diff --git a/README-demo.gif b/README-demo.gif
index 49a1c12..4f2d03d 100644
Binary files a/README-demo.gif and b/README-demo.gif differ
diff --git a/README.md b/README.md
index b3db0d6..4cf02f1 100644
--- a/README.md
+++ b/README.md
@@ -15,10 +15,7 @@ Hux - An x86 32-bit single-CPU toy operating system kernel built from scratch, f
 
 ## Tutorial / Development Doc
 
-I document the whole development process of Hux - its skeleton, related theories, practice pitfalls, plus everything I reckon important as a complete set of tutorials. They can be found at:
-
-- The [**WIKI pages 📝**](https://github.com/josehu07/hux-kernel/wiki) of this repo ✭
-- The Hux kernel dev doc PDF (identical, WIP)
+I document the whole development process of Hux as a complete set of tutorials. They can be found at the [**WIKI pages 📝**](https://github.com/josehu07/hux-kernel/wiki) of this GitHub repository ✭.
 
 If there are any typos / mistakes / errors, please raise an issue!
 
@@ -30,14 +27,13 @@ Requires a Linux host development environment. Tested on Ubuntu Xenial, Bionic,
 Clone the repo, set up a development cross-compilation toolchain following [this wiki page](https://github.com/josehu07/hux-kernel/wiki/02.-The-Very-First-Skeleton), then build Hux by:
 
 ```bash
+$ make clean
 $ make
 ```
 
-Or, if you just want to try out Hux without a development toolchain, download both the released kernel image `hux.iso` and the initial file system image `vsfs.img` to the folder.
-
-(The images have bot been uploaded yet. The first release of Hux is coming soon.)
+Or, if you just want to try out Hux without a development toolchain, download both the released kernel image `hux.iso` and the initial file system image `vsfs.img` to the folder. (The images have bot been uploaded yet. The first release of Hux is coming soon.)
 
-To run Hux in QEMU, do:
+To run Hux in QEMU (currently only supports GUI VGA mode, "nographics" mode is coming soon), do:
 
 ```bash
 $ make qemu
@@ -45,8 +41,6 @@ $ make qemu
 
 You will see the QEMU GUI popping up with GRUB loaded. Choose the "`Hux`" option with <kbd>Enter</kbd> to boot into Hux.
 
-A random screenshot of the system at current progress:
-
 <p align=center> <img src="README-demo.gif" width=720px align=center /> </p>
 
 For development setup & instructions, please check out the wiki pages (recommended). I have every single detail documented there.
@@ -99,11 +93,12 @@ Check the "References" section [here](https://github.com/josehu07/hux-kernel/wik
 - [x] Essential system calls
 - [x] Time-sharing scheduler
 - [x] Basic IDE disk driver
-- [ ] Very simple file system
+- [x] Very simple file system
+- [ ] More user-level utilities
+- [ ] Thorough user-level tests
 - [ ] Caching, swapping, & logging
-- [ ] Multi-threading concurrency
 - [x] Synchronization primitives
+- [ ] Multi-threading concurrency
 - [ ] Move to APIC & IOAPIC
 - [ ] Direct memory access
-- [ ] Summary of current flaws
 - [ ] Extend Hux to Rux with Rust
diff --git a/src/common/printf.c b/src/common/printf.c
index 08fcdb5..ddf56e2 100644
--- a/src/common/printf.c
+++ b/src/common/printf.c
@@ -896,6 +896,8 @@ _vsnprintf(char *buf, size_t count, const char *fmt, va_list va)
         seg_end++;
         fmt++;
     }
+
+    *buf = '\0';
 }
 
 
diff --git a/src/filesys/exec.c b/src/filesys/exec.c
index 94e3159..fcc6f9d 100644
--- a/src/filesys/exec.c
+++ b/src/filesys/exec.c
@@ -175,18 +175,19 @@ exec_program(mem_inode_t *inode, char *filename, char **argv)
