Shell ScriptingProgramming

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Shell Scripting/Programming

• CSCI 2467 Spring 2007
• Instructor Michael Ruth
• Computer Science Department
• University of New Orleans
• mruth_at_cs.uno.edu

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Any Questions Before We Move On?

• Corrections
• The default shell used by the dept is the Korn
  shell
• The file that gets run all the time is not the
  .cshrc but the .kshrc file
• I can never seem to remember all of your
  questions
• please send them to mevia email
• Any other questions before we move on?

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Topics

• Motivation
• Or why learn shell programming?
• What is it?
• How to switch Shells (csh ? ksh, etc)
• Invoking a Shell Script
• Sha-bang!
• How do we know where that is?
• Our first script
• The Korn Shell Overview
• Comments
• Variables
• Control-flow (conditionals, conditional
  execution, looping, etc)
• Command Line Arguments
• Functions (built-in)
• Arrays

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Why Learn Shell Programming

• Automate tasks, such as
• Software install procedures
• Backups
• Administration tasks
• Periodic operations on a database via cron
• Any repetitive operations on files
• A shell script is a "quick and dirty" method of
  prototyping a complex application
• No compilation
• execution

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What is Shell Scripting?

• A shell script is a script written for the shell,
  or command line interpreter, of an operating
  system.
• Scripts are text files interpreted by a shell
  program
• Shell Scripting is by default very shell
  dependent (like duh) so not everything works from
  shell to shining shell
• We will focus on the department shell
• The default shell used by the department is the
  Korn shell (which is the bourne shell )

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First how do we switch shells?

• Remember me saying that shells are programs?
  Well, to switch, simply execute the command
• Shell commands
• csh C Shell, but not the kind we sell at the C
  shore
• ksh Korn Shell (department standard)
• bash Bourne Again Shell
• sh Bourne Shell
• Try switching between them and executing simple
  commands
• Specifically, try the commands which involve the
  environment (set, stty)
• To find out what shell you are in
• echo SHELL
• For the purposes of this course, focus on the
  Korn Shell (ksh)

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Invoking a Shell Script

• There are two basic mechanisms for invoking a
  shell script
• Use the shell command to execute script
• Ex ksh myscript.ksh
• Make the script itself an executable
• Sha-bang mechanism
• The script must be set to be executable (chmod
  755)

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Sha-bang or (!)

• a specific pair of characters that, when used as
  the first two characters on the first line of a
  script, causes Unix to execute that script using
  the interpreter specified by the rest of that
  line
• More precisely, a shebang line consists of a
  number sign and an exclamation point character
  ("!"), followed by the (full) path to the
  interpreter program that will provide the
  interpretation
• The shebang is actually a human-readable instance
  of a magic number in the executable file, the
  magic byte string being 0x23 0x21, the two
  characters' encoding in ASCII.

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More about Sha-Bangs

• !/bin/bash Execute using bash program in the
  /bin/ directory
• !/bin/csh Execute using csh, the C shell
  instead
• !/bin/ksh Execute using the Korn shell
• !/usr/bin/perl Execute using Perl
• Shebang lines can also include specific options
  that will be passed to the interpreter.
• However, implementations differ widely on how
  options are parsed
• !/usr/bin/perl w Execute using Perl with
  warnings enabled

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How do we find the full path?

• which
• Command takes a list of names and determines
  (using path information) what would be executed
  had these names been given as commands
• Ex
• which perl ? /usr/bin/perl
• which ksh ? /usr/bin/ksh
• Also, as a side note the options can be
  discovered via man as usual

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Our First Script
! /usr/bin/ksh echo Hello World!
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Comments

• To comment entire lines of text use the
  operator
• This is all a comment
• This is a comment too

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Setting and Exporting Variables

• varvalue
• Variables are treated as text strings, unless the
  context implies a numeric interpretation.
• Note there must be no spaces around the ""
• set
• Display all the variables currently set in the
  shell
• unset var
• Remove the variable "srcfile"
• var
• Give the variable a null value, (not the same as
  removing it).
• export var
• Added srcfile to the list of variables which will
  be made available to external program through the
  environment.
• If you don't do this, the variable is local to
  this shell instance.
• export
• List all the variables currently being exported -
  this is the environment which will be passed to
  external programs

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Using those Variables

• var
• Prefacing the variable name with causes the
  value of the variable to be substituted in place
  of the name.
• var
• If the variable is not surrounded by whitespace
  (or other characters that can't be in a name),
  the name must be surrounded by "" braces so
  that the shell knows what characters you intend
  to be part of the name

