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Julio Cesar
Asked: 2021-11-09 09:49:24 +0800 CST 2021-11-09 09:49:24 +0800 CST

Problem with thread | python

  • 772

I have a problem with the following code:

import subprocess

process = subprocess.run(
    ['mkdir', 'hola'], stdout = subprocess.PIPE, stderr = subprocess.PIPE, shell = True
)

print(process.stderr)
print(process.stdout)

It does not create the "hello" folder, but the strange thing is that the process gives the following error:

b"mkdir: falta un operando\nPruebe 'mkdir --help' para m\xc3\xa1s informaci\xc3\xb3n.\n" # print(process.stderr)

It's like passing it as the first parameter ['mkdir']instead of ['mkdir', 'hola'], but I don't know.

Also try the following: ['cp', 'file1.txt', 'file2.txt']and something similar happened:

b"cp: falta un fichero como operando\nPruebe 'cp --help' para m\xc3\xa1s informaci\xc3\xb3n.\n"

What's going on? (a few days ago, it worked fine... and I have no idea why it doesn't work now)

python

1 Answers

  1. Best Answer

    When you try to execute a command with the argument shell=True, what you do is run the shell passing the list of arguments. Which command interpreter? It depends on your operating system, how you have it configured; on Linux at least there are plenty of shells to choose from.

    On Linux, the shell is determined by the environment variable $SHELL. On my PC, running Linux Mint, it $SHELLpoints to /bin/bash.

    Now, bashit is called with this format:

    NAME
       bash - GNU Bourne-Again SHell

SYNOPSIS
       bash [options] [command_string | file]

    which tells us that it expects an invocation by optionally passing a string containing the command to execute. In this case, I also have to use the option -cto indicate that a string follows with the command. So if you want to create a directory, I have to type on the command line:

    bash -c "mkdir hola"

    Far-fetched, but illustrates the point.

    Alternativeshell=True

    You only need to invoke the with command shell=Trueif you want the shell to perform some actions of its own, like expanding environment variables or converting ambiguous file specifications (like *.lst) into an explicit list.

    In this case, you must pass a single string with the entire command:['mkdir hola']

    Alternativeshell=False

    In your case, you don't need any of that. You can say shell=False, or omit the e execute parameter by runpassing an array with each element separately:['mkdir', 'hola']

    Example

    I want to list all .txtin my directory.

    I can do it with ls *.txt, but that requires expanding *.txt, which is the shell's job:

    process = subprocess.run(['ls *.txt'], stdout = subprocess.PIPE, stderr = subprocess.PIPE, shell = True)

    produces:

    b'activos.txt\narchivo.txt\ncombo_solucion.txt\ncombos.txt\ncon.txt\ncooldown.txt\ndatos.txt\nhistorico.txt\nmbox-short.txt\nnewfile-combos.txt\nnif.txt\nsentimientos.txt\nTweets.txt\n'

    What happens if I do it withshell=False

    process = subprocess.run(['ls', '*.txt'], stdout = subprocess.PIPE, stderr = subprocess.PIPE, shell = False)

    produces:

    b"ls: cannot access '*.txt': No such file or directory\n"
b''

    because the program lsreceived as an argument *.txtand that is a file that does not exist. Remember that in Linux it is the shell that expands *.txtbefore executing the program.

    Moral: Skipshell=True

    • 3