Starting a Python script from bash with a string placed into stdin

Question

How do I start a Python script with some input already waiting in stdin. For example, say I have this simple script located in main.py, using Python3:

mystr = input()
print(mystr)

If you just called python3 main.py from the terminal, the script would pause and wait for you to type something. After hitting enter it would continue and print out what you typed. But is there a way to place something into stdin before the script executes? That way the script wouldn't pause, it would accept what's already there. I was envisioning something like: python3 main.py | my string but that doesn't work.

Note that I am not asking about command line arguments: I am writing an application which will have to take input from stdin, so I am looking for a convenient solution to test out that environment.

Possible duplicate of [Command Line Arguments In Python](http://stackoverflow.com/questions/1009860/command-line-arguments-in-python) — ZdaR, Feb 18 '17 at 17:44
Or if you have have a file of sample input, `cat my_input | python3 main.py`. — JonathanZ, Feb 18 '17 at 17:51
@Jonathan your solution works. If you turn it into an answer I'll accept it. — J-bob, Feb 20 '17 at 01:37

score 3 · Accepted Answer · answered Feb 21 '17 at 05:40

This doesn't depend on the fact that a Python script is involved, and the same methods work for feeding bytes into the stdin of any process started under bash (you might want to add a bash tag to your question to reflect this):

To feed a short string into it

echo 'my string' | python3 main.py

To feed a file of sample input into it

cat my_input | python3 main.py

You can also feed a file like this

python3 mail.py < my_input

[Note: Some people like to spend a lot of time arguing that method 3 is better than method 2, (search for 'useless use of cat' if you're interested), but I've come to prefer method 2 (for reasons that are beyond the scope of your question). Use whichever makes the most sense to you.]

Starting a Python script from bash with a string placed into stdin

1 Answers1