Add build script, shmd and util

Current functionality
* Executing a compile-time hardcoded command

build.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env sh
+#
+#build shmd
+
+cc -Wall -g -o shmd shmd.c util.c

shmd.c

@@ -0,0 +1,44 @@
+#include "util.h"
+#include <stdio.h>
+#include <stdlib.h>
+
+const char* argv0;
+
+char* execute_command(const char* command) {
+    FILE *fp;
+    fp = popen(command, "r");
+    if (fp == NULL) edie("popen: ");
+
+    char* result = calloc(1, sizeof(char));
+    if (result == NULL) edie("calloc: ");
+    result[0] = '\0';
+
+    size_t bufsize = 81;
+    char* buf = calloc(bufsize, sizeof(char));
+    if (buf == NULL) edie("calloc: ");
+
+    /*
+     * concat allocates a new string on the heap, so free the old memory when we
+     * concat the next line of output.
+     */
+    while (fgets(buf, bufsize, fp) != NULL) {
+        char* tmp = result;
+        result = concat(2, result, buf);
+        free(tmp);
+    }
+    free(buf);
+    pclose(fp);
+
+    return result;
+}
+
+char* process_shmd() {
+    return NULL;
+}
+
+int main(int argc, char* argv[]) {
+    SET_ARGV0();
+    char* result = execute_command("ls -1");
+    printf("%s", result);
+    return EXIT_SUCCESS;
+}

util.c

@@ -0,0 +1,57 @@
+#include "util.h"
+#include <errno.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include <stdlib.h>
+#include <string.h>
+
+extern const char* argv0;
+
+void die(const char* fmt, ...)
+{
+    fmt = concat(2, fmt, "\n");
+    va_list ap;
+
+    va_start(ap, fmt);
+    vfprintf(stderr, fmt, ap);
+    va_end(ap);
+
+    exit(EXIT_FAILURE);
+}
+
+void edie(const char* fmt, ...)
+{
+    fmt = concat(2, fmt, strerror(errno));
+    va_list ap;
+    die(fmt, ap);
+}
+
+void usage()
+{
+    die("usage: %s\n", argv0);
+}
+
+char* concat(int count, ...)
+{
+    va_list ap;
+    int newlen = 1;
+
+    /* Total length of all strings */
+    va_start(ap, count);
+    for (int i = 0; i < count; i++)
+        newlen += strlen(va_arg(ap, char*));
+    va_end(ap);
+
+    char *newstr = calloc(newlen, sizeof(char));
+    if (newstr == NULL) edie("calloc: ");
+
+    /* Concat strings into newstr */
+    va_start(ap, count);
+    for (int i = 0; i < count; i++) {
+        char* s = va_arg(ap, char*);
+        strcat(newstr, s);
+    }
+    va_end(ap);
+
+    return newstr;
+}

util.h

@@ -0,0 +1,8 @@
+#define SHIFT_ARGS() argv++; argc--;
+#define SET_ARGV0() argv0 = argv[0]; SHIFT_ARGS();
+
+void die(const char* fmt, ...);
+void edie(const char* fmt, ...);
+void usage();
+
+char* concat(int count, ...);