Shell Script

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Shell Script Perl Programming

• Lesson 9

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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.

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Our first shell script
vi helloscript
i this is a simple script echo hello
world w q
./helloscript hello world
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Variables
greetinghello echo greeting hello
echo greeting greeting
The set command will let you view all of your
currently defined variables.
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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

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Starting to customize your settings

• In your home directory (/home/) create
  a file called .bash_profile
• Insert into this two lines
• PATH/usr/local/sbin/usr/binPATHHOME/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

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Another variable example
schoolUTSA whereatyou are at school
echo whereat you are at UTSA
What would happen if echo whereat
whereat
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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
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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
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Some simple math
vi add1 let num 1 quotes
important let num num 1 echo num
add1 2 3
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Some loops
vi loop3 again 1 while let again you need the let do echo again howdy
let again again 1 done
loop3 1 howdy 2 howdy 3 howdy
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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 hidden myls x
myls ./myls unary operator expected files in long format
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Reading from a file
vi list Line1 Line2 Line3 end
vi printlist read name while name ! end
do echo name read name done
printlist printlist
list will have to c
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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
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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)

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Our first Perl Program
vi sample !/usr/bin/perl inputline
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
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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
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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 al
l following letters are uppercase \v vertical tab
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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
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Performing Comparisons
11.0 11 greater than 15
15 equals 11.0 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
41 3 3.0 returns 0 they are equal 1
4.0 returns -1 1 21
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 returns 0 bbb cmp aaa returns -1 bbb
aaa What would be returned for 40 lt 8
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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
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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 /jkl/ matches jl,
jkl, jkkl, jkkkl, /jk?l/ matches jl or jkl
only
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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 _at_array
(1, 2, 3) _at_ character specifies var as an
array _at_x (11, my string, 27.44) _at_x
y _at_x is now a list of one element
the value of y Obtaining the length of a
list _at_x (string1, string2, string3) y
_at_a y contains the LENGTH of _at_a
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More about lists and arrays
Array Slices _at_x (1, 2, 3) _at_y _at_x0,1 _at_y
gets slice of _at_x containing 2 values (1,
2). NOTE, position 0 _at_x (1, 2, 3) _at_x4, 5
(10, 20) _at_x now has 6 elements, the
fourth being a null string _at_x (10, 20,
30) y _at_x1 y now has the value 20 _at_x
(10, hello) y Say _at_x1 to your
friends
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More fun with lists and arrays
Using the built-in function sort() _at_x (this,
is, a, test) _at_x sort (_at_x) _at_x is now
(a, is, test, this) Note that sort() is
an ALPHABETIC sort, thus _at_a (100, 50, 2) _at_a
sort (_at_a) _at_a is really (100, 2,
50) The function reverse() reverse the
order _at_a (backwards, is, array,
this) _at_a reverse (_at_a) _at_a is now
(this, array, is, backwards)
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Some final list and array functions
To create a single string from a list or array
variable use join() _at_x join( , this, is,
a, sentence) The first element contains the
character to glue the rest together, _at_x is
thus this is a sentence Youd get the same
results with _at_a (this, is, a) _at_x
join ( , _at_a, sentence) To undo the effects
of join(), use split() _at_y wordsseparatedby
colons _at_x split(//, y) _at_x is now
(words, separated, by,
colons)
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Using Command-line arguments
vi printfirstarg !/usr/bin/perl Print(the
first argument is ARGV0\n)
printfirstarg 1 2 3 The first argument is 1
Note that ARGV0, the first element of the
ARGV array variable, does NOT contain the name
of the program. This is a difference between
Perl and C.
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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)

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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
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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.
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More about files
To read from a file, enclose the name in angle
brackets line 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)
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Subroutines
Subroutines can appear anywhere in the program
but by convention generally go at the
end. !/usr/bin/perl thecount
0 getwords while (words0 ne )
stop when line is empty for (index 0
wordsindex ne index 1) thecount
1 if wordsindex eq the above line
really ? if(wordsindex eq the) thecount
1 getwords print
(Total number of occurrences of the
thecount\n) sub getwords inputline
_at_words split(/\s/, inputline)
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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.

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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 x\n") --x