C++ system() API执行命令报错：sh: 1: Syntax error: "(" unexpected

Hey there, let's break down why you're hitting that sh: 1: Syntax error: "(" unexpected error and how to fix it directly without workarounds.

The Root Cause #

The system() function in C++ defaults to calling /bin/sh instead of your regular interactive bash shell. On Ubuntu, /bin/sh is actually symlinked to dash—a lightweight POSIX-compliant shell that doesn't support bash-specific features like process substitution (<()). That's why the command works perfectly in your terminal (which uses bash) but fails when run via system().

The Direct Fix: Force system() to Use Bash #

To make your command run correctly, you need to explicitly tell system() to execute the command with bash instead of sh. Wrap your entire command in bash -c "...", making sure to handle string escaping properly in C++.

Working C++ Code Example #

Here's a clean implementation using C++11 raw string literals (avoids messy escape characters):

#include <cstdlib>
#include <iostream>

int main() {
    // Raw string literal lets us write the command without escaping quotes
    const char* cmd = R"(bash -c "comm -23 <(apt-mark showmanual | sort -u) <(gzip -dc /var/log/installer/initial-status.gz | sed -n 's/^Package: //p' | sort -u)")";
    
    int exit_code = system(cmd);
    if (exit_code != 0) {
        std::cerr << "Command failed with exit code: " << exit_code << std::endl;
    }
    return 0;
}

If you're stuck with an older C++ standard that doesn't support raw strings, you can manually escape the double quotes:

const char* cmd = "bash -c \"comm -23 <(apt-mark showmanual | sort -u) <(gzip -dc /var/log/installer/initial-status.gz | sed -n 's/^Package: //p' | sort -u)\"";

Quick Notes #

• Why does this work in the terminal? Ubuntu's default interactive shell is bash, which supports process substitution. system() uses the non-interactive sh (dash) by default, hence the mismatch.
• If you wanted to avoid relying on bash entirely, you could rewrite the command using temporary files instead of <(), but that would add unnecessary complexity. Using bash directly is the simplest and most direct fix here.

内容的提问来源于stack exchange，提问作者Ashutosh Pandey