sh.c

#define _GNU_SOURCE
#include <stdarg.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <time.h>
#include "sh.h"

char * vars[NUM_VARS];
const char * var_names[NUM_VARS] = {
    "LOWERDIR",
    "UPPERDIR",
    "MOUNTDIR",
    "LOWERNEW",
    "UPPERNEW",
};

int quote(const char *filename, FILE *output);

FILE* create_shell_script(char *tmp_path_buffer) {
    int tmp_file = mkstemps(tmp_path_buffer, 3); // the 3 is for suffix length (".sh")
    if (tmp_file < 0) { return NULL; }
    fchmod(tmp_file, S_IRWXU); // chmod to 0700
    FILE* f = fdopen(tmp_file, "w");
    if (f == NULL) { return NULL; }
    fprintf(f, "#!/usr/bin/env bash\n");
    fprintf(f, "set -x\n");
    time_t rawtime;
    time (&rawtime);
    fprintf(f, "# This shell script is generated by overlayfs-tools on %s\n", ctime (&rawtime));
    for (int i=0; i < NUM_VARS; i++) {
        if (vars[i]) {
            fprintf(f, "%s=", var_names[i]);
            if (quote(vars[i], f) < 0) { return NULL; }
            if (fputc('\n', f) == EOF) { return NULL; }
	}
    }
    // Non-empty *NEW vars make a backup copy and override *DIR vars
    if (vars[LOWERNEW]) {
        fprintf(f, "rm -rf \"$LOWERNEW\"\n");
        fprintf(f, "cp -a \"$LOWERDIR\" \"$LOWERNEW\"\n");
        fprintf(f, "LOWERDIR=");
        if (quote(vars[LOWERNEW], f) < 0) { return NULL; }
        if (fputc('\n', f) == EOF) { return NULL; }
    }
    if (vars[UPPERNEW]) {
        fprintf(f, "rm -rf \"$UPPERNEW\"\n");
        fprintf(f, "cp -a \"$UPPERDIR\" \"$UPPERNEW\"\n");
        fprintf(f, "UPPERDIR=");
        if (quote(vars[UPPERNEW], f) < 0) { return NULL; }
        if (fputc('\n', f) == EOF) { return NULL; }
    }
    return f;
}

int quote(const char *filename, FILE *output) {
    if (fputc('\'', output) == EOF) { return -1; }
    for (const char *s = filename; *s != '\0'; s++) {
        if (*s == '\'') {
            if (fprintf(output, "'\"'\"'") < 0) { return -1; }
        } else {
            if (fputc(*s, output) == EOF) { return -1; }
        }
    }
    if (fputc('\'', output) == EOF) { return -1; }
    return 0;
}

int substitue(char what, const char *filename, FILE *output) {
    int i;
    for (i=0; i < NUM_VARS; i++)
        if (vars[i] && var_names[i][0] == what)
	    break;
    if (i == NUM_VARS) { return -1; }
    // filename prefix must match the var value
    int prefix = strlen(vars[i]);
    if (strncmp(filename, vars[i], prefix)) { return -1; }
    filename += prefix;
    fprintf(output, "\"$%s\"", var_names[i]);
    return quote(filename, output);
}

int command(FILE *output, const char *command_format, ...) {
    va_list arg;
    va_start(arg, command_format);
    for (size_t i = 0; command_format[i] != '\0'; i++) {
        if (command_format[i] == '%') {
            const char *s = va_arg(arg, char *);
            if (substitue(command_format[++i], s, output) < 0) { return -1; }
        } else {
            if (fputc(command_format[i], output) == EOF) { return -1; }
        }
    }
    va_end(arg);
    if (fputc('\n', output) == EOF) { return -1; }
    return 0;
}