Programming in Unix

1
Programming in Unix

• An Overview of the Significant Programming Tools
  in Common Use

2
Many Tools Are Available

• Many tools are available for programming in Unix
• Some of the more common are
• Traditional compiled languages
• C, C
• Fortran
• Pascal
• Ada
• Assembler (not strictly compiled but)
• Interpreted Languages
• Java

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• Scripting Languages
• Shell (various flavors)
• Perl
• Python
• Tcl/Tk
• Expect
• Scheme

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Compiled Languages

• Most major vendors provide some, if not all, of
  the compiled languages
• Most are also available from GNU
• cc and as are the C compiler and assembler
  provided by most Unix vendors
• GNU provides
• gcc/g
• gas
• F2C
• P2C
• And also has an Ada front end

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• These languages are good for large, production
  code developments or system level programming
• C is the lingua franca of Unix
• The majority of Unix is written in C
• A very mature library for system calls is
  available
• Fortran is the language of scientific and numeric
  programming
• Pascal was a major teaching environment that has
  fallen somewhat out of favor
• Ada was the language of the defense industry for
  a number of years but is also falling out of favor

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Java

• Developed by James Gosling of Sun MicroSystems
• Originally designed for "internet appliances"
• Java is also supported by several vendors
  (primarily Sun)
• The JDK is freely available from the Web
• Borland/Inprise/Corel has also announced Java
  support for Unix platforms based on the Sun JDK
• Java is an object-oriented language that produces
  byte codes, which are then interpreted by a Java
  Virtual Machine (JVM) that is specific to the
  hardware/OS platform it is executing on

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• Java was designed for security
• Common phrase is "the Java sandbox"
• Pointers are not allowed
• Byte codes are validated prior to execution
• Memory is managed by the JVM using garbage
  collection to eliminate memory leaks
• Although original design was for internet
  appliances, Sun has since tried to promote Java
  as the "write once, run anywhere" Holy Grail of
  portable code

8
Perl

• Perl was designed by Larry Wall to help automate
  reporting tasks common to a system admin
• It is also freely available from the Web
• "Practical Extraction and Report Language"
• Pathologically Eclectic Rubbish Lister
• As time went on, Larry added bits and pieces to
  the language and finally released the source to
  the Web
• There it became immensely popular with Sys Admins
  and other scriptors.
• It was enhanced and ported to other systems

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• Perl is now available on most flavors of Unix,
  Windows, Mac, Amiga, Atari ST, VMS, OS/2, and
  MS-DOS
• Perl looks like a combination of sed, shell, and
  awk
• It is extremely useful for scripting sys admin
  tasks but can be hard to read
• Perl is also one of the most popular languages
  for writing CGI scripts
• Since one of its properties is "many ways to do
  anything" it can also be hard to learn

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Python

• Python is another scripting language, written by
  Guido van Rossum in 1990
• It is a portable, interpreted, object-oriented
  programming language influenced by C and Modula-3
• It has a small core language that includes basic
  data types and flow control along with higher
  level types such as
• Strings
• Lists
• Tuples
• Associative arrays
• This makes it fairly easy to learn

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• Object-oriented programming is supported with
  classes and multiple-inheritance
• Exception handling is provided for robust
  error/fault handling
• The real power in Python lies in its
  extensibility
• Python can be extended by writing modules in
  either Python or in compiled languages such as C
  or C
• These modules can define variables, functions,
  new data types and their methods, or simply
  provide a link to existing code libraries

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• The standard Python library includes modules for
  debuggers, profilers, Internet services, and
  graphical user interfaces
• Python is also portable
• It is available for
• Unix
• Windows 3.x
• Windows 95/98/NT
• Mac
• OS/2

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• Python is well suited for
• System administration tasks
• Developing graphical user interfaces
• Database programming
• Integrating other pre-existing components
• Rapid Application Development
• Python is NOT well suited for time critical
  components
• Python is freely available from the Web and has a
  broad support base

