which - once a match is found in $PATH, test that it is a regular file

and can be accessed as executable

world/usr.bin/which/Makefile

@@ -1,5 +1,7 @@
-#  Makefile - hhl - /src/world/bin/which
+#  Makefile - hhl - /src/world/usr.bin/which
 #  Copyright 2020 Nathan Fisher <nfisher.sr@gmail.com>
 #
 progname = which
-include hhl.script.mk
+hhl_source = 1
+onestage   = true
+include hhl.cprog.mk

world/usr.bin/which/src/which.c

@@ -0,0 +1,57 @@
+#include <dirent.h>
+#include <libgen.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+char *__progname;
+
+void usage() {
+  printf("Usage: %s [COMMAND]\n", __progname);
+  exit(1);
+}
+
+int main(int argc, char **argv) {
+  char *path, *seg, *__path;
+  const char sep[2] = ":";
+
+  __progname = basename(argv[0]);
+  if (argc < 2) {
+    fprintf(stderr, "%s: Error: missing argument\n", __progname);
+    usage();
+  } else if (argc > 2) {
+    fprintf(stderr, "%s: Error: missing argument\n", __progname);
+    usage();
+  }
+  path = getenv("PATH");
+  if (path == NULL) {
+    path = "/usr/bin:/bin";
+  }
+  // Save a copy for later use, since strtok will modify the original
+  __path = strndup(path, strlen(path) + 1);
+  seg = strtok(path, sep);
+  while (seg != NULL) {
+    struct dirent *dp;
+    DIR *dir = opendir(seg);
+    if (dir) {
+      while ((dp = readdir(dir)) != NULL) {
+        if (strcmp(argv[1], dp->d_name) == 0) {
+          char *p = malloc(strlen(argv[1] + strlen(seg) + 2));
+          (void)strncpy(p, seg, strlen(seg) + 1);
+          (void)strncat(p, "/", 2);
+          (void)strncat(p, argv[1], strlen(argv[1]) + 1);
+          struct stat path_stat;
+          stat(p, &path_stat);
+          if (access(p, X_OK) == 0 && S_ISREG(path_stat.st_mode)) {
+            printf("%s\n", p);
+            exit(0);
+          }
+        }
+      }
+    }
+    seg = strtok(NULL, sep);
+  }
+  printf("%s: no %s in (%s)\n", __progname, argv[1], __path);
+}

world/usr.bin/which/src/which.sh

@@ -1,2 +0,0 @@
-#!/bin/zsh
-which $@