Detect if stdout is redirected to a pipe (not to a file, character device, terminal, or socket)?

Question

Ideally, this would be scriptable in shell, but Perl or Python would be fine. C code could be helpful, but probably fails cost/benefit.

I recognize that redirection to a FIFO (named pipe) may be indistinguishable from a real pipe, and that is enough of an edge case that I don't really care.

Strict POSIX solutions are best, UNIX/Linux variant-independent are next best, but at least something that works on Darwin (MacOS X) is what I need right now.

Before you write your answer - I already know about test -t - that will tell me whether stdout is a terminal (in which case it is definitely not a pipe - but it will not tell me whether stdout has been redirected to a file, non-terminal character device, or UNIX-domain socket rather than a pipe.

Intended use case: I have a command that should be run within backquotes in the shell, so that it can output commands that set environment variables. I would like the command to abort with an error if stdout is not redirected to a pipe, as in that case it definitely wasn't invoked by eval `mycommand`;.

If there's some special environment variable that a shell will set when running a command within backquotes that would be helpful, but as it is likely to be specific to bash or zsh or something, pipe-detection is more important.

Answer 1

(my comment turned to answer per Alex Dupuy's suggestion) Since you said you can use perl, I guess you can use its -p file test operator (perldoc.perl.org/functions/-X.html); something like perl -e 'exit(-p STDOUT ? 0 : 1);' will tell you if stdout if a pipe or a fifo (without distinguishing between them).

Fwiw, calling that from the shell is exactly the reason why I used perl -e :)

Answer 2

You could do an fstat to the file descriptor and check returning structure, for example, st_mode = 0x1000 (S_IFIFO) indicates a Named Pipe.

Example with Python:

from __future__ import print_function
import sys
import os

print(os.fstat(sys.stdout.fileno()), file=sys.stderr)

Output on windows:

C:> python test_fd.py | more
nt.stat_result(st_mode=4096, st_ino=0L, st_dev=0, st_nlink=0, st_uid=0, st_gid=0, st_size=0L, st_atime=0L, st_mtime=0L, st_ctime=0L)

C:> python test_fd.py > test_fd.txt
nt.stat_result(st_mode=33206, st_ino=16888498602769633L, st_dev=0, st_nlink=1, st_uid=0, st_gid=0, st_size=0L, st_atime= 1401119520L, st_mtime=1401119520L, st_ctime=1401119349L)

Answer 3

The stat command works on MacOS X and (with some finessing of variant implementations) other Linux/Unix variants as well, so it is a solution for me:

FORMOPT=
TYPEFORM=
stat() {
  if [ -z "$FORMOPT" ]; then
    if /usr/bin/stat --version >/dev/null 2>&1; then
      FORMOPT=--format
      TYPEFORM=%F
    else
      FORMOPT=-f
      TYPEFORM=%HT
    fi
  fi
  case $1 in
    type) FORMARG="$FORMOPT $TYPEFORM" ; shift ;;
  esac
  /usr/bin/stat -L $FORMARG "$@"
}

exec 9>&1
case `stat type /dev/fd/9` in
  [Ff]ifo*) echo stdout is a pipe ;;
  *) echo stdout is not a pipe ;;
esac

Answer 4

In Python:

import sys
print sys.stdout.isatty()

Examples:

$ ./a
True
$ ./a | less
False

Answer 5

There is a Linux-specific solution that does what I want:

exec 9>&1
case `readlink /dev/fd/9` in
    pipe:\[*\]) echo stdout is a pipe ;;
    *) echo stdout is not a pipe ;;
esac

but that isn't the answer I am looking for as I want this to run on xBSD as well.

I've made this answer community wiki, so feel free to improve it, but if you have a non-Linux solution (or a Linux-specific solution using another technique), please create a new answer.