Editing Polyglot (computing) (section)

===C, PHP, and Bash===

Two commonly used techniques for constructing a polyglot program are to make use of languages that use different [[Character (computing)|characters]] for [[comment (computer programming)|comments]], and to redefine various tokens as others in different languages. These are demonstrated in this [[public domain]] polyglot written in [[ANSI C]], [[PHP]] and [[Bash (Unix shell)|bash]]:
{{columns-start|num=3}}
'''Highlighted for Bash'''
<syntaxhighlight lang="bash">
 #define a /*
 #<?php
 echo "\010Hello, world!\n";// 2> /dev/null > /dev/null \ ;
 // 2> /dev/null; x=a;
 $x=5; // 2> /dev/null \ ;
 if (($x))
 // 2> /dev/null; then
 return 0;
 // 2> /dev/null; fi
 #define e ?>
 #define b */
 #include <stdio.h>
 #define main() int main(void)
 #define printf printf(
 #define true )
 #define function
 function main()
 {
 printf "Hello, world!\n"true/* 2> /dev/null | grep -v true*/;
 return 0;
 }
 #define c /*
 main
 #*/
</syntaxhighlight>
{{column}}
'''Highlighted for PHP'''
<syntaxhighlight lang="PHP">
 #define a /*
 #<?php
 echo "\010Hello, world!\n";// 2> /dev/null > /dev/null \ ;
 // 2> /dev/null; x=a;
 $x=5; // 2> /dev/null \ ;
 if (($x))
 // 2> /dev/null; then
 return 0;
 // 2> /dev/null; fi
 #define e ?>
 #define b */
 #include <stdio.h>
 #define main() int main(void)
 #define printf printf(
 #define true )
 #define function
 function main()
 {
 printf "Hello, world!\n"true/* 2> /dev/null | grep -v true*/;
 return 0;
 }
 #define c /*
 main
 #*/
</syntaxhighlight>
{{column}}
'''Highlighted for C'''
<syntaxhighlight lang="c">
 #define a /*
 #<?php
 echo "\010Hello, world!\n";// 2> /dev/null > /dev/null \ ;
 // 2> /dev/null; x=a;
 $x=5; // 2> /dev/null \ ;
 if (($x))
 // 2> /dev/null; then
 return 0;
 // 2> /dev/null; fi
 #define e ?>
 #define b */
 #include <stdio.h>
 #define main() int main(void)
 #define printf printf(
 #define true )
 #define function
 function main()
 {
 printf "Hello, world!\n"true/* 2> /dev/null | grep -v true*/;
 return 0;
 }
 #define c /*
 main
 #*/
</syntaxhighlight>
{{columns-end}}

Note the following:
*A hash sign marks a [[preprocessor]] statement in C, but is a comment in both bash and PHP.
*"//" is a comment in both PHP and C and the [[root directory]] in bash.
*Shell [[Redirection (computing)|redirection]] is used to eliminate undesirable outputs.
*Even on commented out lines, the "<code>&lt;?php</code>" and "<code>?&gt;</code>" PHP indicators still have effect.
*The statement "<code>function main()</code>" is valid in both PHP and bash; C #defines are used to convert it into "<code>int main(void)</code>" at compile time.
*Comment indicators can be combined to perform various operations.
*"<code>if (($x))</code>" is a valid statement in both bash and PHP.
*<code>[[printf (Unix)|printf]]</code> is a bash [[shell builtin]] which is identical to the [[printf format string|C printf]] except for its omission of brackets (which the [[C preprocessor]] adds if this is compiled with a [[C compiler]]).
*The final three lines are only used by bash, to call the <code>main</code> function. In PHP the <code>main</code> function is defined but not called and in C there is no need to explicitly call the <code>main</code> function.