From mboxrd@z Thu Jan  1 00:00:00 1970
Message-Id: <200007301954.PAA29225@cse.psu.edu>
Subject: [9fans] pipefile
From: "rob pike" <rob@plan9.bell-labs.com>
Date: Sun, 30 Jul 2000 15:54:30 -0400
To: 9fans@cse.psu.edu
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I had an idea while trying to kill groundhogs today.
The command is called pipefile; it interposes a command
between you and the named file.  Options -r and -w
specify a command to interpose when reading or writing;
-b does both, but in that case the command must handle
bidirectional input; most standard commands such as
cat won't work.
Try this:

	pipefile -w 'tr a-z A-Z' /dev/cons
	rc -i < /dev/cons > /dev/cons >[2] /dev/cons

for a shell reminiscent of olden times.  The origin of the
idea was a way to support Asian input. Consider

	pipefile -r 'tr a-z A-Z' /dev/cons
	rio < /dev/cons > /dev/cons >[2] /dev/cons

I leave the rest to the interested Asian reader.

The command leaves the file in an odd state, and can only
work well on continuous files such as devices or with very
special commands interposed, but I offer it as an interesting
idea.

-rob


#include <u.h>
#include <libc.h>

#define	TEMP	"/n/temp"

void
usage(void)
{
	fprint(2, "usage: pipefile [-r command] [-w command] [-b command] file\n");
	exits("usage");
}

void
connect(char *cmd, int fd0, int fd1)
{
	switch(rfork(RFPROC|RFFDG|RFREND|RFNOWAIT)){
	case -1:
		sysfatal("fork %s: %r", cmd);
		break;
	default:
		return;
	case 0:
		if(fd0 != 0)
			dup(fd0, 0);
		if(fd1 != 1)
			dup(fd1, 1);
		execl("/bin/rc", "rc", "-c", cmd, nil);
		sysfatal("exec %s: %r", cmd);
		break;
	}
}

void
main(int argc, char *argv[])
{
	char *file;
	char *rcmd, *wcmd, *bcmd;
	int fd0, fd1;

	rcmd = wcmd = bcmd = nil;
	ARGBEGIN{
	case 'r':
		if(bcmd != nil)
			usage();
		rcmd = EARGF(usage());
		break;
	case 'w':
		if(bcmd != nil)
			usage();
		wcmd = EARGF(usage());
		break;
	case 'b':
		if(rcmd!=nil || wcmd!=nil)
			usage();
		bcmd = EARGF(usage());
		break;
	default:
		usage();
	}ARGEND

	if(argc!=1 || (rcmd==nil && wcmd==nil && bcmd==nil))
		usage();
	if(rcmd==nil && wcmd!=nil)
		rcmd = "/bin/cat";
	if(wcmd==nil && rcmd!=nil)
		wcmd = "/bin/cat";

	file = argv[0];
	if(bind("#|", TEMP, MREPL) < 0)
		sysfatal("bind pipe %s: %r", TEMP);
	if(bind(TEMP "/data", file, MREPL) < 0)
		sysfatal("bind %s %s: %r", TEMP "/data", file);

	if(bcmd != nil){
		fd0 = open(TEMP "/data1", ORDWR);
		if(fd0 < 0)
			sysfatal("open %s: %r", TEMP "/data1");
		connect(bcmd, fd0, fd0);
		close(fd0);
	}else{
		fd0 = open(TEMP "/data1", OREAD);
		if(fd0 < 0)
			sysfatal("open %s: %r", TEMP "/data1");
		connect(wcmd, fd0, 1);
		fd1 = open(TEMP "/data1", OWRITE);
		if(fd1 < 0)
			sysfatal("open %s: %r", TEMP "/data1");
		connect(rcmd, 0, fd1);
		close(fd0);
		close(fd1);
	}
	unmount(nil, TEMP);
	exits(nil);
}