Helper function for escaping special characters from user-provided strings to be used in calls to system().

safesys.h

#ifndef SAFESYS_H
#define SAFESYS_H

#include <stdlib.h>
#include <stdarg.h>
#ifndef SAFESYS_NOMEM
#include <errno.h>
#define SAFESYS_NOMEM ENOMEM
#endif

static int safesys(const char *format, ...) {
	va_list va; size_t size = 0;
	const char *next, *p, *f, *esc;
	char *d, *cmd, c; int ret, nquot;

	esc = "\t\n\r !\"#$&()*;<=>?[\\]^`{|}~";
	va_start(va, format);
	f = format;
	while ((c = *f++)) if (c == '%') {
		p = next = va_arg(va, const char*);
		nquot = 0;
		while ((c = *p++))
			nquot = c == '\'' ? 
				size += nquot > 1 ? 3 : nquot + 1, 0 :
				nquot + !!strchr(esc, c);
		size += (nquot > 1 ? 2 : nquot) + (p - next) - 2;
	}
	va_end(va);
	size += f - format;

	cmd = malloc(size);
	if (!cmd) return SAFESYS_NOMEM;

#define SAFESYS_COPY \
	if (nquot > 1) *d++ = '\''; \
	while (next < p - 1) { \
		c = *next++; \
		if (nquot == 1 && strchr(esc, c)) \
			*d++ = '\\', nquot = 0; \
		*d++ = c; \
	} \
	if (nquot > 1) *d++ = '\'';

	d = cmd;
	va_start(va, format);
	f = format;
	while ((c = *f++))
		if (c != '%') *d++ = c;
		else {
			p = next = va_arg(va, const char*);
			nquot = 0;
			while ((c = *p++))
				if (c == '\'') {
					SAFESYS_COPY
					nquot = 0; next++;
					*d++ = '\\'; *d++ = '\'';
				} else nquot += !!strchr(esc, c);
			SAFESYS_COPY
		}
	va_end(va);
	*d = 0;

	puts(cmd);
	ret = system(cmd);
	free(cmd);
	return ret;
}

#endif // SAFESYS_H

Minor suggestion:

You might wish to split the code into a safesys.c and a safesys.h to make is easier to link with and to make is less awkward other code to use in more than one .c file.

Again this code is an excellent concept that is, I believe, worthy of refinement.

@lcn2
It's written as a concept because you'll rewrite it anyway.
But there is a working code for escaping single quotes that suits the needs of mkiocccentry.

cd ~/tmp && wc -l -- __unsafe_arg__ is done like this:

safesys("cd ~/tmp && wc -l -- %", __unsafe_arg__);

mkiocccentry does not require double quotes escaping, because if one wants to use $FOO on the command line to invoke mkiocccentry, those variables will be expanded by the shell before mkiocccentry is run.

You might wish to have the code simply return a malloced string that is "safe" and let the caller pass it to system() or popen()

This can be done very simply:

-static int safesys(const char *format, ...) {
+static char* cmdprintf(const char *format, ...) {
 
-	if (!cmd) return SAFESYS_NOMEM;
+	if (!cmd) return cmd;
 
-	puts(cmd);
-	ret = system(cmd);
-	free(cmd);
-	return ret;
+	return cmd;

I just want to keep system() in this gist as an example.