Getting a filehandle

1
Getting a filehandle

• Open for reading
• open(FILEHANDLE, FILENAME)
• open(FILEHANDLE)
• open FILEHANDLE
• Closing
• close (FILEHANDLE)
• close FILEHANDLE
• Do or die function shows the open file error
• open(MYFILE,file) die Cant open file\n

2
Getting a filehandle

• Open for writing or appending
• open(FILEHANDLE, gtFILENAME)
• open(FILEHANDLE,gtgtFILENAME)
• Open for piping
• the command in the pipe will be executed
• open (FILEHANDLE, PIPECMD)
• open (FILEHANDLE, PIPECMD )
• Examples
• open(INPIPE,date )
• today ltINPIPEgt
• close (INPIPE)

3
ARGV and Null Filehandle

• When ltgt is used
• ARGV array is treated as a special filehandle
• Perl shifts through the array
• Examples
• cat f1
• Hello there. Nice day!
• cat f2
• Are you sure about this?
• cat f3
• Are you with me?
• This is kind of hard!
• perl -e while (ltARGVgt) print f1 f2 f3
• Hello there. Nice day!
• Are you sure about this?
• Are you with me?
• This is kind of hard!

4
Example

• !/usr/local/bin/perl
• name ARGV0
• open(FILE, ypcat passwd)
• while (ltFILEgt)
• chop
• _at_words split(,_)
• if words4/name/)
• print Login id ,words0,\n
• print Login Shell ,words6,\n

5
File Testing

• File testing operators
• -r file true if file is readable
• -w file true if file is writable
• -z file true if file is zero in size
• -f file true if file is a plain file
• -T file true if file is a text file
• -B file true if file a binary
• -t file true if file is opened to a tty

6
Subroutines

• Args are passed call by reference
• All variables created within the subroutine are
  global
• The subroutine is called by
• subroutine()
• do subroutine(par1,)
• Define a routine
• sub routinenamebody
• What is _at__
• a special array used for referencing the formal
  params
• the elements are _0,_1,

7
Subroutines

• sub under
• print The values in the _at__ array are,_at__\n
• print The first value is _0\n
• print The last value is ,pop(_at__),\n
• foreach value(_at__)
• value5
• print The value is value\n
• print Give me 5 numbers
• _at_n split( ,ltSTDINgt)
• under(_at_n)
• print Back in main\n
• print The new values are _at_n\n

Give me 5 numbers 1 2 3 4 5 The values in the
_at__array are 1 2 3 4 5 The first value is 1 The
last value is 5 The value is 6 The value is 7 The
value is 8 The value is 9 Back in main The new
values are 6 7 8 9 5
WHY?
8
Directories and Files

• Creating a directory
• mkdir(DIRNAME,MODE)
• perl -e mkdir(joker,0755)
• Other functions
• rmdir(DIR)
• chdir(EXPR)
• chmod(LIST)
• chown(LIST)
• opendir(DIRHANDLE, EXPR) attach to filehandle
• opendir(MYDIR, blahdir)
• readdir(), telldir(),seekdir(),closedir()
• opendir(DIR,..) die Cant open !\n
• _at_parentfiles readdir(DIR)
• closedir(DIR)
• foreach file (_at_parentfiles)
• print file\n

9
System function

• Take a UNIX command as its argument
• send the command to the shell for interpretation
• return control back to calling script
• system(UNIX COMMAND)
• perl -e system(cal 10 1997) print Happy
  Halloween!October 1997S M Tu W Th F S1 2
  Happy Halloween

10
Printf

• Print a formatted strong to a selected file
  handle (STDOUT)
• Like printf() in C language
• printf(FILEHANDLE FORMAT, LIST)
• printf(FORMAT, LIST)
• printf(The number is s and the number is
  d,John, 345)

11
Stdin

• Perl uses ltgt input (read) operator
• get a line from the FILEHANDLE
• we can use chop to remove the newline \n
• Examples
• print What is your name?
• name ltSTDINgt enclose STDIN with input
  operator
• print What is your age?
• age ltgt read using input operator
• chop (name) remove the \n from the string
• Read multiple inputs into arrays
• _at_all ltSTDINgt

12
getc

• Gets a single character into a string from a file
  or STDIN
• getc(FILEHANDLE)
• getc FILEHANDLE
• getc
• Examples
• print Answer y or n
• answer getc
• printf(You say s\n,answer)

