Introduction to Bash Programming, part 3

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Introduction to Bash Programming, part 3

• Ellen Zhang

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Outline

• A review about what we learnt about bash
• Customize your bash environment
• list10 script
• Bash variable

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Shell command line

• Shell command line syntax
• ls R l
• Command name and options separated by space
• Command ends with newline, and

Command name
argument
Options (arguments)
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Bash special characters

• Globbing, filename expansion
• ls , rm , etc.
• Shell expands filename patterns or templates
  containing special characters
• Can be turned off set f, shopt
• Shell special characters
• , ?, ,, , gt, lt, gtgt
• Quotations single quote, double quote

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Shell parameter variables

• If your script is invoked with parameters, shell
  sets parameter variables
• the number of parameters
• 0 the command/script name
• 1 the first parameter given to the script
• 2 the second parameter
• a list of all parameters, seperated by first
  char in IFS
• _at_ a list of all parameters, seperated by space
• Also called positional parameter

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Our first shell script list10

• Our first shell script
• ls -Rl grep - sort -k 5 -nr head -10
• To make it accept path_name
• ls -Rl 1 grep - sort -k 5 -nr head -10
• To make it accept multiple path names
• ls -Rl grep - sort -k 5 -nr head -10

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Bash programming Control Structures

• Similar to other programming language
• Differs in syntax
• Control structures in bash
• if then else fi
• if then elif else fi
• for in do done
• while do done
• until do done
• case in esac

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Specifying conditions

• Shells boolean check command test, or
• if test f list10 if -f list10
• then
• ..
• fi

Important note the spaces before and after
, To put if, then in one line if -f list10
then
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Conditions that one can test

• File conditionals
• -d file true if the file is a directory
• -e file true if the file exists
• -f file true if the file is a regular file
• -r file true if the file is readable -x,-w
• -s file true if the file has nonzero size
• more . Read bash tutorial manuals

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Conditions that one can test(contd)

• String comparison
• string1 string2 strings are equal
• -n string string is not null
• -z string string is null
• Arithmetic comparison
• exp1 eq exp2 true if two expressions are equal
• exp1 ne exp2 true if two expressions are not
  equal
• Others gt, -ge, -lt, -le
• ! exp1 true if exp1 if false

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Special bash scripts
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Shell startup file

• Profile files executed for login shell
• /etc/profile system wide default, setting
  environment for all shell.
• HOME/.bash_profile user-specific bash
  environment default settings
• Initialization files executed for login and
  interactive shell
• /etc/bashrc system wide function and aliases for
  bash
• HOME/.bashrc user-specific initialization files

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Customize your bash environment

• Edit the /.bash_profile file
• Print a greeting message with your account name,
  home directory, date
• echo n Hello
• who am i
• echo n your home is
• pwd
• Echo n today is
• date

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To generate nicer output

• Command substitution substitute output of a
  command (sequence) into another context
• Syntax enclose using backquote, or ()
• As argument for another command
• rm ls .o
• To set a variable
• time1(date) echo times1
• To be used in for construct
• for file in ls do
• done

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Generate adapted welcome msg

• Examples
• echo Welcome, who am i! Your home is pwd
• echo The time is (date)
• You can also use
• echo Welcome, USER

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Even nicer output

• set command, a shell builtin command
• display current variables, set
• set shell options, set f, set n ..
• set position parameters,
• set Hello world echo 1, 2
• Combine command substitution, set
• set who am i
• echo Welcome, 1! You logged in from 5.