             goto fail;
         sp = sp - (strlen(argv[argc]) + 1);
         sp &= 0xFFFFFFFC;   /** Align to 32-bit words. */
-        memcpy((char *) (paddr_top - (USER_MAX - sp)), argv[argc],
+        memcpy((char *) (paddr_top + PAGE_SIZE - (USER_MAX - sp)), argv[argc],
                strlen(argv[argc]) + 1);
         ustack[3 + argc] = sp;
     }
     ustack[3 + argc] = 0;       /** End of argv list. */
 
     ustack[2] = sp - (argc + 1) * 4;       /** `argv` */
-    ustack[1] = argc;                      /** `argv` */
+    ustack[1] = argc;                      /** `argc` */
     ustack[0] = 0x0000DEAD;  /** Fake return address. */
 
     sp -= (3 + argc + 1) * 4;
-    memcpy((char *) (paddr_top - (USER_MAX - sp)), ustack, (3 + argc + 1) * 4);
+    memcpy((char *) (paddr_top + PAGE_SIZE - (USER_MAX - sp)), ustack,
+           (3 + argc + 1) * 4);
 
     /** Change process name. */
     strncpy(proc->name, filename, strlen(filename));
diff --git a/src/filesys/file.c b/src/filesys/file.c
index 46a83df..9400d95 100644
--- a/src/filesys/file.c
+++ b/src/filesys/file.c
@@ -10,6 +10,7 @@
 #include "file.h"
 #include "block.h"
 #include "vsfs.h"
+#include "sysfile.h"
 
 #include "../common/debug.h"
 #include "../common/string.h"
@@ -521,3 +522,17 @@ file_put(file_t *file)
     /** Actually closing, put inode. */
     inode_put(inode);
 }
+
+
+/** Get metadata information of a file. */
+void
+file_stat(file_t *file, file_stat_t *stat)
+{
+    inode_lock(file->inode);
+
+    stat->inumber = file->inode->inumber;
+    stat->type = file->inode->d_inode.type;
+    stat->size = file->inode->d_inode.size;
+
+    inode_unlock(file->inode);
+}
diff --git a/src/filesys/file.h b/src/filesys/file.h
index 0a499d1..2f52241 100644
--- a/src/filesys/file.h
+++ b/src/filesys/file.h
@@ -12,6 +12,7 @@
 #include <stddef.h>
 
 #include "vsfs.h"
+#include "sysfile.h"
 
 #include "../common/spinlock.h"
 #include "../common/parklock.h"
@@ -73,5 +74,7 @@ file_t *file_get();
 void file_ref(file_t *file);
 void file_put(file_t *file);
 
+void file_stat(file_t *file, file_stat_t *stat);
+
 
 #endif
diff --git a/src/filesys/sysfile.c b/src/filesys/sysfile.c
index 9e7b683..998da9f 100644
--- a/src/filesys/sysfile.c
+++ b/src/filesys/sysfile.c
@@ -193,3 +193,20 @@ syscall_exec(void)
     }
     return SYS_FAIL_RC;
 }
+
+/** int32_t fstat(int32_t fd, file_stat_t *stat); */
+int32_t
+syscall_fstat(void)
+{
+    int32_t fd;
+    file_stat_t *stat;
+
+    if (!sysarg_get_int(0, &fd))
+        return SYS_FAIL_RC;
+    if (!sysarg_get_mem(1, (char **) &stat, sizeof(file_stat_t)))
+        return SYS_FAIL_RC;
+
+    if (!filesys_fstat(fd, stat))
+        return SYS_FAIL_RC;
+    return 0;
+}
diff --git a/src/filesys/sysfile.h b/src/filesys/sysfile.h
index f712ace..971c0aa 100644
--- a/src/filesys/sysfile.h
+++ b/src/filesys/sysfile.h
@@ -19,6 +19,15 @@
 #define CREATE_DIR  0x2
 
 
+/** For the `fstat()` syscall. */
+struct file_stat {
+    uint32_t inumber;
+    uint32_t type;
+    uint32_t size;
+};
+typedef struct file_stat file_stat_t;
+
+
 int32_t syscall_open();
 int32_t syscall_close();
 int32_t syscall_create();
@@ -28,6 +37,7 @@ int32_t syscall_write();
 int32_t syscall_chdir();
 int32_t syscall_getcwd();
 int32_t syscall_exec();
+int32_t syscall_fstat();
 
 
 #endif
diff --git a/src/filesys/vsfs.c b/src/filesys/vsfs.c
index 36ff24c..84ddc23 100644
--- a/src/filesys/vsfs.c
+++ b/src/filesys/vsfs.c