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Tcl/Tk

• Tool Command Language/Tool Kit
• Developed in late '80s by John Ousterhout at UC
  Berkeley
• Born out of a frustration with building
  (generally bad) command languages over and over
  for different design tools
• Tk evolved about the same way
• Driven by desire to build interactive graphical
  interfaces without a huge monolithic underpinning
• Took component approach
• Allows construction if interfaces by assembling
  groups of small components

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• Tcl is a scripting language designed for
  providing command language "front ends" for
  either existing tools or new developments
• Consists of a library of C function calls
• Available for free via the Web and supported by
  tens of thousands of developers
• Even though it is a library of C function calls,
  it is possible to create useable graphical
  interfaces consisting only of Tcl script

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• Tcl has many advantages
• Since it can be interpreted by the Tcl shell
  tclsh, it provides a rapid development
  environment
• No edit-compile-execute-debug cycle
• It also makes it easy for your apps to provide a
  powerful scripting language without having to
  write a lot of code
• It makes an excellent "glue language" for
  integrating multiple existing packages into one
  seamless application

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• Tk is a toolkit for the X window System
• It extends the Tcl core language with commands
  for building Motif-like graphical user interfaces
• Like Tcl, Tk is implemented as a library of C
  functions
• Individual programmers can extend the base Tk
  features with new user interface widgets written
  in C
• Tk can also be interpreted by the windowing shell
  wish

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Expect

• Expect is a Tcl based scripting language for
  automating interactive applications such as
• telnet
• ftp
• passwd
• fsck
• Expect makes it almost trivial to automate these
• Expect is also useful for testing interactive
  applications
• By using Tk with Expect, you can also wrap
  interactive applications with an X Window GUI

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• Expect was written by Don Libes in the late '80s
  and early '90s
• Expect is freely available from the Web
• It supports various flavors of Unix and Windows
• Expect is primarily useful for those situations
  where you "expect" something from a program and
  then provide a response

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Example Expect Script

• minimal script to read a web page
• match_max 100000
• set timeout -1
• spawn telnet host 80
• expect "Escape character is \n"
• send "GET page\r\n"
• expect
• puts "expect_out(buffer)"

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Scheme Shell (scsh)

• Scheme shell (not to be confused with Scheme, the
  full language) was written by a very strange
  gentleman named Olin Shivers in the early '90s
• It came about from a flame fest in which Shivers
  made the claim it would be easier to do shell
  programming from Scheme that to continue using
  shell
• The serious Unix folks kindly expressed their
  doubts so Shivers decided to prove them wrong!
• Probably only take a week or two to add some OS
  interfaces and a few macros
• 26 months later, Scheme shell was released

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• According to its author, scsh is pronounced
  "skishhhh"
• scsh is freely available via the Web as open
  source
• It runs on most Unix platforms without problem

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• scsh is a Unix shell that uses the Scheme
  language as its scripting language
• Provides the following features
• A complete POSIX interface
• Very complete support for networking, with high
  and low level interfaces
• An additional network package containing an HTTP
  server and SMTP support is also available
• Powerful string manipulation functions
• Pattern matching
• Filename manipulations
• Awk-like macros
• Regular expressions

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• Additionally, since scsh is based upon a "real"
  programming language, it has serious data
  structures and control structures

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Comparison of Several Scripts

• The following scripts, in sh, Perl, Python, and
  scsh, all print a list of all the executables in
  the current PATH to stdout

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sh

• !/bin/sh
• IFS''
• for d in PATH do
• for f in d/ do
• -x f -a ! -d f echo f
• done
• done

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Perl

• !/usr/local/bin/perl
• foreach dir (split(//, ENV'PATH'))
• opendir(DIR, dir) or die "can't opendir
  dir !"
• map -x "dir/_" ! -d "dir/_" print
  "_\n" readdir(DIR)
• closedir DIR

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Python

• !/usr/local/bin/python
• import os, string, stat
• for d in string.split(os.environ'PATH'
  , '')
• for f in os.listdir(d)
• mode os.lstat(d '/'
  f)stat.ST_MODE
• if not stat.S_ISDIR(mode)
• print f

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scsh

• !/usr/local/bin/scsh -s
• !
• (map (lambda (f) (display f) (newline))
• (apply append
• (map directory-files ((infix-splitter "")
  (getenv "PATH")))))