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To test your settings

• Reloggin
• Type bash to create an another sub-shell
• Current shell is not altered
• Run a script from current shell to change
  settings of current shell
• source .bashrc , or . .bashrc

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Outline

• A review about what we learnt about BASH
• Customize your bash environment
• list10 script
• Bash variables

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Script list10

• !/bin/bash
• echo
• echo 1
• echo 0
• ls -Rl grep - sort -k 5 -nr head -10

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Handling command line option

• To accept an optional arguments
• list_n largest_num path1 path2 path3
• display 20 largest files under a directory
• list_n -20 zhang/documents
• If the number argument (starting with -) is not
  given, then list 10 largest files

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Coming back to list10

• To test if first argument is followed by a
  number
• if "1" -0-9
• then
• echo "list10 with number argument "
• else
• echo "list10 without number argument"
• fi
• Double brackets allow for pattern matching

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Our list10 script

• if "1" -0-9
• then
• list10 with number argument
• how to implement this ?
• else
• list10 withoutnot number argument
• all arguments are paths
• ls Rl grep - sort -k 5 nr head
  -10
• fi

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Bash Programming Loop Structure

• for construct to loop through a range of values,
  which can be any set of strings
• for student in jack, john, alice
• do
• echo Hello, student gt greeting_student
• write student lt greeting_student
• done

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Final list10 script

• if "1" -0-9
• then
• for path in
• do
• if path ! 1
• then
• path_list"path_list
  path"
• fi
• done
• ls -Rl path_list sort -k 5 -nr head
  1
• else
• ls -Rl grep - sort -k 5 -nr
  head -10
• fi

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Another example

• Save student account name in a file, all.txt
• for student in cat all.txt
• do
• echo Hello, student gt greeting_student
• grep student student_rec.txt
  gtgtgreeting_student
• write student lt greeting_student
• rm greeting_student
• done
• exit

How to avoid using the temporary file,
greeting_student ?
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Outline

• A review about what we learnt about BASH
• Customize your bash environment
• list10 script
• Bash variables

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SHELL Variables

• Different types of variables, set
• Environment variables HOME, PATH, PS1, PS2
• Parameter variables 1, ,
• User defined variables student,
• Declare variables by using them, e.g.,
• for path in
• Or
• x1 variable x is set to 1
• Note no spaces before and after

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SHELL Variables

• Access variable by preceding it with
• echo x
• Need quote marks if there are spaces
• greetingHello world need to quotation
• echo greeting
• Hello world

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Read variable value from input

• read timeofday
• Morning
• echo Good timeofday!
• Good Morning!
• read greeting
• Good morning dont need to quote
• echo greeting
• Good morning
• echo greeting is \greeting.

What will be the output ?
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Variables value type string

• Variables values are stored as strings
• number75
• echo number
• 75
• x2 y3
• z1xy z2xy
• echo z1 z2 What will be the output?

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Arithmetic Evaluation

• To evaluate an arithmetic expression
• Use expr command
• x1
• xexpr x 1 increment x by 1
• Or x(expr x1)
• Or x(( x1))
• Other operations
• Comparison , lt, gt, gt, lt, !
• Arithmetic , -, , /,

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A bash based calculator

• First, lets implement addition
• echo calculate xy
• echo n x
• read x
• echo n y
• read y
• echo xy expr x y

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Exerciseswork on the following problems,
finish them up at your own time and submit by
Monday 8pm.
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Compare string variable

• Remember the conditionals for testing strings
  , !, -n , -z
• Exercise 1
• Write a script that read from standard input a
  string, and check if its the same as your secret
  password secret if yes, print out welcome!
  print out Go away if not.

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Compare string variable

• Exercise 2
• Use while construct to rewrite the script, so
  that user can keep on trying until getting it
  right.
• while condition do
• statements
• done

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Exercise 3 A Simple Calculator

• To perform addition, subtraction,
• echo "evaluate binary operation on x,y"
• echo -n "x"
• read x
• echo -n "op (, -, , /, )"
• read op
• echo -n "y"
• read y
• Whats next ? Hint, case construct
• Evaluate multiple expressions, until user select
  to exit

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Exercise4 loop10

• Write a script that wake up every 5 seconds,
  print out a message Get up and do some
  exercise.
• Extend the above script so that it loops for 10
  times.

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Next two classes

• here document
• More advanced shell scripts
• Regular expression
• Sed, grep, awk,