Escapes a string for use with bash shell, formatted as $'...'.

shell_escape.c

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

/**
  * Escapes a string for use with bash shell, formatted as $'...'.
  *
  * @param input  the input string you want to escape.
  *
  * @return the escaped string (which must be free'd after use).
  */
char* shell_escape(const char* input)
{
	int         c;
	const char *p;
	char       *q;
	char       *result;
	size_t      length;

	/* If the input is NULL, then return NULL */
	if (input == NULL)
		return NULL;

	/* First, compute how much space will be required for the escaped string. */
	p = input, length = 4;
	while ((c = *p++ & 255) != 0) {
		length++;
		if (c < 32 || c >= 128) {
			length += 3;
		} else if (c == '"' || c == '\'' || c == '?') {
			length++;
		}
	}

	/* Then allocate a char array for the result. */
	if ((result = (char *) malloc(length)) == NULL)
		return NULL;

	/* Finally, escape the string. */
	p = input, q = result;
	*q++ = '$';
	*q++ = '\'';
	while ((c = *p++ & 255) != 0) {
		if (c < 32 || c >= 128) {
			*q++ = '\\';
			*q++ = (c >> 6 & 3) + '0';
			*q++ = (c >> 3 & 7) + '0';
			*q++ = (c >> 0 & 7) + '0';
		} else if (c == '"' || c == '\'' || c == '?') {
			*q++ = '\\';
			*q++ = c;
		} else {
			*q++ = c;
		}
	}
	*q++ = '\'';
	*q++ = 0;

	/* We should have arrived at a string whose length is equal to the value
	   of the variable “length”. */
	assert((q - result) == length);

	return result;
}