Write a simple UNIX ls clone

exercises/coreutils/README.md

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+# 🛠️ Coreutils Reimplementations
+
+This directory contains reimplementations of classic **GNU Coreutils** commands in C,  
+using only **POSIX system calls** and the standard C library.
+
+The goal is to gain hands-on experience with **Linux system programming**, while practicing  
+error handling, portability, and clean coding.
+
+---
+
+## 📂 Structure
+Each command is implemented in its own folder:
+```
+
+coreutils/
+├── ls/        # list directory contents
+├── cat/       # concatenate files
+├── cp/        # copy files
+├── echo/      # echo a string
+├── sleep/     # wait X millis
+````
+
+Each subfolder contains:
+- `main.c` → source code
+- `Makefile` → to build the command
+
+---
+
+## 🎯 Learning Objectives
+- Practice **file and directory operations**
+- Reinforce knowledge of **system calls**: `open`, `read`, `write`, `stat`, `opendir`, `unlink`, `link`, etc.
+- Develop robust error handling
+- Understand the **UNIX philosophy**: small tools, combined power
+
+---
+
+## ⚙️ Build & Run
+Example (`ls`):
+
+```bash
+cd ls
+make
+./ls .
+````
+
+---
+
+*Last updated: 2025-09-18*

exercises/coreutils/ls/Makefile

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+CC      := gcc
+CFLAGS  := -Wall -Wextra -pedantic -std=c11 -pthread -g
+TARGET  := ls
+SRC     := main.c
+OBJS    := $(SRC:.c=.o)
+
+.PHONY: all clean valgrind
+
+all: $(TARGET)
+
+$(TARGET): $(OBJS)
+	$(CC) $(CFLAGS) -o $@ $^
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c $< -o $@
+
+clean:
+	rm -f $(TARGET) $(OBJS)
+

exercises/coreutils/ls/main.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <dirent.h>
+#include <errno.h>
+#include <string.h>
+#include <fcntl.h>
+#include <inttypes.h>
+#include <limits.h>
+#include <sys/stat.h>
+
+char filetype(mode_t mode);
+char *getpermissions(mode_t mode);
+
+int main(int argc, char *argv[])
+{
+  char *dirname = ".";
+  if (argc > 1)
+    dirname = argv[1];
+
+  DIR *dir = opendir(dirname);
+  if (dir == NULL) {
+    int errsv = errno;
+    fprintf(stderr, "%s: cannot access %s: %s\n", argv[0], dirname, strerror(errsv));
+    return EXIT_FAILURE;
+  }
+
+  struct stat statbuf;
+  struct dirent *dirp;
+  while ((dirp = readdir(dir)) != NULL) {
+    // Skip current and previous directory symbols
+    if (strcmp(dirp->d_name, ".") == 0 ||
+	strcmp(dirp->d_name, "..") == 0)
+      continue;
+
+    char path[PATH_MAX];
+    snprintf(path, sizeof(path), "%s/%s", dirname, dirp->d_name);
+    if (stat(path, &statbuf) < 0) {
+      int errsv = errno;
+      fprintf(stderr, "%s: cannot stat %s: %s\n", argv[0], path, strerror(errsv));
+      continue;
+    }
+    
+    char type = filetype(statbuf.st_mode);
+    char *permissions = getpermissions(statbuf.st_mode);
+    
+    /*
+     * Format output:
+     * TYPE: d (directory) | f (regular file) | l (symbolic link) | o (others)
+     * PERMISSIONS (ex. rwxrwxrwx)
+     * NR HARD LINKS
+     * USER
+     * GROUP
+     * SIZE
+     * NAME
+     */
+    printf("%c%s %lu %u %u %6lu %s\n",
+	   type, permissions, statbuf.st_nlink,
+	   statbuf.st_uid, statbuf.st_gid, statbuf.st_size,
+	   dirp->d_name);
+
+    if (permissions != NULL) {
+      free(permissions);
+      permissions = NULL;
+    }
+  }
+
+  if (closedir(dir) == -1) {
+    int errsv = errno;
+    fprintf(stderr, "%s: cannot close %s: %s\n", argv[0], dirname, strerror(errsv));
+    return EXIT_FAILURE;
+  }
+}
+
+char filetype(mode_t mode)
+{
+  if (S_ISREG(mode)) return '-';
+  if (S_ISDIR(mode)) return 'd';
+  if (S_ISLNK(mode)) return 'l';
+  if (S_ISCHR(mode)) return 'c';
+  if (S_ISBLK(mode)) return 'b';
+  if (S_ISFIFO(mode)) return 'p';
+#ifdef S_ISSOCK
+  if (S_ISSOCK(mode)) return 's';
+#endif
+  return '?';
+}
+
+char checkpermission(mode_t mode, int perm, char c)
+{
+  return (mode & perm) ? c : '-';
+}
+
+char *getpermissions(mode_t mode)
+{
+  char *permissions = malloc(9 + 1);
+
+  /* User */
+  permissions[0] = checkpermission(mode, S_IRUSR, 'r');
+  permissions[1] = checkpermission(mode, S_IWUSR, 'w');
+  permissions[2] = checkpermission(mode, S_IXUSR, 'x');
+
+  /* Group */
+  permissions[3] = checkpermission(mode, S_IRGRP, 'r');
+  permissions[4] = checkpermission(mode, S_IWGRP, 'w');
+  permissions[5] = checkpermission(mode, S_IXGRP, 'x');
+
+  /* Other */
+  permissions[6] = checkpermission(mode, S_IROTH, 'r');
+  permissions[7] = checkpermission(mode, S_IWOTH, 'w');
+  permissions[8] = checkpermission(mode, S_IXOTH, 'x');
+
+  permissions[9] = '\0';
+  return permissions;
+}