Shell Scripting Lab 2 Advanced Operating System Spring Advanced Operating System - Spring Lab 2

Today Objectives Review on Shell interpreter Know basic shell scripting (Tcl) Know cons and pros of shell scripting 2 Advanced Operating System - Spring Lab 2

Shell (review) Is a utility program with the Linux system that serves as an interface between the user and the kernel Plays an important role of command interpretation for the kernel 3 Advanced Operating System - Spring Lab 2

Remember: How the Shell works?

Shell (review) … cont. The shell as a Command Interpreter : Reads the command Locates the file in the directories containing utilities Loads the utility into memory Executes the utility 5 Advanced Operating System - Spring Lab 2

The shell creates a child shell for the execution of a utility The shell requests the kernel for any hardware interaction Shell (review) … cont. 6 Advanced Operating System - Spring Lab 2

Unix Shell There are many shells, some of them are Bourne Shell (sh/bash): sh is the executable filename for this shell C Shell (csh/tcsh ): csh is the executable filename for this shell Korn Shell: The executable filename is ksh Restricted Shell: Is typically used for guest logins To know the Type of your Shell write “echo $SHELL” The command to change your shell is ……… ?! 7 Advanced Operating System - Spring Lab 2

Shell Scripts Stores a sequence of frequently used Linux commands in a file Enables the shell to read the file and execute the commands in it Allows manipulation of variables, flow-of-control and iteration constructs that make programming possible 8 Advanced Operating System - Spring Lab 2

To run the script save it with any name (i.e. hello) Give it execute permission (chmod …) Then run it (i.e../hello) Hint : to know the path to your shell write on the shell  which shellname (i.e. which tclsh) Comment Commands ….. Hello World Example #!/bin/tclsh # My first script puts "Hello World!” Directory for the shell that will interpret the script 9 Advanced Operating System - Spring Lab 2

Exercise 1... (7 mins ) Open server.c and client.c, take 2 mins to understand what they do Check the IPC resources used (ipcs..) take notes how many message queues there Run server program in the background ( … &) Run the client program Enter the following three messages separately (message1, message2, message3) Repeat last 2 steps What happened ? And why ? Remove message queues (ipcrm) 10 Advanced Operating System - Spring Lab 2

Exercise 1... cont. Imagine that you repeat this task every time you run your program or your program crashes …. Boring and Timing Consuming isn’t it !! Don't you think a quick script could make our life easier :) 11 Advanced Operating System - Spring Lab 2

So Why Script Programming? A working knowledge of shell scripting is essential to understand the behavior of your system.  Consider that as a Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and set up services. Writing shell scripts is easy to learn, simple to make, easier to debug and faster (no compilation, write and run ). A shell script is a "quick and dirty" method of prototyping a complex application. Whenever you find yourself doing the same task over and over again you should use shell scripting 12 Advanced Operating System - Spring Lab 2

Scripting Language Scripting language like any other language, it has  Conditional statements (if …)  Loops (for, while, until, …)  Functions  Variables  Can do arithmetic operation  Very easy string handling ...etc As mentioned before it is much easier to learn, to use to debug but Take Care all comes at a cost Scripts are interpreted while normal language are pre-compiled which make them much more faster Also Scripts in general doesn't have such structures A good reference for bash scripting is Prog-Intro-HOWTO.html 13 Advanced Operating System - Spring Lab 2

Tcl (Tool Command Interpreter) Extensible You can easily add new commands and libraries to a Tcl interpreter. Embeddable You can use Tcl as a scripting language within another larger application. Orthogonal The syntax is simple and consistent - Tcl code is easy to write and also easy to read. Modern All the modern programming constructs are supported, including simple objects and namespaces. Multiplatform The Tcl interpreter has been ported to more platforms that Java. The same scripts can be run on everything from IBM Big-Iron to real-time embedded kernels. Powerful The Tcl architects have provided powerful and well-thought generalizations that make it fast and easy to write what would otherwise be complex applications. 14 Advanced Operating System - Spring Lab 2

15 Basic operations print to screen ( puts ) puts –nonewline "Hello, world!" puts "!!" assignment ( set ) set income puts "income is $income" (using '$' to get the value of a variable) mathematical expressions ( expr ) set a 10.0 expr $a + 5 expr int($a/3) Advanced Operating System - Spring Lab 2