13
argv

• _at_ARGV is the argument array
• !/usr/bin/perl
• printf(Number of args d\n,ARGV1)
• print _at_ARGV\n
• a ARGV0
• b ARGV1
• c ARGV2
• printf(The name of the scripts is s\n,0)
• printf(Argument s s s\n,a,b,c)

14
Array Functions

• chop(LIST)
• chops off last char of each string in the list
• pop(ARRAY)
• pops off last element of the array
• push(ARRAY,X)
• similar to stack operation
• join(DELIMITER, LIST)
• joins the elements of the array into a single
  string and separates each one by the delimiter
• split(DELIMITER,EXP)
• splits up a string by some delimiter and returns
  an array

15
Special Arrays

• The ENV contains environment variables
• the key is the name of the environment variable
• the value is what is assigned to it
• Example
• foreach key (keys(ENV))
• printkey\n
• print Your login name ENVLOGNAME\n
• pwd ENVPWD
• print pwd

16
Operators

• Most of the operators are borrowed from C
• Mixing data types
• x 12!! 4\n strings are converted
  into a number
• print x
• y ZAP . 5.5 concatenate two strings
• Output
• 16
• ZAP5.5
• Perl tests whether operator expects string or
  number.
• If numeric operator, then perl converts string to
  floating point value. Leading whitespace or
  trailing non-numeric characters, they will be
  ignored. Converts to zero if it cant figure out
  what to do.

17
Operators

• Assignment operators
• ,,-,ltlt,
• Relational operators
• numeric gt,gt,lt,lt,,!,ltgt
• string gt,ge,lt,le,eq,ne,cmp
• Logical operators
• ,,!
• x ? y z
• String operators and functions
• s1 . s2 s1 x num
• substr(str1,offset,len) length(EXPR)
• index(str,substr)

18
Conditional Constructs

• if (expr) block
• if (expr) block else block
• if (expr) block elseif (expr) block else
  block
• unless (expr) block
• unless (expr) block else block
• unless (expr) block elseif (expr) block .
• while (expr) block eval expr before executing
  block
• until (expr) block
• do block while (expr) eval expr after
  executing block
• do block until (expr)

19
Loops

• for (expr1 exp2 exp3)block
• foreach var (array) block
• _ used if var is not present
• _at_color(red, green, blue)
• foreach (_at_color)
• print _
• _ black
• print \n_at_color
• red green blue
• black black black

20
Command line value and loops

• Example
• print "ARGV is the subscript of the ",
• "last command arg.\n"
• Iterate on numeric subscript 0 to ARGV
• for (i0 i lt ARGV i)
• print "Argument i is ARGVi.\n"
• A variation on the preceding loop
• foreach item (_at_ARGV)
• print "The word is item.\n"
• A similar variation, using the
• "Default Scalar Variable" _
• foreach (_at_ARGV)
• print "Say _.\n"

• Demonstration
• gt perl example5.pl Gooood evening, Class!
• 2 is the subscript of the last command arg.
• Argument 0 is Gooood.
• Argument 1 is morning,.
• Argument 2 is Class!.
• The word is Gooood.
• The word is morning,.
• The word is Class!.
• Say Gooood.
• Say morning,.
• Say Class!.

21
Standard I/O

• print STDOUT "Tell me something "
• while (input ltSTDINgt)
• print STDOUT "You said, quote input
  endquote\n"
• chop input
• print STDOUT "Without the newline
  input endquote\n"
• if (input eq '') print STDERR
  "Null input!\n"
• print STDOUT "Tell me more, or D to
  end\n"
• print STDOUT "That's all!\n"
• Note 1 The while statement's condition is an
  assignment statement assign the next record from
  standard input to the variable input. On end of
  file, this will assign not a null value but an
  "undefined" value. An undefined value in the
  context of a condition evaluates to "false".
• Note 2 Data records are by default terminated by
  a newline character "\n" which in the above
  example is included as the last character of
  variable input. The "chop" function removes the
  last character of its argument. Perl 5 introduces
  a "chomp" function that removes the last
  characters of a variable only if they are the
  currently defined end-of-record sequence, which
  is defined in the special variable /.

22
File I/O

• !/usr/local/bin/perl
• Function Reverse each line of a file
• 1 Get command line values
• if (ARGV !1)
• die "Usage 0 inputfile outputfile\n"
• (infile,outfile) _at_ARGV
• if (! -r infile)
• die "Can't read input infile\n"
• if (! -f infile)
• die "Input infile is not a plain file\n"
• 2 Validate files
• Or statements "" short-circuit, so that
• if an early part evaluates as true, Perl
• doesn't bother to evaluate the rest.
• Here, if the file opens successfully,
• we don't abort
• open(INPUT,"ltinfile")
• die "Can't input infile !"

