Introduction to Bash Programming, part 4

1
Introduction to Bash Programming, part 4

• Ellen Zhang

2
Use Arsc

• Type in user name and password once, clear
  Terminal button to open PuTTy window
• Open one terminal first
• I will write a script to that terminal
• Open another terminal

3
Write the script to all students

• !/bin/bash
• Usage write given file to all students listed
  in a file
• write cat /CSRU3130/submissions/student_accounts
  .txt lt 1

Example at /Demo/Examples/loop.sh
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Outline

• Review
• Execution of shell script
• Bash variables
• Conditions
• Control Structure
• Arithmetic, working with numbers
• Function
• Here document

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Execution of shell script

• !/bin/bash
• an endless loop, to illustrate how script is
  run.
• cd oddvariable1
• echo "PWD is set to PWD"
• echo "oddvariableoddvariable
• echo -n press any key to continue
• read x
• while 1 Endless loop.
• do
• echo endless loop, type Ctrl-c to abort. . ."
• done
• )
• Script will run forever,
• or at least until terminated by a Ctl-C.

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Execution of shell script

• bash ./myscript or ./myscript
• a subshell is started to interpret and run script
• Type command ps -ef grep USER grep bash
• to check your own bash processes
• Current shells environment is not altered,
  including current directory, variables
• pwd echo PWD echo oddvariable

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Execution of shell script

• Run a script from current shell source myscript,
  or . myscript
• Any changes to environment is made at current
  shell
• Try running myscript using above commands, use
• ps ef grep bash grep USER to check of
  bash processes
• Notice current directory is changed, and
  oddvariable remains set

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SHELL Variables (1)

• Different types of variables, set
• Environment variables HOME, PATH, PS1, PS2
• Parameter variables ,0, 1, ,_at_
• User defined variables student,
• Declare variables by using them, e.g.,
• x1 variable x is set to 1
• Note no spaces before and after

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Shell variables (2)

• Do not need to declare a variable
• All variables are string (stored as strings)
• Set a variables value
• X3 messagehello world
• To read from standard input a string, and set it
  to a variable
• echo Please input value for x echo n x
• read x

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Shell variables (3)

• Refer to a variables value add a before
  variable name
• echo The value of x is x
• If x is not defined, x returns a empty string
• Unset a variable, unset command
• unset x

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Shell variables (4)

• Shell variables created in a shell is not
  available in subshells (including interactive
  shell and shell running scripts)
• x123
• bash
• echo x
• To make a variable available to all subshells
• export xabc strHello
• export PATHPATH/bin

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Outline

• Review
• Execution of shell script
• Bash variables
• Conditions
• Control Structure
• Exercises problem from last class
• Some useful bash built-in commands
• export, break, , , etc.
• Here document

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Bash programming Control Structures

• 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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If/then/else/fi Check password input

• echo n "Enter your password "
• read input
• if input "secret"
• then
• echo "welcomed --
• echo to the secret world!
• else
• echo "go away!"
• fi

Please pay attention to the style. use
indention use comments
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Specifying conditions

• We use test command, or to specify conditions
• if test input secret or
  input "secret"
• then
• echo "welcomed --
• echo to the secret world!
• else
• echo "go away!"
• fi

Important note the spaces before and after
, To put if, then in one line if -f list10
then
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Check password script2

• !/bin/bash
• while "input" ! "secret"
• do
• echo "Enter your password"
• read input
• done
• echo "welcome!"

What will happen if no quotation on input ?
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Alternative way

• while 1
• do
• echo -n "Enter your password"
• read input
• if input "secret"
• then
• break
• else
• echo -n "Try again... "
• fi
• done

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Check password script2 use until

• !/bin/bash
• until "input "secret"
• do
• echo -n"Enter your password "
• read input
• done
• echo "welcome!"

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Conditions that one can test

• 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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String pattern matching

• Double brackets allow for pattern matching
• To test if first argument is followed by a
  number
• if "1" -0-9
• then
• .
• To check if input is abc followed by numbers
• if "input" abc0-9
• then
• .

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test about files

• 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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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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Outline

• Review
• Execution of shell script
• Bash variables
• Conditions
• Control Structure
• Arithmetic, working with numbers
• Some useful bash built-in commands
• export, break, , , etc.
• Here document

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Variables are strings

• Variables values are stored as strings
• number75
• echo number
• 75
• x2 y3
• z1xy z2xy
• echo z1 z2 What will be the output?
• xy 23

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Arithmetic Evaluation

• To evaluate an arithmetic expression use expr
  command
• x1
• expr x 100 Need space before/after
• 101
• xexpr x 1 need space before/after
• x(expr x 1) need space before/after
• x((x1)) dont need space
• Other operations supported , -, , /,

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Exercise4 loop10

• count1
• limit10
• while count -lt limit
• do
• sleep 5 echo Get up and do some exercise
• countexpr count 1
• done

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Another solution

• for (( i0ilt10i ))
• do
• sleep 5 echo "Get up and get some
  excercise!"
• done

Courtesy of Chris Belsole
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Exercise 3 A Simple Calculator

• echo "evaluate binary operation on x,y"
• echo -n "x"
• read x
• echo -n "op (, -, , /, )"
• read op
• echo -n "y"
• read y

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First attempt

• if op ""
• then
• echo ((xy))
• elif op "-"
• then
• echo ((x-y))
• elif op "/"
• then
• echo ((x/y))
• elif op ""
• then
• echo ((xy))
• else
• echo ((xy))
• fi

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Exercise 3 A Simple Calculator
variable

• case op in
• ) echo "xyexpr x y"
• -) echo "x-yexpr x - y"
• "") echo "xyexpr x "" y"
• /) echo "x/yexpr x / y"
• ) echo "xyexpr x y"
• ) echo "Not recognized operation"
• esac

pattern

• On first matching pattern, execute corresponding
  statement.
• Last (wildcards) match the all the rest input
  what happen if put it first?
• Note that we need to quote on 3rd line.

