system("command") produces an error; but it works when invoked directly from Bash prompt

Question

I am trying to run the following bash command from my C++ program:

diff <(cat /etc/passwd) <(ls -l /etc)

with the following C++ statement:

system("diff <(cat /etc/passwd) <(ls -l /etc)");

The command works fine when running it directly from the Linux shell but when running it from my program, I get:

sh: 1: Syntax error: "(" unexpected

That's referring to the (

I have tried escaping the ( with a \, but that creates more issues:

system("diff <\\(cat /etc/passwd\\) <\\(ls -l /etc\\)");

sh: 1: cannot open (cat: No such file

All I want is to run the following from my C++ program:

diff <(cat /etc/passwd) <(ls -l /etc)

I can create a file and run it, but I leave that as a last option.

Answer 1

As mentioned system() creates a new standard shell sh and executes the commands. Since <() is a bash specific feature it can't be interpreted by sh.

You can circumvent this by calling bash explicitly and use the -c option:

system("bash -c \"diff <(cat /etc/passwd) <(ls -l /etc)\"");

or using a raw string literal:

system(R"cmd(bash -c "diff <(cat /etc/passwd) <(ls -l /etc)")cmd");

---

Here's the relevant part of the system(3) call manual page:

The system() library function uses fork(2) to create a child process that executes the shell command specified in command using execl(3) as follows:

 execl("/bin/sh", "sh", "-c", command, (char *) 0);

system() returns after the command has been completed.

Answer 2

The system(3) call invokes /bin/sh to process the command. If you want specifically use bash features, you need to insert bash -c in front of the command string, which will run bash and tell it to process the remainder of the string.

system("bash -c \"diff <(cat /etc/passwd) <(ls -l /etc)\"");