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Ruby Internals: Exec

Today I was curious to learn how Ruby implements the exec command to evaluate code in the shell. It didn't take me much time to find the definition of rb_proc_exec_e inside process.c. Here's the interesting part (which doesn't apply for windows):

rb_proc_exec_e(const char *str, VALUE envp_str)
{
  while (*str == ' ' || *str == '\t' || *str == '\n')
    str++;

  if (!*str) {
      errno = ENOENT;
      return -1;
  }

  before_exec();
  if (envp_str)
      execle("/bin/sh", "sh", "-c", str, (char *)NULL, (char **)RSTRING_PTR(envp_str));
  else
      execl("/bin/sh", "sh", "-c", str, (char *)NULL);
  preserving_errno(after_exec());

  return -1;
}

This function accepts a string containing the command to execute and a process environment variable, which may be null.

The first part of the function starts with sanitizing user input by removing whitespaces, tabs and newlines from the beginning of the string It then returns -1 if the string is empty.

Before and after exec are routines which help with threading, but the interesting part is how Ruby uses execl and execle in a clever way to execute commands without having to parse arguments directly. This is helpful because both execl and execle accept a variable number of arguments which are passed as arguments to the program you're running (the first parameter). So if you wanted to execute ls -a in C, you would probably use something similar to:

execl("/bin/sh", "ls", "-", (char *)NULL);

Where (char *)NULL tells execl that the arguments are finished. However, parsing user input this way in C is tedious, so Ruby uses the -c flag to tell bash to interpret the next parameter as the command itself, so it doesn't have to parse and send arguments like we did.

To summarize, when you do exec "ls -a" in Ruby what you're really doing is:

/bin/sh -c "ls -a"

As usual, it pays to know how your tools work (in this case bash). This tip could have saved me some time when programming in C and Objective-C and had to deal with user input.

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