$EDITOR as unix filter
It is really, really strange, but none of major text editors – neither vim, nor emacs work as filter – I mean, you can’t pipe text into stdin, edit and have edited version on stdout. That is because stdout is expected to be tty, so editor writes control sequences on stdout, and we have expected behaviour. But we can do better – use stderr descriptor as tty, and save stdout for piping.
Here is code of little C99, POSIX-2008 program, that invokes your `$EDITOR’ as filter.
/* edit-stream.c --- invoke $EDITOR as filter */
/* Copyright (C) 2016 Dmitry Bogatov <KAction@gnu.org> */
/* This program is free software; you can redistribute it and/or */
/* modify it under the terms of the GNU General Public License */
/* as published by the Free Software Foundation; either version 3 */
/* of the License, or (at your option) any later version. */
/* This program is distributed in the hope that it will be useful, */
/* but WITHOUT ANY WARRANTY; without even the implied warranty of */
/* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the */
/* GNU General Public License for more details. */
/* You should have received a copy of the GNU General Public License */
/* along with this program. If not, see <http://www.gnu.org/licenses/>. */
#define _POSIX_C_SOURCE 200809L
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/wait.h>
/* stdio.h would be overkill */
#define write2(str) write(2, str, sizeof(str))
int
main(int argc, char **argv)
{
int return_value = 1;
const char *tty = ttyname(2); /* POSIX.1-2001, did you knew? */
if (!tty) {
write2("stderr is not tty");
return 1;
}
int in = open(tty, O_RDONLY);
if (in == -1) {
write2("failed to open tty for reading");
return 1;
}
char tempname[] = "/tmp/edit-stream.XXXXXX";
int tempfd = mkstemp(tempname);
if (tempfd == -1) {
write2("failed to open temporary file");
goto err_close_tty;
}
/* POSIX does not mandates modes of temporary file. */
if (fchmod(tempfd, 0600) == -1) {
write2("failed to change mode of temporary file");
goto err_remove_tempfile;
}
char buffer[2048];
ssize_t bytes;
while ((bytes = read(0, buffer, sizeof(buffer))) > 0) {
/* Here actually must be loop, since write(2) does not guarates
* that it will be able to write everything. But I am reckless.
*/
if (write(tempfd, buffer, bytes) != bytes) {
write2("failed to write data to temporary file");
goto err_remove_tempfile;
}
}
if (fsync(tempfd) == -1) {
write2("failed to fsync temporary file");
goto err_remove_tempfile;
}
if (close(tempfd) == -1) {
write2("failed to close temporary file");
goto err_remove_tempfile;
}
if (dup2(in, 0) == -1) {
write2("failed to set tty as stdin");
goto err_remove_tempfile;
}
int stdout = dup(1);
if (stdout == -1) {
write2("failed to create copy of stdout");
goto err_remove_tempfile;
}
/* Descriptor 2 (stderr) still points to tty */
if (dup2(2, 1) == -1) {
write2("failed to set tty as stdout");
goto err_close_stdout;
}
const char *editor = getenv("EDITOR");
if (!editor) {
write2("EDITOR is not set");
goto err_close_stdout;
}
pid_t pid = fork();
if (pid == 0) {
close(stdout);
close(tempfd);
close(in);
execlp(editor, editor, tempname, NULL);
_exit(129);
}
int status;
if (wait(&status) == -1) {
write2("wait failed");
goto err_close_stdout;
}
if (!(WIFEXITED(status) && WEXITSTATUS(status) == 0)) {
write2("editor invocation failed");
goto err_close_stdout;
}
int tempfd_r = open(tempname, O_RDONLY);
if (tempfd_r == -1) {
write2("failed to open for reading edited temporary file");
goto err_close_stdout;
}
while ((bytes = read(tempfd_r, buffer, sizeof(buffer))) > 0) {
if (write(stdout, buffer, bytes) != bytes) {
write2("failed to write data to stdout");
goto err_close_tempfile_read;
}
}
return_value = 0;
/* Clean up on error is best efford. Descriptors are closed, files
are unlinked, but nothing is checked. */
err_close_tempfile_read:
close(tempfd_r);
err_close_stdout:
close(stdout);
err_remove_tempfile:
close(tempfd);
unlink(tempname);
err_close_tty:
close(in);
close(0);
close(1);
close(2);
return return_value;
}