Writing short utilities in other languages

C / C++

You can of course write short command-line-driven utilities and CGI scripts in any programming language.

Compiling a C++ program in UNIX / Linux

 edit prog.cxx
 g++ prog.cxx

This creates a binary executable file called a.out
To make the output a file called prog do this:

 g++ prog.cxx -o prog

You may need to: chmod +x prog
Then run:

 prog (args)

g++ - GNU C++ compiler
gcc - GNU C-only compiler
CC - Sun C++ compiler
cc - Sun C-only compiler
cxx - DEC C++ compiler

Accessing the command-line from C++

In a HLL, calling other programs (and access to the command-line in general) is more awkward than in a command-line-oriented script.
In C/C++ you use the system() call:

  #include <stdlib.h>

  main()
  {
   system ( "rm file7.txt" );
  }

which is obviously more complex than the Shell:

  rm file7.txt

You also have to compile the program, and keep track of 2 files - the source and the binary. In Shell, there is only 1 file. Also, the above is alright if filenames are static. But consider where the file name is variable. In Shell:

  for i in 96 97 98 99 00
  do
   rm $i.log $i.txt
  done

In C++ this is much more complex:

  #include <stdio.h>
  #include <stdlib.h>

  main()
  {
   char buf [ 30 ];

   for ( int i=96; i<=99; i++ )
   {
    sprintf ( buf, "rm %d.log %d.txt", i, i );
    system ( buf );
   }

   system ( "rm 00.log 00.txt" );
  }

And if you want to get directory listings you really want to use Shell.
Q. How would you code the following Shell script in C++?

  for i in */*doc */*xls
  do
   cp $i $HOME/backups/$i
  done

Access to environment variables is also more awkward in the HLL:

  #include <stdlib.h>

  char *homestring = getenv ( "HOME" );

Disadvantages of C++ compared to shell (for writing short utilities):
1. Constructing command-lines in general, with environment variables, variable and wildcard filenames, and piping and redirection, is more cumbersome.
2. The programs have to be recompiled any time you move to a new system.
Advantages of C++ compared to shell:
1. Execution speed much faster.
2. High-level programming features - types, data structures, objects, classes.
  Shell is only useful for simple programs.

Data structures and arithmetic in C++ v. Shell

On the plus side of course, is that sophisticated data structures exist in the HLL and not in Shell.
For instance, there are no types in Shell (or at least the "vanilla" shells). You cannot do:

  int i=0;

  if ( condition ) i++;

All you have in Shell are simple TAG=VALUE pairs of text strings. So you can only set simple flags:

 flag=0

 if condition
 then
  flag=1
 fi

Arithmetic in Shell

Although in fact there is a way to do arithmetic comparisons. For instance, test if the argument is less than 50:

 if test $1 -lt 50

And using the "expr" command, you can do arithmetic:


# x = $1 + $2

x=`expr $1 + $2`

So in fact, for the flag above, we can write the following in Shell:


flag=0
echo $flag

while test $flag -lt 30
do
 flag=`expr $flag + 1`
 echo $flag
done

which is equivalent to the following in C++:

int flag = 0;
cout << flag << "\n";

while ( flag < 30 )
{
 flag++;
 cout << flag << "\n";
}

The C++ program will run a lot faster of course!

So despite the fact that Shell environment variables are only text strings, and have no types, we can use other programs (test, expr) that interpret their text string arguments in certain ways, and so we can use them as numeric types after all.

But this is only the start of it of course. In C++, you also have large data types like arrays, recursive function calls, object-oriented classes with inheritance, libraries of useful functions, and all the other equipment of a HLL. In Shell, you need to construct your functionality by piping together lots of tools at the command-line. This can get very slow and cumbersome for large, complex programs.

And obviously for any serious application (for instance, anything with a windowed user interface, or anything with threads) you turn to a HLL.

Question - Why does the C++ program run a lot faster?

Hint:

$ which expr
/bin/expr

$ ls -l /bin/expr
-r-xr-xr-x   1 bin      bin        20988 May  3  1996 /bin/expr

flag++;
= say 3 machine instructions (what 3?)
30 times this = 90 machine instructions.

flag=`expr $flag + 1`
= say 300 machine instructions (why?)
30 times this = 9000 machine instructions.