16 Some useful commands unset : destroy a variable unset num info : check whether the named variable has been defined if {![info exists num]} { set num 0 } incr num Advanced Operating System - Spring Lab 2

17 Special characters # : single-line comments, similar to "//" in C ;# : in-line comments, just like "//" in C \ : escape character, same function as in C : also used to break a long line of code to two lines $ : get the value of a variable var : name of variable $var : value of variable [] : evaluate command inside brackets Advanced Operating System - Spring Lab 2

18 Control structures (1) if then else set income if {$income > 30000} { puts "$income -- high" } elseif {$income > 20000} { puts "$income -- middle" } else { puts "$income -- low" } while loops set i 0 while {$i < 100} { puts "I am at count $i" incr i } Advanced Operating System - Spring Lab 2

19 Control structures (2) for loops for {set i 0} {$i < 100} {incr i} { puts "I am at count $i and going up" } for {set i 100} {$i > 0} {set i [expr $i - 1]} { puts "I am at count $i and going down" } foreach loops set lstColors {red orange yellow green blue purple} foreach c $lstColors { puts $c } Advanced Operating System - Spring Lab 2

20 Control structures (3) foreach loops (con't) set lstColors {red orange yellow green blue purple} foreach {a b c} $lstColors { puts "$c--$b--$a" } set lstFoods {apple orange banana lime berry grape} foreach f $lstFoods c $lstColors { puts "a $f is usually $c" } foreach {a b} $lstFoods c $lstColors { puts "$a & $b are foods. $c is a color." } Advanced Operating System - Spring Lab 2

21 Procedures(1) procedure calls (embedded commands) set b [expr $a + 5] puts "The value of b is $b" create your own procedure (called by value only) proc foo {a b c} { return [expr $a * $b - $c] } puts [expr [foo 2 3 4] + 5] proc bar { } { puts "I'm in the bar procedure" } bar Advanced Operating System - Spring Lab 2

22 Procedures (2) procedure calls (variable number of arguments) proc sum {args} { set s 0 foreach arg $args { incr s $arg } return $s } puts [sum ] puts [sum 1 2] puts [sum 4] Advanced Operating System - Spring Lab 2

23 Procedures (3) procedure calls (implicit variable) proc power {a {b 2}} { if {$b == 2} { return [expr $a * $a] } set value 1 for {set i 0} {$i<$b} {incr i} { set value [expr $value * $a] } return $value } set v1 [power 5] set v2 [power 5 4] puts "5^2 is $v1" puts "5^4 is $v2" Advanced Operating System - Spring Lab 2

24 Variable scope local and global variables set a 5 set b 6 set c 7 proc var_scope { } { global a set a 3 set b 2 set ::c 1 } var_scope puts "The value for a b c is: $a $b $c" Advanced Operating System - Spring Lab 2

25 Variable scope local and global variables set a 5 set b 6 set c 7 proc var_scope { } { upvar a x set x 3 set b 2 set ::c 1 } var_scope puts "The value for a b c is: $a $b $c" Advanced Operating System - Spring Lab 2

Exercise 2... (30 min) 1. Write the function change, which takes a whole number and set of coins (integer numbers too) as its arguments and prints a list that represents how to "make change" for that number using the set of coins (or notes) for any decimal currency. first number is the whole number then followed by the set of coins. your function should have the minimum number of coins possible. (10 mins) proc change {n args} {...} Examples : change => change => change => Advanced Operating System - Spring Lab 2

Exercise Write the command vardefault, which takes two arguments, the name of a variable, and a default value. If the variable is defined in the calling scope, vardefault returns the variable's value. If it is not defined, vardefault returns the default value. (10 min) proc vardefault {var default} {...} Examples : set x 45 vardefault x 0 => 45 proc foo {} { Vardefault x 0 } foo => 0 To write this function, you'll need to use upvar to achieve call by name, and you'll need a way to test whether or not a variable is defined (use info exists). 27 Advanced Operating System - Spring Lab 2

