Shell Script & Perl Programming

Shell Script &Perl Programming

Shell Programming • A shell script is a text file that contains Linux (UNIX) commands, which you enter using any standard editor. • You can then execute the commands in the file by using the filename as an argument to any dot (.) command.

Our first shell script $ vi helloscript i# this is a simple script echo “hello world” <esc> :w :q $ ./helloscript hello world $

Perl Variables • The primary distinction in Perl is not between data types (e.g. string, integer, real) but rather between “singular” and “plural” data. • Singular (scalar): strings and numbers • Plural (array): lists of strings or numbers • Variables do not need to be predefined

Variable example $welcome = “Hello world \n”; #set a variable print $welcome #print the variable Notice that - we don’t predefine the variable - the $ tells Perl that this is a scalar variable

Perl variable types Type Character example Is a name for… Scalar $ $cents An individual value (number or string) Array @ @large A list of values, keyed by number Hash % %interest A group of values, keyed by string Subroutine & &calcval A “callable” piece of Perl code Typeglob * *struck Everything named struck

Another Variables Example $ greeting=“hello” $ echo $greeting hello $ echo greeting greeting $ The set command will let you view all of your currently defined variables.

Some interesting variables • Use the set command to see some of your defined variables (e.g. after you assign the value to “greeting” in previous slide). • Also check out some other important variables (type “set | more”) • PATH • PS1

Starting to customize your settings • In your home directory (“/home/<username>) create a file called ‘.bash_profile’ • Insert into this two lines: • PATH=/usr/local/sbin:/usr/bin:$PATH:$HOME/bin:. • PS1=‘> ‘ • The first tells the system where to look for programs (commands) to execute, the second sets your prompt • Default probably PS1=‘\s-\v\$ ‘, use set to see • \s tells it to print what shell you are using • \v tells it to print what version • $ is the prompt you are telling it to use

Another variable example > school=“UTSA” > whereat=“you are at $school” > echo $whereat you are at UTSA > What would happen if > echo whereat whereat >

Variables and scripts > vi greetvar echo Please enter a greeting: read greeting echo “The greeting you entered was $greeting” #note optional quote > greetvar Please enter a greeting: howdy The greeting you entered was howdy >

Arguments to scripts > vi greetargs echo The first argument of $0 is: $1 echo the second argument is: $2 echo the third: $3 echo and finally: $4 > greetargs num1 number2 N3 “simply N4” The first argument of ./greetargs is: num1 the second argument is: number2 the third: N3 and finally: simply N4 > greetargs num1 num2 num3 The first argument of ./greetargs is: num1 the second argument is: num2 the third: num3 and finally: >

Some simple math > vi add1 let “num = $1” # quotes important let “num = num + 1” echo $num > add1 2 3 >

Some loops > vi loop3 again = 1 while let “again <= 3 “ # you need the let do echo $again howdy let “again = again + 1 “ done > loop3 1 howdy 2 howdy 3 howdy >

Conditional statements > vi myls if [ $1 = a ] # watch spaces, need after [ and before ] then ls -al # lists all files including hidden else ls -l # lists files in long format fi > myls a <lists files in long format including hidden> > myls x <list files in long format> > myls ./myls: [: =: unary operator expected <lists files in long format> >

Reading from a file > vi list Line1 Line2 Line3 end > > vi printlist read name while [ $name != end ] do echo $name read name done > printlist < list Line1 Line2 Line3 > printlist list #will have to ^c

Script for adding users THE FILE 500 Abarshay Jbell Abelvis end #this is a script to create a bunch of accounts read grpid read name let "userid = grpid + 1“ while [ "$name" != end ] do echo $name $userid $grpid adduser -u $userid -g $grpid -d /home/$name -p "" -r $name mkdir /home/$name chown $name /home/$name let "userid = userid + 1" read name done

Perl • Practical Extraction and Report Language • Designed to provide convenience of shell scripts and power and flexibility of a programming language. • Perl programs are interpreted and executed directly like shell scripts. • Similar in many respects to the C programming language • Freeware, available from Free Software Foundation (comes with Linux)

Our first Perl Program $ vi sample #!/usr/bin/perl $inputline = <STDIN>; # input text print ("$inputline"); # output the text $ ./sample test line test line $ - # symbol indicates comment, except #! in first line which indicates the location of the program interpreter. - notice each line ends with a semi-colon - variable treated in a similar manner as in shell scripts

Quotes and Escape Sequences #!/usr/bin/perl $x = "a string"; $y = "This is $x"; # becomes "This is a string” $z = 'This is $x'; # remains 'This is $x’ print ("$x\n"); print ("$y\n"); print ("$z\n"); print ('$z\n'); print ("\a\LFREDDIE \Umary\n"); print ("a quote \"in a string\"\n"); $ ./sample a string This is a string This is $x $z\nfreddie MARY a quote "in a string” $

Some more escape sequences \a Bell (beep) \b backspace \e escape \E cancel effect of \L, \U, \Q \L all following letters are lowercase \n newline \Q do not look for special pattern characters \r carriage return \t tab \U all following letters are uppercase \v vertical tab