@@ -664,6 +664,21 @@ filesys_exec(char *path, char **argv)
 }
 
 
+/** Get metadata information about an open file. */
+bool
+filesys_fstat(int8_t fd, file_stat_t *stat)
+{
+    file_t *file = _find_process_file(fd);
+    if (file == NULL) {
+        warn("fstat: cannot find file for fd %d", fd);
+        return false;
+    }
+
+    file_stat(file, stat);
+    return true;
+}
+
+
 /** Flush the in-memory modified bitmap block to disk. */
 bool
 inode_bitmap_update(uint32_t slot_no)
diff --git a/src/filesys/vsfs.h b/src/filesys/vsfs.h
index 014f2ae..3d2fb5e 100644
--- a/src/filesys/vsfs.h
+++ b/src/filesys/vsfs.h
@@ -13,6 +13,8 @@
 #include <stdbool.h>
 #include <stddef.h>
 
+#include "sysfile.h"
+
 #include "../common/bitmap.h"
 
 
@@ -126,6 +128,8 @@ bool filesys_getcwd(char *buf, size_t limit);
 
 bool filesys_exec(char *path, char **argv);
 
+bool filesys_fstat(int8_t fd, file_stat_t *stat);
+
 bool inode_bitmap_update(uint32_t slot_no);
 bool data_bitmap_update(uint32_t slot_no);
 
diff --git a/src/interrupt/syscall.c b/src/interrupt/syscall.c
index ad9b146..8b70385 100644
--- a/src/interrupt/syscall.c
+++ b/src/interrupt/syscall.c
@@ -46,7 +46,8 @@ static syscall_t syscall_handlers[] = {
     [SYSCALL_WRITE]     syscall_write,
     [SYSCALL_CHDIR]     syscall_chdir,
     [SYSCALL_GETCWD]    syscall_getcwd,
-    [SYSCALL_EXEC]      syscall_exec
+    [SYSCALL_EXEC]      syscall_exec,
+    [SYSCALL_FSTAT]     syscall_fstat
 };
 
 #define NUM_SYSCALLS ((int32_t) (sizeof(syscall_handlers) / sizeof(syscall_t)))
diff --git a/src/interrupt/syscall.h b/src/interrupt/syscall.h
index 763fa53..27a7968 100644
--- a/src/interrupt/syscall.h
+++ b/src/interrupt/syscall.h
@@ -37,6 +37,7 @@
 #define SYSCALL_CHDIR   17
 #define SYSCALL_GETCWD  18
 #define SYSCALL_EXEC    19
+#define SYSCALL_FSTAT   20
 
 
 /**
diff --git a/user/init.c b/user/init.c
index 71a20be..8180990 100644
--- a/user/init.c
+++ b/user/init.c
@@ -4,6 +4,7 @@
 
 
 #include <stdint.h>
+#include <stddef.h>
 
 #include "lib/syscall.h"
 #include "lib/debug.h"
@@ -22,8 +23,9 @@ main(void)
 
     if (shell_pid == 0) {
         /** Child: exec the command line shell. */
-        char *argv[1];
-        argv[0] = 0;
+        char *argv[2];
+        argv[0] = "shell";
+        argv[1] = NULL;
         exec("shell", argv);
         error("init: failed to exec the shell program");
 
diff --git a/user/lib/printf.c b/user/lib/printf.c
index cdd552e..d7e7c07 100644
--- a/user/lib/printf.c
+++ b/user/lib/printf.c
@@ -632,6 +632,8 @@ _vsnprintf(char *buf, size_t count, const char *fmt, va_list va)
         seg_end++;
         fmt++;
     }
+
+    *buf = '\0';
 }
 
 /** Wrapper over string printing for `printf()` and `cprintf()`. */
diff --git a/user/lib/syscall.h b/user/lib/syscall.h
index c25bf57..893b25c 100644
--- a/user/lib/syscall.h
+++ b/user/lib/syscall.h
@@ -24,6 +24,29 @@
 #define CREATE_FILE 0x1
 #define CREATE_DIR  0x2
 
+/** Struct & type code for `fstat()`. */
+#define INODE_TYPE_EMPTY 0
+#define INODE_TYPE_FILE  1
+#define INODE_TYPE_DIR   2
+
+struct file_stat {
+    uint32_t inumber;
+    uint32_t type;
+    uint32_t size;
+};
+typedef struct file_stat file_stat_t;
+
+/** Struct of a directory entry, see `vsfs.h`. */
+#define DENTRY_SIZE 128
+#define MAX_FILENAME 100
+
+struct dentry {
+    uint32_t valid;
+    uint32_t inumber;
+    char filename[DENTRY_SIZE - 8];
+} __attribute__((packed));
+typedef struct dentry dentry_t;
+
 
 /**
  * Externed from ASM `syscall.s`.
