使用Python将变量传递给bash命令

Question

I have the next code:

from subprocess import Popen, PIPE

p = Popen("C:/cygwin64/bin/bash.exe", stdin=PIPE, stdout=PIPE)

path = "C:/Users/Link/Desktop/folder/"

p.stdin.write(b"cd " + str.encode(path)))

p.stdin.close()
out = p.stdout.read()
print(out)

The output is b''

Is there any way to pass a variable to the bash command p.stdin.write(b"cd " + path)

I ask because the way it is written above don't work. Output is null, just like Cygwin started and nothing else.

EDIT

As long as I see the question is not so clear, I'll add this scenario:

I am on Windows and I am using Python 3.6. I have a bash cmd that requieres Cygwin to be executed. This cmd may have a variable in his string, which will change after every execution. Immagine a for loop which executes a command.

For example (an ImageMagick command):

convert image.jpg -resize 1024x768 output_file.jpg

How can I execute this cmd from Python with output_file.jpg as variable ?

Answer 1

Bash doesn't run in interactive mode by default unless it detects that standard input and output are connected to a terminal. You PIPE d these in, therefore they're definitely not connected to a terminal.

Bash does not display any prompts in non-interactive mode, hence you see nothing. You can force it to be interactive with -i switch.

However, even then, it is not going to write to stdout but stderr ; you can try piping stderr to stdout

from subprocess import Popen, PIPE, STDOUT
p = Popen(["C:/cygwin64/bin/bash.exe", "-i"], stdin=PIPE, stdout=PIPE, stderr=STDOUT)

and you will capture the prompts and such.

Or use your original approach with a command that does produce output - here pwd that prints the current working directory:

p.stdin.write(b"cd " + path.encode() + b"\n")
p.stdin.write(b"pwd")

---

It is tricky to talk to an interactive process like this though - read too little => deadlock. Write too much => deadlock. This is why Popen has the .communicate method for providing all of input at once and getting the stdout and stderr afterwards.

Answer 2

As it seems you are using the Cygwin python, than you should use proper Posix paths and not Windows-like ones

Instead of

p = Popen("C:/cygwin64/bin/bash.exe", stdin=PIPE, stdout=PIPE)

use

p = Popen("/bin/bash.exe", stdin=PIPE, stdout=PIPE)