In linux, how can I test whether the output of a program is going to a live terminal or to a file?

Question

When you use git it seems to magically know whether standard out is going through a pipe or into a file vs when it is being displayed to the console. For example, if you have colors enabled and you do

git status

it will colorize the output for different categories of files being listed. However, if you do

git status | less

or

git status > status.txt

it removes the linux color formatting and you only see plain, uncolored text.

How does git detect whether the output of its commands are going to file vs going to the terminal?

Answer 1

isatty(fd) returns whether the fd refers to a terminal or something else:

http://linux.die.net/man/3/isatty

It's part of the GNU C library.

Answer 2

This is a C Code to demonstrate how to detect if standard output is redirected:

int main(int argc, char **argv){
    if (!isatty(fileno(stdout))){
      fprintf(stdout, "argv, argc, someone is redirecting me elsewhere...\n");
      return 1;
    }
    /* rest of C code here... */
}

That is how git knows whether the output is going to the terminal or to a file.

Hope this helps, Best regards, Tom.

Answer 3

Can confirm that's what git relies on:

$ grep -ir "isatty" ./*
./builtin-commit.c:     if (isatty(0))
./builtin-config.c:         stdout_is_tty = isatty(1);
./builtin-pack-objects.c:   progress = isatty(2);
./builtin-prune-packed.c:   int opts = isatty(2) ? VERBOSE : 0;
./builtin-revert.c: if (isatty(0))
./builtin-shortlog.c:   if (!nongit && !rev.pending.nr && isatty(0))
./builtin-unpack-objects.c: quiet = !isatty(2);
./color.c:      stdout_is_tty = isatty(1);
./compat/winansi.c: if (!isatty(fileno(stream)))
./compat/winansi.c: if (!isatty(fileno(stream)))
./pack-redundant.c: if (!isatty(0)) {
./pager.c:  if (!isatty(1))
./pager.c:  if (isatty(2))
./remote-curl.c:    options.progress = !!isatty(2);
./transport.c:  args.no_progress = args.quiet || (!transport->progress && !isatty(1));
./transport-helper.c:   int no_progress = v < 0 || (!t->progress && !isatty(1));
./wt-status.c:   * will have checked isatty on stdout).

Run against the git source tree.

Note that fds 0=stdin, 1=stdout, 2=stderr by default, but these can of course be re-directed or closed (typically if you are a daemon you close your file descriptors and re-open the ones you want).