@@ -50,6 +73,7 @@ extern int32_t write(int32_t fd, char *src, uint32_t len);
 extern int32_t chdir(char *path);
 extern int32_t getcwd(char *buf, uint32_t limit);
 extern int32_t exec(char *path, char **argv);
+extern int32_t fstat(int32_t fd, file_stat_t *stat);
 
 
 #endif
diff --git a/user/lib/syscall.s b/user/lib/syscall.s
index 4ec9595..fcbddf1 100644
--- a/user/lib/syscall.s
+++ b/user/lib/syscall.s
@@ -39,3 +39,4 @@ SYSCALL_LIBGEN  write,   SYSCALL_WRITE
 SYSCALL_LIBGEN  chdir,   SYSCALL_CHDIR
 SYSCALL_LIBGEN  getcwd,  SYSCALL_GETCWD
 SYSCALL_LIBGEN  exec,    SYSCALL_EXEC
+SYSCALL_LIBGEN  fstat,   SYSCALL_FSTAT
diff --git a/user/lib/syslist.s b/user/lib/syslist.s
index 50d5e4b..d8319ee 100644
--- a/user/lib/syslist.s
+++ b/user/lib/syslist.s
@@ -30,3 +30,4 @@ SYSCALL_WRITE   = 16
 SYSCALL_CHDIR   = 17
 SYSCALL_GETCWD  = 18
 SYSCALL_EXEC    = 19
+SYSCALL_FSTAT   = 20
diff --git a/user/ls.c b/user/ls.c
new file mode 100644
index 0000000..31c637b
--- /dev/null
+++ b/user/ls.c
@@ -0,0 +1,128 @@
+/**
+ * Command line utility - list directory.
+ */
+
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stddef.h>
+
+#include "lib/syscall.h"
+#include "lib/printf.h"
+#include "lib/debug.h"
+#include "lib/string.h"
+
+
+#define CONCAT_BUT_SIZE 300
+
+
+static char *
+_get_filename(char *path)
+{
+    int32_t idx;
+    for (idx = strlen(path) - 1; idx >= 0; --idx) {
+        if (path[idx] == '/')
+            return &path[idx + 1];
+    }
+    return path;
+}
+
+static void
+_print_file_stat(char *filename, file_stat_t *stat)
+{
+    bool is_dir = stat->type == INODE_TYPE_DIR;
+    printf("%5u %s %8u ", stat->inumber, is_dir ? "D" : "F", stat->size);
+    cprintf(is_dir ? VGA_COLOR_LIGHT_BLUE : VGA_COLOR_LIGHT_GREY,
+            "%s\n", filename);
+}
+
+
+static void
+_list_directory(char *path)
+{
+    int8_t fd = open(path, OPEN_RD);
+    if (fd < 0) {
+        warn("ls: cannot open path '%s'", path);
+        return;
+    }
+
+    file_stat_t stat;
+    if (fstat(fd, &stat) != 0) {
+        warn("ls: cannot get stat of '%s'", path);
+        close(fd);
+        return;
+    }
+
+    /** Listing on a regular file. */
+    if (stat.type == INODE_TYPE_FILE) {
+        _print_file_stat(_get_filename(path), &stat);
+        close(fd);
+        return;
+    }
+
+    /**
+     * Listing a directory, then read out its content, interpret as an
+     * array of directory entries.
