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I'm trying to get the script that asks for the input to work when stdin is redirected. It could be that this is not even possible, but I would like to understand why. Here is an example:

$ echo 'q = raw_input("question ?"); print "\ngot: ", q' | python -  < <(echo answer)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'answer' is not defined

I know I can get it to work with this:

$ python <(echo 'q = raw_input("question ?"); print "\ngot: ", q') < <(echo answer)
question ?
got:  answer

but this is a process substitution. Is it possible to get it to work only with redirection?

  • I'm not sure I get the main point here, is there some reason to not use python -c or a program saved in a file? Or does your script need to read something from stdin and prompt the user at the same time? Which would lead to the usual idea of giving the input from a file named at the script command line. Is there some reason you don't want that? – ilkkachu Mar 26 '17 at 20:19
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Yes, maybe. One way is to open up the TTY and interact with that:

#!/usr/bin/env expect
package require Tcl 8.5

gets stdin line; puts "stdin: $line"

set tty [open /dev/tty r+]
chan configure $tty -buffering none

puts -nonewline $tty "nimi sina li seme? "
set name [gets $tty]

gets stdin line; puts "stdin: $line"
puts $tty "name: $name"

Which has results thusly given the above saved in ./prompt, chmod +x, installed expect, etc.

$ (echo foo; echo bar) | ./prompt
stdin: foo
nimi sina li seme? Bob
stdin: bar
name: Bob
$ 
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$ echo 'q = ...' | python -  < <(echo answer)

This isn't going to work since you're redirecting to python's stdin twice. The (later) redirection of < <(echo answer) takes effect, and python gets only the word answer as the script to run.

Opening /dev/tty (or even /dev/stderr) can work even if stdin is redirected.

$ echo foo | python -c 'import sys; 
     s = sys.stdin.readline(); 
     print "from stdin:", s; 
     t = open("/dev/tty", "r"); 
     s = t.readline(); 
     print "from tty:", s;'

Of course this requires that the script is running with a tty, and not from cron or similar.

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