How to use a wildcard expression within system() - c++

Question

The following command runs fine on my embedded Linux (Beaglebone Black):
echo bone_pwm_P9_21 > /sys/devices/bone_capemgr.?/slots
But not when using this small C++ program:
#include <stdlib.h>
#include <string>
int main {
system(std::string("echo bone_pwm_P9_21 > /sys/devices/bone_capemgr.?/slots").c_str());
return 0;
}
The problem involves the '?' question mark, that is used as a wildcard.
When the question mark, in the std::string that is passed to system(), is replaced with a normal character, the system() function evaluates the command perfect.
Solutions I've tried without success:
replace ? with \?
replace ? with *

Answer 1

Apart from your code not being compilable, this fails because system(3) runs sh, often a minimal shell provided by dash or busybox.
Meanwhile, your interactive login uses bash, ksh or some other more comfy shell.
dash and busybox sh do not do glob expansion on redirections, while bash and ksh do. Here's a demonstration of the behavior you want courtesy of bash:
$ touch file.txt
$ bash -c 'echo "bash contents" > *.txt'
$ cat file.txt
bash contents
Meanwhile, the problem you're having with e.g. dash:
$ dash -c 'echo "and now dash" > *.txt'
$ ls
*.txt file.txt
$ cat '*.txt' # Instead of expanding, the path was taken literally
and now dash
$ cat file.txt
bash contents
To fix this, you can (in order of preference)
Write your C program in C code instead of shell script
Call a better shell with execve.
Rewrite to not write to a glob, e.g. echo "stuff" | tee *.txt > /dev/null
Call a better shell with system, e.g. bash -c "echo stuff > *.txt"

Answer 2

NOTE: As πάντα ῥεῖ pointed out the system() command calls the shell which will usually do the expansion when presented with the correct wildcard: *. This answer is thereful more appropriate if you want the control to make each system() call separately or if the underlying shell is limited.
Original answer:
Perhaps you could use wordexp for this to construct your strings before you make the system() call:
#include <string>
#include <vector>
#include <iostream>
#include <wordexp.h>
std::vector<std::string> expand_env(const std::string& var, int flags = 0)
{
std::vector<std::string> vars;
wordexp_t p;
if(!wordexp(var.c_str(), &p, flags))
{
if(p.we_wordc)
for(char** exp = p.we_wordv; *exp; ++exp)
vars.push_back(exp[0]);
wordfree(&p);
}
return vars;
}
int main()
{
for(auto&& s: expand_env("$HOME/*")) // <= Note the wildcard '*'
std::cout << s << '\n';
}
In your specific case you could perhaps use something like this:
int main()
{
std::vector<std::string> devices = expand_env("/sys/devices/bone_capemgr.*/slots");
for(std::vector<std::string>::size_type i = 0; i < devices.size(); ++i)
system(("echo bone_pwm_P9_21 > " + devices[i]).c_str());
}