+.\" @(#)u4 4.1 (Berkeley) %G%
+.\"
+.SH
+IV. PROGRAMMING
+.PP
+There will be no attempt made to teach any of
+the programming languages available
+but a few words of advice are in order.
+One of the reasons why the
+.UC UNIX
+system is a productive programming environment
+is that there is already a rich set of tools available,
+and facilities like pipes, I/O redirection,
+and the capabilities of the shell
+often make it possible to do a job
+by pasting together programs that already exist
+instead of writing from scratch.
+.SH
+The Shell
+.PP
+The pipe mechanism lets you fabricate quite complicated operations
+out of spare parts that already exist.
+For example,
+the first draft of the
+.UL spell
+program was (roughly)
+.P1
+.ta .6i 1.2i
+cat ... \f2collect the files\f3
+| tr ... \f2put each word on a new line\f3
+| tr ... \f2delete punctuation, etc.\f3
+| sort \f2into dictionary order\f3
+| uniq \f2discard duplicates\f3
+| comm \f2print words in text\f3
+ \f2 but not in dictionary\f3
+.P2
+More pieces have been added subsequently,
+but this goes a long way
+for such a small effort.
+.PP
+The editor can be made to do things that would normally
+require special programs on other systems.
+For example, to list the first and last lines of each of a
+set of files, such as a book,
+you could laboriously type
+.P1
+ed
+e chap1.1
+1p
+$p
+e chap1.2
+1p
+$p
+.ft R
+etc.
+.P2
+But you can do the job much more easily.
+One way is to type
+.P1
+ls chap* >temp
+.P2
+to get the list of filenames into a file.
+Then edit this file to make the necessary
+series of editing commands
+(using the global commands of
+.UL ed ),
+and write it into
+.UL script .
+Now the command
+.P1
+ed <script
+.P2
+will produce
+the same output as the laborious hand typing.
+Alternately
+(and more easily),
+you can use the fact that the shell will perform loops,
+repeating a set of commands over and over again
+for a set of arguments:
+.P1
+for i in chap*
+do
+ ed $i <script
+done
+.P2
+This sets the shell variable
+.UL i
+to each file name in turn,
+then does the command.
+You can type this command at the terminal,
+or put it in a file for later execution.
+.SH
+Programming the Shell
+.PP
+An option often overlooked by newcomers
+is that the shell is itself a programming language,
+with variables,
+control flow
+.UL if-else , (
+.UL while ,
+.UL for ,
+.UL case ),
+subroutines,
+and interrupt handling.
+Since
+there are
+many building-block programs,
+you can sometimes avoid writing a new program
+merely by piecing together some of the building blocks
+with shell command files.
+.PP
+We will not go into any details here;
+examples and rules can be found in
+.ul
+An Introduction to the
+.ul
+.UC UNIX
+.IT Shell ,
+by S. R. Bourne.
+.SH
+Programming in C
+.PP
+If you are undertaking anything substantial,
+C is the only reasonable choice of programming language:
+everything in
+the
+.UC UNIX
+system
+is tuned to it.
+The
+system
+itself
+is written in C,
+as are most of the programs that run on it.
+It is also a easy language to use
+once you get started.
+C is introduced and fully described in
+.ul
+The C Programming Language
+by
+B. W. Kernighan and D. M. Ritchie
+(Prentice-Hall, 1978).
+Several sections of the manual
+describe the system interfaces, that is,
+how you do I/O
+and similar functions.
+Read
+.ul
+UNIX Programming
+for more complicated things.
+.PP
+Most input and output in C is best handled with the
+standard I/O library,
+which provides a set of I/O functions
+that exist in compatible form on most machines
+that have C compilers.
+In general, it's wisest to confine the system interactions
+in a program to the facilities provided by this library.
+.PP
+C programs that don't depend too much on special features of
+.UC UNIX
+(such as pipes)
+can be moved to other computers that have C compilers.
+The list of such machines grows daily;
+in addition to the original
+.UC PDP -11,
+it currently includes
+at least
+Honeywell 6000,
+IBM 370,
+Interdata 8/32,
+Data General Nova and Eclipse,
+HP 2100,
+Harris /7,
+VAX 11/780,
+SEL 86,
+and
+Zilog Z80.
+Calls to the standard I/O library will work on all of these machines.
+.PP
+There are a number of supporting programs that go with C.
+.UL lint
+checks C programs for potential portability problems,
+and detects errors such as mismatched argument types
+and uninitialized variables.
+.PP
+For larger programs
+(anything whose source is on more than one file)
+.UL make
+allows you to specify the dependencies among the source files
+and the processing steps needed to make a new version;
+it then checks the times that the pieces were last changed
+and does the minimal amount of recompiling
+to create a consistent updated version.
+.PP
+The debugger
+.UL adb
+is useful for digging through the dead bodies
+of C programs,
+but is rather hard to learn to use effectively.
+The most effective debugging tool is still
+careful thought, coupled with judiciously placed
+print statements.
+.PP
+The C compiler provides a limited instrumentation service,
+so you can find out
+where programs spend their time and what parts are worth optimizing.
+Compile the routines with the
+.UL \-p
+option;
+after the test run, use
+.UL prof
+to print an execution profile.
+The command
+.UL time
+will give you the gross run-time statistics
+of a program, but they are not super accurate or reproducible.
+.SH
+Other Languages
+.PP
+If you
+.ul
+have
+to use Fortran,
+there are two possibilities.
+You might consider
+Ratfor,
+which gives you the decent control structures
+and free-form input that characterize C,
+yet lets you write code that
+is still portable to other environments.
+Bear in mind that
+.UC UNIX
+Fortran
+tends to produce large and relatively slow-running
+programs.
+Furthermore, supporting software like
+.UL adb ,
+.UL prof ,
+etc., are all virtually useless with Fortran programs.
+There may also be a Fortran 77 compiler on your system.
+If so,
+this is a viable alternative to
+Ratfor,
+and has the non-trivial advantage that it is compatible with C
+and related programs.
+(The Ratfor processor
+and C tools
+can be used with Fortran 77 too.)
+.PP
+If your application requires you to translate
+a language into a set of actions or another language,
+you are in effect building a compiler,
+though probably a small one.
+In that case,
+you should be using
+the
+.UL yacc
+compiler-compiler,
+which helps you develop a compiler quickly.
+The
+.UL lex
+lexical analyzer generator does the same job
+for the simpler languages that can be expressed as regular expressions.
+It can be used by itself,
+or as a front end to recognize inputs for a
+.UL yacc -based
+program.
+Both
+.UL yacc
+and
+.UL lex
+require some sophistication to use,
+but the initial effort of learning them
+can be repaid many times over in programs
+that are easy to change later on.
+.PP
+Most
+.UC UNIX
+systems also make available other languages,
+such as
+Algol 68, APL, Basic, Lisp, Pascal, and Snobol.
+Whether these are useful depends largely on the local environment:
+if someone cares about the language and has worked on it,
+it may be in good shape.
+If not, the odds are strong that it
+will be more trouble than it's worth.
projects / unix-history / commitdiff