Native shebang
Simple shebangs can help with scripting, e.g. "#!/usr/bin/env python" at the top of a Python script will allow it to be run in a terminal as "./script.py".
The task Multiline shebang largely demonstrates how to use "shell" code in the shebang to compile and/or run source-code from a 3rd language.
However in this task Native shebang task we are go native. In the shebang, instead of running a shell, we call a binary-executable generated from the original native language, e.g. when using C with gcc "#!/usr/local/bin/script_gcc" to extract, compile and run the native "script" source code.
Other small innovations required of this Native shebang task:
- Cache the executable in some appropriate place in a path, dependant on available write permissions.
- Generate a new cached executable only when the source has been touched.
- If a cached is available, then run this instead of regenerating a new executable.
Difficulties:
- Naturally, some languages are not compiled. These languages are forced to use shebang executables from another language, eg "#!/usr/bin/env python" uses the C binaries /usr/bin/env and /usr/bin/python. If this is the case, then simply document the details of the case.
- In a perfect world, the test file (e.g. echo.c) would still be a valid program, and would compile without error using the native compiler (e.g. gcc for text.c). The problem is that "#!" is syntactically incorrect on many languages, but in others it can be parsed as a comment.
- The "test binary" should be exec-ed and hence retain the original Process identifier.
Test case:
- Create a simple "script file" (in the same native language) called "echo" then use the "script" to output "Hello, world!"
ALGOL 68
Note: With Algol68G the option "-O3" will compile the script file to a ".so" file, this ".so" file is a binary executable and dynamically loaded library. Also note that this ".so" will only be generated if the ".a68" source file has been touched. File: echo.a68<lang algol68>#!/usr/bin/a68g --script #
- -*- coding: utf-8 -*- #
STRING ofs := ""; FOR i FROM 4 TO argc DO print((ofs, argv(i))); ofs:=" " OD</lang>Test Execution:
$ ./echo.c Hello, world!
Output:
Hello, world!
C
File: script_gcc.c <lang c>#!/usr/local/bin/script_gcc.sh /* Optional: this C code initially is-being/can-be boot strapped (compiled) using bash script_gcc.sh */
- include <errno.h>
- include <libgen.h>
- include <stdarg.h>
- include <stdio.h>
- include <stdlib.h>
- include <string.h>
- include <sys/stat.h>
- include <unistd.h>
/* the actual shebang for C target scripts is:
- !/usr/local/bin/script_gcc.c
- /
/* general readability constants */ typedef char /* const */ *STRING; typedef enum{FALSE=0, TRUE=1} BOOL; const STRING ENDCAT = NULL;
/* script_gcc.c specific constants */
- define DIALECT "c" /* or cpp */
const STRING
CC="gcc", COPTS="-lm -x "DIALECT, IEXT="."DIALECT, OEXT=".out";
const BOOL OPT_CACHE = TRUE;
/* general utility procedured */ char strcat_out[BUFSIZ];
STRING STRCAT(STRING argv, ... ){
va_list ap;
va_start(ap, argv);
STRING arg;
strcat_out[0]='\0';
for(arg=argv; arg != ENDCAT; arg=va_arg(ap, STRING)){
strncat(strcat_out, arg, sizeof strcat_out);
}
va_end(ap);
return strndup(strcat_out, sizeof strcat_out);
}
char itoa_out[BUFSIZ];
STRING itoa(int i){
sprintf(itoa_out, "%d", i); return itoa_out;
}
time_t modtime(STRING filename){
struct stat buf; if(stat(filename, &buf) != EXIT_SUCCESS)perror(filename); return buf.st_mtime;
}
/* script_gcc specific procedure */ BOOL compile(STRING srcpath, STRING binpath){
int out;
STRING compiler_command=STRCAT(CC, " ", COPTS, " -o ", binpath, " -", ENDCAT);
FILE *src=fopen(srcpath, "r"),
*compiler=popen(compiler_command, "w");
char buf[BUFSIZ];
BOOL shebang;
for(shebang=TRUE; fgets(buf, sizeof buf, src); shebang=FALSE) if(!shebang)fwrite(buf, strlen(buf), 1, compiler);
out=pclose(compiler); return out;
}
void main(int argc, STRING *argv, STRING *envp){