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Some Preset Variables

• USER, LOGNAME
• Preset to the currently logged-in username.
• PATH
• The list of directories that will be searched for
  external commands. You can change this in a
  script to make sure you get the programs you
  intend, and don't accidentally get other versions
  which might have been installed.
• EDITOR
• If set, this contains the name of the program
  which the user prefers to use for text file
  editing. A program which needs to have the user
  manually edit a file might choose to start up
  this program instead of some built-in default
  (e.g. "crontab -e". This also determines the
  default command-line-editing behaviour in
  interactive shells.
• PWD
• Always set the current working directory
  (readonly)
• OLDPWD
• The previous directory (before the most recent cd
  command)

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Conditionals (Strings)

• -n "string"
• true if string has non-zero length
• -z "string"
• true if string has zero length
• variable text
• True if variable matches text.
• variable
• True if variable comes before (lexically)
  textSimilarly, comes after

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Conditionals (Math)

• variable -eq number
• True if variable, interpreted as a number, is
  equal to number.
• variable -ne number
• True if variable, interpreted as a number, is
  not equal to number.Similarly, -lt less than,
  -le less than or equal, -gt greater than, -ge
  greater than or equal

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Conjunction junction Whats your function!
(conditionals)

• AND
• conditional conditional
• conditional a conditional
• OR
• conditional conditional
• conditional o conditional
• NEGATION
• ! conditional

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Conditionals File Tests

• -e file
• True if file exists (can be of any type).
• -f file
• True if file exists and is an ordinary file.
• -d file
• True if file exists and is a directory.
• -r file
• True if file exists and is readableSimilarly, -w
  writable, -x executable, -L is a symlink.
• -s file
• True if file exists and has size greater than
  zero

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Conditional Execution (if)

• Standard if/else constructs
• In the following discussion list implies a group
  of statements
• if (condition)
• then
• list
• elif (conditional)
• then
• list
• else
• list
• fi

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Conditional Execution (case)

• Case statements are exactly what you think they
  are
• The pattern follows the same rules as the
  filename metacharacters
• case word in
• pattern)
• list
• esac
• This needs an example

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Case Example
case fn in .dat) echo Processing a .dat
file .sas) echo Processing a
.sas file ) catch all
pattern echo Unsure how to deal with fn"
esac
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Looping Constructs (while/until)

• While operates precisely as expected
• while (condition)
• do
• list
• done
• Until is a bit like its opposite
• until (condition)
• do
• list
• done

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Last Loopy Construct

• For is more like a foreach statement
• for identifier in word list
• do
• list
• done
• Example
• for file in .dat
• do
• echo Processing file
• done

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Command Line Arguments

• Officially termed positional parameters
• The number of parameters is in
• The actual values are stored in 1 through n
• All values are stored in , _at_
• Use these in the for loop construct to loop over
  the command line arguments
• Lastly, 0 stores the command line along with the
  path

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Command line example
!/usr/bin/ksh for clop in _at_ do echo
clop done
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Shell Functions (intro)

• You can write functions in shell scripts
• A function ( procedure) must be defined before
  it is called, because it is interpreted at run
  time
• It knows all the variables from the calling shell
  except the commandline arguments.
• But has it's own command line arguments so that
  one can call it with different values from
  different places in the script. It has an exit
  status but cannot return a value like a c
  function can

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Creating a function

• There are two ways to create functions
• function foo
• commands...
• foo()
• commands...

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Calling the function

• To call it just put it's name in the script
• foo
• To give it arguments do
• foo arg1 arg2
• The arguments are there in the form of 1...n
  and for all at once like in the main code
• And the main 1 is not influenced bye the 1 of a
  particular function
• The return statement exits the function
  immediately with the specified return value as an
  exit status

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Arrays

• Set and use an array like
• arrname14
• To fill in
• print arraname1
• To print out
• arrname
• Get all elements
• arrname
• Get the number of elements

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Others

• Read lines of text interactively
• Read command
• echo "What is your name?"
• read myname
• echo myname
• The math evaluator is very useful.
• Everything inside the double-parens gets
  evaluated with basic math functions.
• For example
• four((2 2))
• eight((four 4))
• print ((four eight))

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Resources

• Unix Shell Programming Patterns
• http//c2.com/cgi/wiki?UnixShellPatterns
• SHELLdorado
• http//www.shelldorado.com
• Microsoft Services for UNIX
• http//www.microsoft.com/windows/sfu/
• Dartmouth has a great tutorial
• http//www.dartmouth.edu/rc/classes/ksh/
• Useful Korn Shell Tutorial
• http//b62.tripod.com/doc/docksh.htmregex
• Incredible BASH Shell Tutorial
• http//tldp.org/LDP/abs/html/index.html

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The future

• Perl Regular Expressions
• Homework 1 will be delivered THURSDAY!!!
• An amazing announcement will be made as well!
• Introduction to C Programming

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Questions?