• if ( -e outfile)
• print STDERR "outfile exists!\n"
• until (ans eq 'r' ans eq 'a'ans eq 'e'
  )
• print STDERR "rplc, append,or exit? "
• ans getc(STDIN)
• if (ans eq 'e') exit
• if (ans eq 'a') mode'gtgt'
• else mode'gt'
• open(OUTPUT,"modeoutfile")
• die "Can't output outfile !"
• 3 read reverse each line, output it.
• while (ltINPUTgt)
• chop _ _ reverse _
• print OUTPUT _,"\n"
• 4 Done!
• close INPUT,OUTPUT

23
Pipe I/O and System Calls

• !/usr/local/bin/perl
• Report on disk usage under specified files
• The Unix command "du -sk ..." (on BSD Unix, "du
  -s ...")
• produces a series of lines
• 1942 bin
• 2981 etc
• listing the K bytes used under each file or
  directory. It doesn't show other
• information, such as the modification date or
  owner. This program gets du's kbytes
• and filename, and merges this info with other
  useful information for each file.
• files join(' ',_at_ARGV)
• The trailing pipe "" directs command output
  into our program
• if (! open (DUPIPE,"du -sk files sort -nr
  "))
• die "Can't run du! !\n"
• printf "8s -8s -16s 8s s\n",
  'K-bytes','Login','Name','Modified','File'
• while (ltDUPIPEgt) parse the du info
• (kbytes, filename) split
• Call system to look up file info like
  "ls" does
• (dev,ino,mode,nlink,uid,gid,rdev,
  size,atime,mtime,ctime)

24
Pipe I/O and System Calls (conti.)

• Call system to associate login name with uid
• if (uid ! previous_uid)
• (login,passwd,uid,gid,quota,comment,r
  ealname,dir,shell)
• getpwuid(uid)
• previous_uid uid
• Convert the modification-time to readable
  form
• (sec,min,hour,mday,mon,myear)
  localtime(mtime)
• mmonth mon1
• printf "8s -8s -16s 02s-02d-02d s\n",
• kbytes, login,
  realname, myear, mmonth, mday, filename
• while loop
• Demonstration Output
• K-bytes Login Name
  Modified File
• 40788 c527100 Fred Flintstone 95-10-05
  c527100
• 32685 c565060 Peter Parker 95-10-05
  c565060
• 24932 c579818 Clark Kent 95-10-06
  c579818
• 15388 c576657 Lois Lane 95-10-06
  c576657
• 9462 c572038 Bruce Wayne 95-10-06
  c572038

25
Regular Expressions

• What is a regular expression (in Perl)?
• a sequence or pattern of characters that is
  matched against a string of text when performing
  searches and replacements
• Pattern binding operators
• var /EXP/
• true if var contains EXP
• var !/EXP/
• !(var EXP)
• VARs/old/new/
• substitue
• name Tommy Tuttle
• print Hello Tommy\n if name /Tom/
• print name\n if name ! /Tom/
• name s/T/M/
• print name.\n

26
Regular Expressions

• . match any character except newline
• a-z0-9 match any single char in set
• a-z match any single char not in set
• \d match one digit
• \D same as 0-9
• \w match an alphanumeric (word) character
• \W match a non-alphanumeric character
• \s match whitespace
• \S match non whitespace
• match beginning of line
• match end of line
• X? match 0 or 1 X
• X match 0 or more Xs
• X match 1 or more Xs
• Xn,m match at least n but not more than m Xs
• abc match a,b,c respectively
• waswerewill match was, were or will

27
m and s operator

• Searches a string for a pattern match
• m/pattern/i /pattern/i
• m is optional, i is case sensitive
• Substitutions
• s/old/new/ s/old/new/g
• g stands for global change on a line
• Examples
• cat tmp.file
• John Doe
• Jane Doe
• Frank Smith
• perl -ne s/Doe/Francois/ print tmp.file
• John Francois
• Jane Francois
• Frank Smith

28
tr and y operators

• Translates characters on a one-to-one
  correspondence, returns the number of characters
  replaced or deleted
• tr/oldlist/newlist/ y/oldlist/newlist/ (for
  sed users)
• ARGV1y/A-Z/a-z/ upper to lowercase
• cnt tr/// count the stars in _
• cnt tr/0-9// count the digits
• tr/a-z/ / replace lowercase chars with space