+     */
+    char concat_buf[CONCAT_BUT_SIZE];
+    strncpy(concat_buf, path, CONCAT_BUT_SIZE - 2);
+    if (CONCAT_BUT_SIZE - 1 - strlen(concat_buf) < MAX_FILENAME) {
+        warn("ls: path '%s' too long", path);
+        close(fd);
+        return;
+    }
+    char *name_buf = &concat_buf[strlen(concat_buf)];
+    *(name_buf++) = '/';
+    *name_buf = '\0';
+
+    dentry_t dentry;
+    while (read(fd, (char *) &dentry, sizeof(dentry_t)) == sizeof(dentry_t)) {
+        if (dentry.valid != 1)
+            continue;
+
+        strncpy(name_buf, dentry.filename, MAX_FILENAME);
+        int8_t inner_fd = open(concat_buf, OPEN_RD);
+        if (inner_fd < 0) {
+            warn("ls: cannot open path '%s'", concat_buf);
+            close(fd);
+            return;
+        }
+
+        file_stat_t inner_stat;
+        if (fstat(inner_fd, &inner_stat) != 0) {
+            warn("ls: cannot get stat of '%s'", concat_buf);
+            close(inner_fd);
+            close(fd);
+            return;
+        }
+
+        _print_file_stat(dentry.filename, &inner_stat);
+        close(inner_fd);
+    }
+
+    close(fd);
+}
+
+
+void
+main(int argc, char *argv[])
+{
+    if (argc < 2) {
+        _list_directory(".");
+        exit();
+    }
+
+    if (argc == 2) {
+        _list_directory(argv[1]);
+        exit();
+    }
+
+    /** If multiple directories, display as multiple sections. */
+    for (int i = 1; i < argc; ++i) {
+        printf("%s:\n", argv[i]);
+        _list_directory(argv[i]);
+        if (i < argc - 1)
+            printf("\n");
+    }
+    exit();
+}
diff --git a/user/shell.c b/user/shell.c
index a9194f5..2aeb6cb 100644
--- a/user/shell.c
+++ b/user/shell.c
@@ -11,7 +11,14 @@
 #include "lib/printf.h"
 #include "lib/debug.h"
 #include "lib/string.h"
-#include "lib/malloc.h"
+#include "lib/types.h"
+
+
+#define CWD_BUF_SIZE  256
+#define LINE_BUF_SIZE 256
+#define MAX_EXEC_ARGS 32
+
+#define ENV_PATH "/"
 
 
 /** A fancy welcome logo in ASCII. */
@@ -29,172 +36,149 @@ _shell_welcome_logo(void)
             "\n");
 }
 
+/** Compose & print the prompt. */
+static void
+_print_prompt(void)
+{
+    char username[5] = "hush";
+    cprintf(VGA_COLOR_GREEN, "%s", username);
+
+    cprintf(VGA_COLOR_DARK_GREY, ":");
+
+    char cwd[CWD_BUF_SIZE];
+    memset(cwd, 0, CWD_BUF_SIZE);
+    if (getcwd(cwd, CWD_BUF_SIZE - MAX_FILENAME - 1) != 0)
+        error("shell: failed to get cwd");
+    size_t ret_len = strlen(cwd);
+    if (cwd[ret_len - 1] != '/') {
+        cwd[ret_len] = '/';
+        cwd[ret_len + 1] = '\0';
+    }
+    cprintf(VGA_COLOR_CYAN, "%s", cwd);
+
+    cprintf(VGA_COLOR_DARK_GREY, "$ ");
+}
+
+
+/** Built-in cmd `cd`: change directory. */
+static void
+_change_cwd(char *path)
+{
+    if (path == NULL)   /** empty path, go to "/". */
+        path = "/";
+
+    if (chdir(path) != 0)
+        warn("shell: cd to path '%s' failed", path);
+}
+
+/** Fork + exec external command executable. */
+static void
+_fork_exec(char *path, char **argv)
+{
+    int8_t pid = fork(0);
+    if (pid < 0) {
+        warn("shell: failed to fork child process");
+        return;
+    }
+
+    if (pid == 0) {     /** Child. */
+        /**
+         * Try the executable under current working directory if found.
+         * Otherwise, fall through to try the one under ENV_PATH (which
+         * is the root directory "/").
+         */
+        if (open(path, OPEN_RD) < 0) {      /** Not found. */
+            char full[CWD_BUF_SIZE];
+            snprintf(full, CWD_BUF_SIZE, "%s/%s", ENV_PATH, path);
+            exec(full, argv);
+        } else 
+            exec(path, argv);
+
+        warn("shell: failed to exec '%s'", path);
+        exit();
+    
+    } else {            /** Parent shell. */
+        int8_t pid_w = wait();
+        if (pid_w != pid)
+            warn("shell: waited pid %d does not equal %d", pid_w, pid);
+    }
+}
+
 
-__attribute__((unused))
+/**
+ * Parse a command line. Parse the tokens according to whitespaces,
+ * interpret the first one as the executable filename, and together
+ * with the rest as the argument list.
