Python中将命令行命令转为os.system/subprocess调用报错的问询

Hey there, let's sort out why your command is throwing those errors!

The Root Cause #

When you split your full command string into a list and pass it to subprocess.call(), you're telling Python to run the cat command directly with all those parts as arguments. That means cat is trying to open files named |, sort, >, etc.—which obviously don't exist, hence the "No such file or directory" errors.

The pipe (|) and redirection (>) are shell features, not arguments to cat. By default, subprocess doesn't use the shell to parse these, so they get treated as regular filename arguments.

Solution 1: Use shell=True (Quick Fix) #

If you want to run the command exactly as you would in a terminal, you can tell subprocess to use the shell to parse the entire command string. Here's how:

import subprocess

command = "cat /home/norman/conf/second.txt | sort | uniq > /home/norman/conf/third.txt"
subprocess.call(command, shell=True)

Note: Be careful with shell=True if your command includes user-provided input—it can lead to shell injection vulnerabilities. Since your command uses fixed paths, this is safe here.

Solution 2: Avoid Shell (Recommended, More Secure) #

For better security and control, you can chain the commands using subprocess pipes directly, without relying on the shell. This is the preferred approach for production code:

import subprocess

# Step 1: Read the file with cat
cat_process = subprocess.Popen(
    ["cat", "/home/norman/conf/second.txt"],
    stdout=subprocess.PIPE
)

# Step 2: Pipe cat's output to sort
sort_process = subprocess.Popen(
    ["sort"],
    stdin=cat_process.stdout,
    stdout=subprocess.PIPE
)
cat_process.stdout.close()  # Allow cat to receive SIGPIPE if sort exits early

# Step 3: Pipe sort's output to uniq, and write to the output file
with open("/home/norman/conf/third.txt", "w") as output_file:
    uniq_process = subprocess.Popen(
        ["uniq"],
        stdin=sort_process.stdout,
        stdout=output_file
    )
sort_process.stdout.close()

# Wait for the final process to finish
uniq_process.wait()

Alternatively, you can use subprocess.run with a controlled shell invocation:

import subprocess

with open("/home/norman/conf/third.txt", "w") as outfile:
    subprocess.run(
        ["sh", "-c", "cat /home/norman/conf/second.txt | sort | uniq"],
        stdout=outfile
    )

Either way, this avoids treating shell operators as command arguments, which fixes your original error.

内容的提问来源于stack exchange，提问作者Buteman