Exercise Write the iota function, which takes a numeric argument n and returns a list of numbers of length n which are the numbers from 0 to n-1. the iota function can take an optional second argument which is an increment to separate the integer values. The default increment is 1. iota always returns a list whose length is equal to its first argument. (10 min) proc iota {n {inc 1}} {...} Examples : iota 10 => iota 10 3  Advanced Operating System - Spring Lab 2

Exercise 2 …. (optional) 4. Write the function bits which takes two parameters, both numbers, and returns a binary representation of the first number as a list of bits (each bit is a 0 or a 1), from most significant at the left to least siginifcant at the right. The result list should contain as many bits as specified by the second number, which should be an optional parameter, using leading zero bits if necessary (assume that the bit length requested is long enough to represent the number). If the second parameter isn't specified, use the minimal number of bits (no leading zeros). You'll probably want to use the expr command's bit-fiddling operators for this. proc bits {howmany num} {...} Examples bits 0777  bits 0644  bits 19  bits 1 3  Advanced Operating System - Spring Lab 2

30 Lists in Tcl Everything is a list! Many ways to create a list set myList [list a b c] set myList "a b c" set myList {a b c} set myList [list $a $b $c] set myList {$a $b $c} set myList [list a b c] set myList "a b c" set s Hello puts "The length of $s is [string length $s]." => The length of Hello is 5. puts {The length of $s is [string length $s].} => The length of $s is [string length $s]. Advanced Operating System - Spring Lab 2

31 List operations set lstStudents [list "Fan" "Kristy" "Susan"] puts [lindex $lstStudents 0] puts [lindex $lstStudents end] puts [llength lstStudents] (unexpected result!) puts [llength $lstStudents] lappend $lstStudents "Peter" (wrong!) lappend lstStudents "Peter" puts [linsert lstStudents 2 "Tom"] (wrong!) puts [linsert $lstStudents 2 "Tom"] set lstStudents [linsert $lstStudents 2 "Tom"] set lstStudents [lreplace $lstStudents 3 3 "Rachel"] set lstStudents [lreplace $lstStudents end end] set lstStudents [lsort –ascii $lstStudents] puts [lsearch $lstStudents "Peter"] Advanced Operating System - Spring Lab 2

32 Lists of lists (of lists…) set a [list [list x y z]] puts [lindex $a 0] puts [lindex [lindex $a 0] 1] puts [lindex [lindex $a 1] 0] (unexpected result) set a [list x [list [list y] [list z]]] => How to get to the z? set arg1 [list g [list f [list h [list i X]]] [list r Y] k] set arg2 [list g [list f [list h [list i Y]]] [list r b] L] set both [list $arg1 $arg2] puts $both Advanced Operating System - Spring Lab 2

33 Array operations Associative arrays (string as index) set color(rose) red set color(sky) blue set color(medal) gold set color(leaves) green set color(blackboard) black puts [array exists color] (tests if an array with the name "color" exists) puts [array exists colour] puts [array names color] (returns a list of the index strings) foreach item [array names color] { puts "$item is $color($item)" } (iterating through array) set lstColor [array get color] (convert array to list) array set color $lstColor (convert list to array) Advanced Operating System - Spring Lab 2

34 Regular expressions regsub set stmt "Fan is one of Shania’s fans" regsub –nocase "fan" $stmt "Kristy" newStmt ?switches? exp string subSpec ?varName? puts "$newStmt" regsub –nocase –all "fan" $stmt "Kristy" newStmt puts "$newStmt" regexp (returns 1 if the regular expression matches the string, else returns 0) puts [regexp –nocase "fan" $stmt] ?switches? regexp string format puts [format "%s is a %d-year-old" Fan 26] formatString ?arg arg...? Advanced Operating System - Spring Lab 2

35 String operations set statement " Fan is a student " set statement [string trim $statement] puts [string length $statement] puts [string length statement] puts [string index $statement 4] puts [string index $statement end] puts [string first "is" $statement] (string last) puts [string first $statement "is"] puts [string range $statement 4 end] puts [string replace $statement 9 end "professor"] puts [string match "*student" $statement] (* ? []) Advanced Operating System - Spring Lab 2

