -n
"number..." numberexpr number
1 done echo ""
gt test.csh gt Testing123 gt
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Test conditions

• Below are several different tests that can be
  done on numbers or strings - the left box is what
  were used to, and the right box is the bash
  script code.

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Readin, Riting, and Rithmatic

• There are two ways in a bash script to do simple
  arithmatic (add , subtract -, multiply ,
  divide /, and modulus )
• The first way is using the expr command
• x(expr 3 4) -or- xexpr 3 4
• The second is using double parenthesis
• x((3 4))
• Note that both ways deal only with integers -
  neither handles deciman values.

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Please enter your name

• In some cases, you might need to prompt for user
  input - this is done with the read command.
• Format read variable_name

!/bin/bash echo What is your name? read
name echo Hello name.
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References

• http//www.linuxnewbie.org/nhf/Programming/Introdu
  ction_to_bash_Shell_Scripting.html
• http//pegasus.rutgers.edu/elflord/unix/bash-tute
  .html
• http//www.devhood.com/tutorials/tutorial_details.
  aspx?tutorial_id468