+ */
+static int
+_parse_tokens(char *line, char **argv)
+{
+    size_t argi;
+    size_t pos = 0;
+
+    for (argi = 0; argi < MAX_EXEC_ARGS - 1; ++argi) {
+        while (isspace(line[pos]))
+            pos++;
+        if (line[pos] == '\0') {
+            argv[argi] = NULL;
+            return argi;
+        }
+
+        argv[argi] = &line[pos];
+
+        while(!isspace(line[pos]))
+            pos++;
+        if (line[pos] != '\0')
+            line[pos++] = '\0';
+    }
+
+    argv[argi] = NULL;
+    return argi;
+}
+
+/**
+ * Handle a command line.
+ *   - If is a built-in command, handle directly;
+ *   - Else, fork a child process to run the command executable.
+ */
 static void
-_test_child_workload(void)
+_handle_cmdline(char *line)
 {
-    int32_t res = 0;
-    for (int32_t i = 12345; i < 57896; ++i)
-        for (int32_t j = i; j > 12345; --j)
-            res += (i * j) % 567;
-    printf("res %d: %d\n", getpid(), res);
+    char *argv[MAX_EXEC_ARGS];
+    int argc = _parse_tokens(line, argv);
+    if (argc <= 0)
+        return;
+    else if (argc >= MAX_EXEC_ARGS)
+        warn("shell: line exceeds max num of args %d", MAX_EXEC_ARGS);
+
+    if (strncmp(argv[0], "cd", 2) == 0)
+        _change_cwd(argv[1]);   /** Could be NULL. */
+    else
+        _fork_exec(argv[0], argv);
 }
 
 
 void
-main(void)
+main(int argc, char *argv[])
 {
-    _shell_welcome_logo();
+    (void) argc;    // Unused.
+    (void) argv;
 
-    printf("Hello from the exec'ed shell program!\n");
+    _shell_welcome_logo();
 
-    char cmd_buf[256];
-    memset(cmd_buf, 0, 256);
+    char cmd_buf[LINE_BUF_SIZE];
+    memset(cmd_buf, 0, LINE_BUF_SIZE);
 
     while (1) {
-        printf("temp shell $ ");
+        _print_prompt();
         
-        if (kbdstr(cmd_buf, 256) < 0)
-            warn("shell: failed to get keyboard string");
+        if (kbdstr(cmd_buf, LINE_BUF_SIZE) < 0)
+            error("shell: failed to get keyboard string");
         else
-            printf("%s", cmd_buf);
+            _handle_cmdline(cmd_buf);
 
-        memset(cmd_buf, 0, 256);
+        memset(cmd_buf, 0, LINE_BUF_SIZE);
     }
 
-    // char dirname[128] = "temp";
-    // char filepath[128] = "temp/test.txt";
-    // char filename[128] = "test.txt";
-
-    // printf("[P] Created dir '%s' -> %d\n", dirname, create(dirname, CREATE_DIR));
-    // printf("[P] Created file '%s' -> %d\n", filepath, create(filepath, CREATE_FILE));
-    // printf("[P] Changed cwd to '%s' -> %d\n", dirname, chdir(dirname));
-
-    // int8_t pid = fork(0);
-    // assert(pid >= 0);
-
-    // if (pid == 0) {
-    //     // Child.
-    //     char cwd[100];
-    //     printf("[C] Called getcwd -> %d\n", getcwd(cwd, 100));
-    //     printf("    cwd: %s\n", cwd);
-    //     int8_t fd = open(filename, OPEN_WR);
-    //     printf("[C] Opened file '%s' -> %d\n", filename, fd);
-    //     printf("[C] Written to fd %d -> %d\n", fd, write(fd, "AAAAA", 5));
-    //     printf("    src: %s\n", "AAAAA");
-    //     exit();
-
-    // } else {
-    //     // Parent.