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Putting multiple patterns together

• echo is it morning ? (yes or no)
• read timeofday
• case timeofday in
• Yes y yes ) echo Good morning
• N n ) echo Good afternoon
• ) echo Sorry, invalid answer
• esac

The means or.
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Another solution

• while 1
• do
• ans(( xopy ))
• echo "xopyans"
• echo "Are you finished (y/n)"
• read choice
• if choice y
• then
• exit
• fi
• echo ""
• Done
• Courtesy of Chris Belsole

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Outline

• Review
• Execution of shell script
• Bash variables
• Conditions
• Control Structure
• Arithmetic, working with numbers
• Something new
• Lists, break, , , etc.
• Here document

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Check password maximum 5 tries

• tries0 limit5
• while "input" ! "secret tries lt
  limit
• do
• echo "Enter your password"
• read input tries((tries1))
• done
• if input secret
• then
• echo "welcome!
• else
• echo retry limit (5) reached
• fi

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AND list construct

• We used to connect two test commands together
• Generally,
• Statement1 statement2 statement3
• Each statement is executed if it returns true,
  next statement is executed. Until finding first
  statement that returns false, or all statements
  return true
• Note the statement can be any command, such as
  echo (which always return true)

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OR list construct

• Generally,
• Statement1 statement2 statement3
• Each statement is executed if it returns false,
  next statement is executed. Until finding first
  statement that returns true (the construct then
  return true), or all statements return false (the
  construct then returns false).
• Note the statement can be any command, such as
  echo (which always return true)

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Functions

• One can define functions to increase modularity
  and readability of shell scripts
• More efficient than breaking large scripts into
  many smaller ones Why ?
• foo()
• Echo in function foo
• echo start script
• foo
• echo end script

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About functions

• Need to be defined first, and then can be called
• Parameter passing
• ! /bin/bash
• calsum()
• echo expr 1 2
• x1y2
• calsum x y

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About functions

• Result returning
• Through setting a variable
• Use return command
• Use echo command
• ! /bin/bash
• calsum()
• echo expr 1 2
• x1y2
• calsum x y
• zcalsum x y

calsum() zexpr 1 2 x1y2 cals
um x y echo zz
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About functions

• Local variable its scope is within the function
• ! /bin/bash
• calsumsqr()
• local sumexpr 1 2
• echo expr sum sum
• x1y2
• calsum x y
• zcalsum x y

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Here document

• A special way to pass input to a command here
  document, i.e., from the shell script itself
• !/bin/bash
• cat ltlt!FUNKY!
• Hello
• This is a here
• Document
• !FUNKY!

Here document starts with ltlt, followed by a
special string which is repeated at the end of
the document.
Note the special string should be chosen to be
rare one.
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Here document2

• Benefits store codes and data together, easier
  to maintain
• Example 411 script
• grep ltlt End
• Dial-a-joke 212-976-3838
• Dial-a-prayer 212-246-4200
• Dial santa 212-976-141
• End

/Demo/Examples/411
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A case study bundle program (P 97)

• Suppose a friend asks for copies of shell files
  in your bin directory
• cd /user/you/bin
• for i in .sh
• gt do
• gt echo This is file i
• gt cat i
• gt done mail yourfriend_at_hotmail.com

Pipeline input/output redirection can be
applied to a for, while, until loop.
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Make it better ?

• Construct a mail message that could automatically
  unpack itself, i.e., to generate the original
  files packed inside
• A shell script contains instructions for
  unpacking, and files themselves
• Use here document mechanism
• Generate this shell script using another script

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Bundle script

• !/bin/bash
• echo To unbundle, bash this file
• for i
• do
• echo echo i 1gt2
• echo cat gti ltltEnd of i
• cat i
• echo End of i
• done

/Examples/writescript bundle.sh
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An example bundle file

• Try it out
• ./bundle.sh bundle.sh 411 gt junk
• Inside junk

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Inside junk file
end of bundle.sh echo 411 1gt2 cat gt411 ltlt'End of
411' grep "" ltltEnd dial-a-joke
212-976-3838 dial-a-prayer 000000000 dial santa
8900999 today is date end end of 411

• To unbundle, bash this file
• echo bundle.sh 1gt2
• cat gtbundle.sh ltlt'End of bundle.sh'
• !/bin/bash
• echo 'To unbundle, bash this file'
• for i in _at_ or for i
• do
• echo "echo i 1gt2"
• echo "cat gti ltlt'End of i'"
• cat i
• echo "End of i"
• done

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Summary

• We so far covered basic features of shell
  variables, functions, conditions and constructs.
• Next class
• Some powerful commands for searching, stream
  processing text files grep, sed, awk
• Regular expressions