36 File operations set fRead [open source.txt r] set fWrite [open target.txt w] while {![eof $fRead]} { set strLine [gets $fRead] ;#or gets $fRead strLine regsub –nocase –all "fan" $strLine "kristy" strLine puts $fWrite $strLine } close $fRead close $fWrite ################ source.txt ################ Fan is a CSE student. Fan is also one of Shania’s fans. Kristy and Fan are classmates. Advanced Operating System - Spring Lab 2

37 Miscellaneous commands eval: execute a command dynamically built up in your program set Script { set Number1 17 set Number2 25 set Result [expr $Number1 + $Number2] } eval $Script exec: execute external programs Advanced Operating System - Spring Lab 2

38 Common pitfalls Missing $ or extraneous $ Using {a} vs "a" vs [list a] Creating list items that are empty lists a b {} d Advanced Operating System - Spring Lab 2

When not to use shell scripts need data structures, such as linked lists or trees need to generate or manipulate graphics or GUIs need direct access to system hardware. procedures involving heavy-duty math operations. resource-intensive tasks, especially where speed is a factor (sorting, hashing, etc.) complex applications, where structured programming is a necessity (need typechecking of variables, function prototypes, etc.) situations where security is important 39 Advanced Operating System - Spring Lab 2

Exercise 3 …. (60 min) 1.Write the function strlenlist, which takes one argument, a list of strings, and returns a list of equal length as a result, in which each element is the string length of the corresponding element of the argument list. (10 mins ) proc strlenlist {L} {...} Examples : strlenlist { } => strlenlist {foo bar antidisestablishmentarianism} => Advanced Operating System - Spring Lab 2

Exercise 3 …. (60 min) 2. Write the function choose, which takes three parameters: three equal-length lists, the first two of arbitrary values, and the third a list of boolean values (zeros and ones). choose returns a list (also of the same length) where each element is chosen from one of the first two lists, depending on the corresponding boolean value. Elements of the first parameter are chosen for 1's (true), while elements of the second parameter are chosen for 0's (false). choose is one of many useful functions that operate on bit vectors. (10 mins ) proc choose {trues falses bools} {...} Examples : choose [list a b c d] [list A B C D] [list ] => A b c D 41 Advanced Operating System - Spring Lab 2

Exercise 3 …. (60 min) 3. Write a function to find all lines containing a certain word in a file. It takes filename and the word to search for as an argument, and returns line number : the line this word occurs at proc findword { filename word} { …} Examples : ################ source.txt ################ Fan is a CSE student. Fan is also one of Shania’s fans. Kristy and Fan are classmates. findword source.txt is  1 : Fan is a CSE student  2 : Fan is also on of Shania’s fans. (Hint : it could be done with one line of code ) 42 Advanced Operating System - Spring Lab 2

Exercise 3 …. (60 min) A sequence of strings is generated as follows: the first two strings are given and each successive string is made by joining together the previous two strings (in their original order). For example, if the first string is abc and the second string is cde, the third string will be abccde and the fourth will be cdeabccde. The task for this question is to determine the ith character in the sequence of strings (not the ith string). In the previous example the 2nd character is b, the 5th is d, as is the 11th. The 13th character, which happens to be the 1st character of the 4th term, is c. (bonus) Input The first line will contain the first string, of between 1 and 10 characters, using lower case letters. The second line will contain the second string, again between 1 and 10 lower case letters. The third line will be a single integer i (1 <= i <= 2^30). Please note that there can be any number of sets of input, each consisting of the specified three lines of input. Sample input fibonacci revenge Output Output the ith character in each input sequence, separated by lines. Sample output e 43 Advanced Operating System - Spring Lab 2

44 Tcl references and resources Help file Search site: List of Tcl commands with man pages Tcl examples: All code in this tutorial (plus some) and expected output ftp://cslu.ece.ogi.edu/pub/kristina/ tutorial.tcl, tutorial_output.txt, source.txt Advanced Operating System - Spring Lab 2

45 Tcl editors Any editor nedit emacs TextPad TCL/TK (5) Advanced Operating System - Spring Lab 2

Other References Linux Shell Scripting Tutorial - A Beginner's handbook - Vivek G. Gite Advanced Bash-Scripting Guide: A complete guide to shell scripting, using Bash by Mendel Cooper CSCI6303 – Principles of I.T Advanced Operating System - Spring Lab 2