STRING binpath,
srcpath=argv[1],
argv0_basename=STRCAT(basename((char*)srcpath /*, .DIALECT */), ENDCAT),
*dirnamew, *dirnamex;
argv++; /* shift */
/* Warning: current dir "." is in path, AND * /tmp directories are common/shared */
STRING paths[] = {
dirname(strdup(srcpath)), /* not sure why strdup is required? */
STRCAT(getenv("HOME"), "/bin", ENDCAT),
"/usr/local/bin",
".",
STRCAT(getenv("HOME"), "/tmp", ENDCAT),
getenv("HOME"),
STRCAT(getenv("HOME"), "/Desktop", ENDCAT),
/* "/tmp" ... a bit of a security hole */
ENDCAT };
for(dirnamew = paths; *dirnamew; dirnamew++){
if(access(*dirnamew, W_OK) == EXIT_SUCCESS) break;
}
/* if a CACHEd copy is not to be kept, then fork a sub-process to unlink the .out file */
if(OPT_CACHE == FALSE){
binpath=STRCAT(*dirnamew, "/", argv0_basename, itoa(getpid()), OEXT, ENDCAT);
if(compile(srcpath, binpath) == EXIT_SUCCESS){
if(fork()){
sleep(0.1); unlink(binpath);
} else {
execvp(binpath, argv);
}
}
} else {
/* else a CACHEd copy is kept, so find it */
time_t modtime_srcpath = modtime(srcpath);
for(dirnamex = paths; *dirnamex; dirnamex++){
binpath=STRCAT(*dirnamex, "/", argv0_basename, OEXT, ENDCAT);
if((access(binpath, X_OK) == EXIT_SUCCESS) && (modtime(binpath) >= modtime_srcpath))
execvp(binpath, argv);
}
}
binpath=STRCAT(*dirnamew, "/", argv0_basename, OEXT, ENDCAT); if(compile(srcpath, binpath) == EXIT_SUCCESS) execvp(binpath, argv);
perror(STRCAT(binpath, ": executable not available", ENDCAT)); exit(errno);
}</lang>
Test Source File: echo.c <lang c>#!/usr/local/bin/script_gcc.c
- include <stdio.h>
- include <string.h>
- include <stdlib.h>
int main(int argc, char **argv, char **envp){
char ofs = '\0';
for(argv++; *argv; argv++){
if(ofs)putchar(ofs); else ofs=' ';
fwrite(*argv, strlen(*argv), 1, stdout);
}
putchar('\n');
exit(EXIT_SUCCESS);
}</lang>
Test Execution:
$ ./echo.c Hello, world!
Test Output:
Hello, world!
UNIX Shell
Note: this Native shebang task does not exactly apply to bash because bash is interpretive, but as a skeleton template the following script is an example of how compiled languages can implement the shebang. Also: this bash code can be used to automatically compile the C code in /usr/local/bin/script_gcc.c above.
File: script_gcc.sh <lang bash>#!/bin/bash
- Actual shebang when using bash:
- !/usr/local/bin/script_gcc.sh
- Alternative shebang when using bash:
- !/bin/bash /usr/local/bin/script_gcc.sh
- CACHE=No # to turn off caching...
- Note: this shell should be re-written in actual C! :-)
DIALECT=c # or cpp CC="gcc" COPTS="-lm -x $DIALECT" IEXT=.$DIALECT OEXT=.out
ENOENT=2
srcpath="$1"; shift # => "$@"
- basename="$(basename "$srcpath" ."$DIALECT")"
basename="$(basename "$srcpath")"
- Warning: current dir "." is in path, AND */tmp directories are common/shared
paths="$(dirname "$srcpath") $HOME/bin /usr/local/bin . $HOME/tmp $HOME $HOME/Desktop"
- /tmp
while read dirnamew; do
[ -w "$dirnamew" ] && break
done << end_here_is $paths end_here_is
compile(){
sed -n '2,$p' "$srcpath" | "$CC" $COPTS -o "$binpath" -
}
if [ "'$CACHE'" = "'No'" ]; then
binpath="$dirnamew/$basename-v$$$OEXT" if compile; then ( sleep 0.1; exec rm "$binpath" ) & exec "$binpath" "$@" fi
else
while read dirnamex; do
binpath="$dirnamex/$basename$OEXT"
if [ -x "$binpath" -a "$binpath" -nt "$srcpath" ];
then exec "$binpath" "$@"; fi
done << end_here_is
$paths end_here_is
binpath="$dirnamew/$basename$OEXT" if compile; then exec "$binpath" "$@"; fi
echo "$binpath: executable not available" 1>&2 exit $ENOENT
fi</lang> Test Source File: echo.c <lang c>#!/usr/local/bin/script_gcc.sh
- include <stdio.h>
- include <string.h>
- include <stdlib.h>
int main(int argc, char **argv, char **envp){
char ofs = '\0';
for(argv++; *argv; argv++){
if(ofs)putchar(ofs); else ofs=' ';
fwrite(*argv, strlen(*argv), 1, stdout);
}
putchar('\n');
exit(EXIT_SUCCESS);
}</lang>
Test Execution:
$ ./echo.c Hello, world!
Test Output:
Hello, world!