Cat – Command to Output File Content to Stdout

To build an interactive application you can open /dev/tty, it will return a file descriptor to the controlling terminal:

int ttyfd = open("/dev/tty", O_RDWR);

You can use it instead of STDIN_FILENO or STDOUT_FILENO (those could be redirected to something different than the terminal when the program is started).

Here is some example:

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main() {
  int ttyfd = open("/dev/tty", O_RDWR);
  printf("fd: %d\n", ttyfd);
  write(ttyfd, "hello tty!\n", 11);
  return 0;
}

When invoked with ./test >out, it should print the hello message on the terminal and something like fd: 3 in the out file.

If grep o produces color output, then either grep is an alias to grep --color=auto or grep --color=always (or possibly more options), or GREP_OPTIONS is set to a value that contains --color=auto or --color=always. Since $GREP_OPTIONS is empty, it must be the alias.

Since grep o | less -R doesn't show colors, the alias must be to grep --color=auto (a sensible choice). With the alias, the grep command always receives the --color option on the command line, and this takes precedence over the environment variable.

If you want to use the environment variable, remove the alias definition from your ~/.bashrc, or for one session run unalias grep. You can replace alias grep='grep --color=auto' by export GREP_OPTIONS='--color=auto': they have essentially the same meaning, except that:

• setting GREP_OPTIONS to a different value only overrides the latter;
• the alias only kicks in when you run grep from an interactive shell, whereas setting GREP_OPTIONS also applies when grep is run from scripts and other applications.

Never put --color=always or most other options in GREP_OPTIONS: it would break many programs that parse the output of grep. --color=auto is about the only safe option to put in GREP_OPTIONS. For anything else, use the alias. Future versions of GNU grep will drop support for the option for this reason.

Note that the alias definition goes into ~/.bashrc (it's a shell setting), whereas the environment variable definition goes into ~/.profile (it's a session setting). See Is there a ".bashrc" equivalent file read by all shells?

If you want to run the unaliased command just once, run \grep instead of grep (quoting any part of the name bypasses the alias lookup).