Arithmetic in bash

bash has some nicer syntax:

#!/usr/bin/bash

for (( i=1 ; i <= 10 ; i++ ))
do
 echo $i
done

j=5		 
echo $j
(( j++ ))
echo $j
(( j = $j + 1 ))
echo $j

Arithmetic Evaluation (see also "let")

Data types and arithmetic in Korn shell

#!/usr/bin/ksh

integer i

i=1
echo $i
i=i+10
echo $i
i=i-4
echo $i
i=i*4
echo $i
i=i/2
echo $i

Conclusion

I tend to write my utilities in Shell if possible. For the more complex utilities, I move up to C++. For the simpler utilities I try to express them as aliases (see below).

Often, I use both. I surround a C++ utility with a small Shell wrapper that prepares the filenames and environment variables, calls the C++ program, and then possibly does some processing of the output.

In my CGI scripts, I surround Shell utilities with C++ input pre-processing wrappers.

C++ Reference

Perl

Perl is an interpreted language designed to give much of the functionality of a language like C++ in the interpreted world of Shell - with direct access to the command-line.

Perl is popular in particular for CGI scripts (but by no means necessary).

Perl

aliases

For short one-line "scripts" or command-lines, instead of a shell script it is more efficient to use aliases in your .cshrc / .bashrc file.

Summary - What language should I use?

I want to customise my system, and automate many tasks.
Like any programmer, I am always starting to write programs. How should I approach writing small custom utilities?

Very simple customisation - Check out program preferences or command-line arguments.
1-liner utilities - aliases
Command-line utilities with some logic - Shell
Complex command-line utilities - Perl
Small applications doing lots of calculations - C++ (or HLL of your choice)
Complex applications - Before investing a load of time in writing it yourself, maybe look online for freeware, shareware, or even a full commercial product.

Packages to allow shell scripts pop up GUI dialogs (and retrieve results):

Windows command line

The DOS command-line on Microsoft Windows also has a scripting language. You put your commands in a "batch file" with a name like PROG.BAT, and then to run it type PROG.

Some things you can do:

There are pipes:
```
 dir | find "string" | sort
```
Redirection:
```
 dir > file
```
Command-line arguments, and string compare:
```
  if '%1'=='' echo No arg.
```
Some equivalents of UNIX utilities (but rather few, and with usually a lot less functionality):
```
grep - FIND
sort - SORT
```

FOR loops:

  for %%i in (*.html) do call secondprog %%i

Environment variables:


set myvar=string
echo myvar is %myvar%  

  echo path is %path%

  copy %1 %homepath%\backup

multiple statements in block:


@echo off
rem the above line prevents the script echoing back the commands
rem is a comment

for %%i in (*.*) do (
  echo %%i
  dir %%i
)

IF and ELSE:


IF EXIST "C:\Program Files\Mozilla Firefox" (
  echo Firefox exists

) ELSE (
  echo Firefox missing
)

CALL of another batch file

But the scripting language is a bit more primitive than Shell, and the standard utilities are fewer, and have less options, than on UNIX/Linux systems.
People who like command lines tend to migrate to UNIX/Linux.

Similar to UNIX, on DOS/Windows you may or may not make use of this programmable command-line (you can survive without ever going near it). If you use it you might adopt a similar policy - write your utilities if possible as batch files, and only turn to a HLL for the more complex utilities.

For example:

Earlier in my career, I spent most of my time on DOS/Windows systems, and wrote all my utilities in a combination of DOS BAT files and Pascal EXE's.
For the last number of years, I have spent most of my time on UNIX systems, and have a similar system, with all my utilities in a combination of Shell scripts and C++ binaries.

You may prefer to use a combination of Perl scripts and Java programs (respectively). The principle is the same.

DOS batch files (and here)
List of DOS commands
UnixHelp
- DOS to UNIX quick reference
Advanced Bash-Scripting Guide
- DOS BAT syntax v. UNIX shell syntax
Also:
- UNIX clear - DOS cls
- UNIX pwd - DOS cd (with no argument)
Change prompt:
- prompt $p$g$s
- prompt switches
More powerful Windows command-line:
- Windows PowerShell - now installed by default on Windows 7.
- Windows Script Host - write scripts in JScript (Microsoft JavaScript) and VBScript
- Windows Resource Kits - many command-line tools.

Missing DCU student