Performing Math $ vi math #!/usr/bin/perl $a = 15; print ("$a\n"); $a = 4 + 5.1; print ("$a\n"); $a = 17 - 6.2; print ("$a\n"); $a = 2.1 * 6; print ("$a\n"); $a = 48 / 1.5; print ("$a\n"); $a = 2 ** 3; print ("$a\n"); $a = 21 % 5; print ("$a\n"); $a = - $a; print ("$a\n"); $ $ ./math 15 9.1 10.8 12.6 32 8 1 -1 $

Performing Comparisons 11.0 < 16 # less than 16 > 11 # greater than 15 == 15 # equals 11.0 <= 16 # less than or equal to 16 >= 11 # greater than or equal to 15 != 14 # not equal to $a || $b # logical OR; true if either is non-zero $a && $b # logical AND; true only if both are non-zero ! $a # logical NOT; true if $a is zero 4 <==> 1 # returns 1; 4>1 3 <==> 3.0 # returns 0; they are equal 1 <==> 4.0 # returns -1; 1<4

Performing Comparisons - strings “aaa” lt “bbb” # less than “bbb” gt “aaa” # greater than “aaa” eq “aaa” # equals “aaa” le “bbb” # less than or equal to “bbb” ge “aaa” # greater than or equal to “aaa” ne “bbb” # not equal to “aaa” cmp “bbb” # similar to <==>, returns 1; “aaa” < “bbb” “aaa” cmp “aaa” # returns 0 “bbb” cmp “aaa” # returns -1; “bbb” > “aaa” What would be returned for “40” lt “8”

Some assignment operators $a = 9; # most common assignment operator $a += 1; # equivalent to $a = $a + 1; $a -= 1; # same as $a = $a - 1; ++$a # same as $a += 1; $a++ # same as $a += 1; --$a # same as $a -= 1; $a-- # same as $a -= 1; be careful on the last four of these $a = 7; $b = ++$a; # $a and $b are both set to 8 $b = $a++; # $a is now 8, but $b is 7

Some cool things to do with strings The . operator joins the second operand to the first operand: $a = “be” . “witched”; # $a is now “bewitched” The x operator makes n copies of a string: $a = “t” x 5; # $a is now “ttttt” The =~ operator matches patterns and can substitute with s: $x =~ /jkl/ # returns true if “jkl” is in $x $val =~ s/abc/def/; #replaces abc with def The * character matches 0 or more, ? Matches zero or 1 copy /jk*l/ # matches jl, jkl, jkkl, jkkkl, … /jk?l/ # matches jl or jkl only

Lists and Arrays A list is a collection of values enclosed in parentheses. ( 1, 5.3, “hello”, 2) # contains four values () # an empty list Perl allows you to store lists in array variables @array = (1, 2, 3); # @ character specifies var as an array @x = (11, “my string”, 27.44); @x = $y; # @x is now a list of one element # the value of $y Obtaining the length of a list @x = (“string1”, “string2”, “string3”); $y = @a; # $y contains the LENGTH of @a

More about lists and arrays Array Slices @x = (1, 2, 3); @y = @x[0,1]; # @y gets slice of @x containing 2 # values (1, 2). NOTE, position 0 @x = (1, 2, 3); @x[4, 5] = (10, 20); # @x now has 6 elements, the # fourth being a null string @x = (10, 20, 30); $y = @x[1]; # $y now has the value 20 @x = (10, “hello”); $y = “Say @x[1] to your friends”;

More fun with lists and arrays Using the built-in function sort() @x = (“this”, “is”, “a”, “test”); @x = sort (@x); # @x is now (“a”, “is”, “test”, “this”) Note that sort() is an ALPHABETIC sort, thus @a = (100, 50, 2); @a = sort (@a); # @a is really (“100”, “2”, “50”) The function reverse() reverse the order @a = (“backwards”, “is”, “array”, “this”); @a = reverse (@a); # @a is now # (“this”, “array”, “is”, “backwards”)

Some final list and array functions To create a single string from a list or array variable use join() @x = join(“ “, “this”, “is”, “a”, “sentence”); The first element contains the character to “glue” the rest together, @x is thus “this is a sentence” You’d get the same results with @a = (“this”, “is”, “a”); @x = join (“ “, @a, “sentence”); To undo the effects of join(), use split() @y = “words::separated::by::colons”; @x = split(/::/, $y); # @x is now # (“words”, “separated”, “by”, “colons”)

Using Command-line arguments $ vi printfirstarg #!/usr/bin/perl Print(“the first argument is $ARGV[0]\n”); $ printfirstarg 1 2 3 The first argument is 1 $ Note that $ARGV[0], the first element of the $ARGV array variable, does NOT contain the name of the program. This is a difference between Perl and C.

Controlling program flow if ($x == 14) { print(“\$x is 14\n”); } Two way branching if ($x == 14) { print(“\$x is 14\n”); } else { print(“\$x is not 14\n”); }

Files To open a file, use the function open(); open(MYFILE, “/home/gbwhite/testfile”); The second argument is the name of the file you want to open. You can supply the full pathname or just the filename itself. By default, Perl assumes you will open a file for reading. If you want to open one for writing, put a > character in front of your filename: open(MYFILE, “>/home/gbwhite/writefile”); If you want to append instead of overwriting, use >>

More about handling files The open() function returns true if it succeeded, false otherwise. You can use the || (logical OR) function to test it: open(MYFILE, “/home/gbwhite/testfile”) || die(“unable to open /home/gbwhite/testfile\n”); This works because the right side of a logical OR only is executed if the left side is false.

More about files To read from a file, enclose the name in angle brackets: $line = <MYFILE>; This statement reads a line of input. To write to a file, print MYFILE (“this is a test\n”, “this too\n”); Closing a file: close(MYFILE);

Subroutines Subroutines can appear anywhere in the program but by convention generally go at the end. #!/usr/bin/perl $thecount = 0; &getwords; while ($words[0] ne “”) { # stop when line is empty for ($index = 0; $words[$index] ne “”; $index += 1) { $thecount +=1 if $words[$index] eq “the”; #above line really  if($words[$index] eq “the”) {$thecount +=1;} } &getwords; } print (“Total number of occurrences of the: $thecount\n”); sub getwords{ $inputline = <>; @words = split(/\s+/, $inputline); }

Reading from and Writing to Files • To access a file on a UNIX file system from within your Perl program, you must perform the following steps: • Your program must open the file. This tells the system that your Perl program wants to access the file. • The program can either read from or write to the file, depending on how you have opened the file. • The program can close the file. This tells the system that your program no longer needs access to the file.

Looping Constructs $ ./loops $x is now 1 $x is now 2 $x is now 3 $x is now 4 $x is now 5 and $x is now 6 and $x is now 5 and $x is now 4 and $x is now 3 and $x is now 2 and $x is now 1 $ $ vi loops #!/usr/bin/perl $x = 1; while ($x <= 5) { print ("\$x is now $x\n"); ++$x; } until ($x <= 0) { print ("and \$x is now $x\n"); --$x; } $

Arrays • To assign a value (or list of values) to an array, simply group them together. @weekdays = (“Mon”, “Tue”, “Wed”, “Thu”, “Fri”); • Arrays are zero-based like C. You could also have accomplished the above by: @weekdays[0] = “Mon”; @weekdays[1] = “Tue”; @weekdays[2] = “Wed”; @weekdays[3] = “Thu”; @weekdays[4] = “Fri”; • Another example @home[0] = “couch”; @home[1] = “chair”; @home[2] = “table”; @home[3] = “stove”;

Hashes • An unordered set of scalars, accessed by some string value associated with each scalar %longday = (“Mon”, “Monday”, “Tue”, “Tuesday”, “Wed”, “Wednesday”, “Thu”, “Thursday”, “Fri”, “Friday”); • Somewhat confusing to look at so Perl provides another way to initialize %longday =(“Mon” => “Monday”, “Tue” => “Tuesday”, “Wed” => “Wednesday”, “Thu” => “Thursday”, “Fri” => “Friday”);

Arrays –vs– Hashes

Let’s do a little Perl Review • Write a Perl program that will read in a set of scores from a file and calculate and print the average score. • The file is of the format: Fred 23 Mary 24 Terry 25

The code #!/usr/bin/perl open(GRADES, "grades") or die "Can't open grades\n"; $total = 0; $scores = 0; while ($line = <GRADES>) { ($student, $grade) = split(" ", $line); $total += $grade; $scores++; } close(GRADES); $average = $total / $scores; print "Average: $average\n";

STDIN and STDOUT #!/usr/bin/perl print STDOUT “Enter a number: “; $number = <STDIN>; print STDOUT “The number is $number.\n”; $ ./printnum Enter a number: 4 The number is 4 . $

Angle Operator <> • Encloses the file handle that you want to read lines from. • The empty angle operator, <>, will read lines from all the files specified on the command line. • If no files were specified on the command line then <> reads from STDIN.

Chop • Remember the extra line at the end of our output for the previous example. This happens because the line-reading operation does not automatically remove the newline from the input line. • The chop command will indiscriminately remove (and return) the last character of the string.

chop example #!/usr/bin/perl print STDOUT “Enter a number: “; $number = <STDIN>; chop($number) print STDOUT “The number is $number.\n”; $ ./printnum Enter a number: 4 The number is 4. $

Executing UNIX commands from Perl • The backtick( `) character is the command input operator. e.g. • $info = `finger root`; • Another way is to use system • system(“finger root”); • These two are not exactly the same as the first example places the result in variable $info.

pop and push • push (array, value-list) adds element(s) to the end of the array. • pop (array) removes the last element from the end of an array. #!/usr/bin/perl @stack = (“top”, “middle”, “bottom”); $var = pop(@stack); print “var = $var\n”; push(@stack, “next”); print “stack = @stack\n”); $./pop_push_example var = bottom stack = top middle next $

Some String Operations • substr(str, start-pos, length) returns a substring of the specified string, beginning at the start position for the length specified. • substr(str, start-pos, length) = string replace the specified section of the string with the assigned string. • length(str) find the length of a string • index(str, pattern) find the position of a pattern in a string

Shell Script & Perl Programming