-    //     assert(wait() == pid);
-    //     printf("[P] Changed cwd to '%s' -> %d\n", "./..", chdir("./.."));
-    //     int8_t fd = open(filepath, OPEN_RD);
-    //     printf("[P] Opened file '%s' -> %d\n", filepath, fd);
-    //     char buf[6] = {0};
-    //     printf("[P] Read from fd %d -> %d\n", fd, read(fd, buf, 5));
-    //     printf("    dst: %s\n", buf);
-    //     printf("[P] Closing fd %d -> %d\n", fd, close(fd));
-    //     printf("[P] Removing file '%s' -> %d\n", filepath, remove(filepath));
-    //     printf("[P] Removing dir '%s' -> %d\n", dirname, remove(dirname));
-    // }
-
-    // printf("Files done!\n");
-
-    // int8_t i;
-
-    // printf("parent: forking...\n");
-    // for (i = 1; i <= 3; ++i) {
-    //     int8_t pid = fork(i*4);
-    //     if (pid < 0)
-    //         error("parent: forking child i=%d failed", i);
-    //     if (pid == 0) {
-    //         // Child.
-    //         _test_child_workload();
-    //         exit();
-    //     } else
-    //         printf("parent: forked child pid=%d, timeslice=%d\n", pid, i*4);
-    // }
-
-    // printf("parent: waiting...\n");
-    // for (i = 1; i <= 3; ++i) {
-    //     int8_t pid = wait();
-    //     printf("parent: waited child pid=%d\n", pid);
-    // }
-
-    // printf("On-stack buffer of size 8200...\n");
-    // char buf[8200];
-    // buf[0] = 'A';
-    // buf[1] = '\0';
-    // printf("Variable buf @ %p: %s\n", buf, buf);
-
-    // printf("\nOn-heap allocations & frees...\n");
-    // char *buf1 = (char *) malloc(200);
-    // printf("Buf1: %p\n", buf1);
-    // char *buf2 = (char *) malloc(4777);
-    // printf("Buf2: %p\n", buf2);
-    // mfree(buf1);
-    // char *buf3 = (char *) malloc(8);
-    // printf("Buf3: %p\n", buf3);
-    // mfree(buf3);
-    // mfree(buf2);
-
-    // char kbd_buf[100];
-    // printf("Please enter an input str: ");
-    // kbdstr(kbd_buf, 100);
-    // printf("Fetched str from keyboard: %s\n", kbd_buf);
-
-    // int32_t mypid = getpid();
-    // printf(" Parent: parent gets pid - %d\n", mypid);
-    // sleep(2000);
-
-    // cprintf(VGA_COLOR_LIGHT_GREEN, "\n Round 1 --\n");
-    // printf("  Parent: forking child 1\n");
-    // int32_t pid1 = fork(0);
-    // if (pid1 < 0) {
-    //     cprintf(VGA_COLOR_RED, "  Parent: fork failed\n");
-    //     _fake_halt();
-    // }
-    // if (pid1 == 0) {
-    //     // Child.
-    //     printf("  Child1: entering an infinite loop\n");
-    //     _fake_halt();
-    // } else {
-    //     // Parent.
-    //     printf("  Parent: child 1 has pid - %d\n", pid1);
-    //     sleep(1500);
-    //     printf("  Parent: slept 1.5 secs, going to kill child 1\n");
-    //     kill(pid1);
-    //     wait();
-    //     printf("  Parent: killed child 1\n");
-    // }
-
-    // cprintf(VGA_COLOR_LIGHT_GREEN, "\n Round 2 --\n");
-    // printf("  Current uptime: %d ms\n", uptime());
-    // printf("  Going to sleep for 2000 ms...\n");
-    // sleep(2000);
-    // printf("  Current uptime: %d ms\n", uptime());
-
-    // cprintf(VGA_COLOR_LIGHT_GREEN, "\n Round 3 --\n");
-    // printf("  Parent: forking child 2\n");
-    // int32_t pid2 = fork(0);
-    // if (pid2 < 0) {
-    //     cprintf(VGA_COLOR_RED, "  Parent: fork failed\n");
-    //     _fake_halt();
-    // }
-    // if (pid2 == 0) {
-    //     // Child.
-    //     printf("  Child2: going to sleep 2 secs\n");
-    //     sleep(2000);
-    //     exit();
-    // } else {
-    //     // Parent.
-    //     printf("  Parent: child 2 has pid - %d\n", pid2);
-    //     wait();
-    //     printf("  Parent: waited child 2\n");
-    // }
-
-    // cprintf(VGA_COLOR_GREEN, "\n Cases done!\n");
-
     exit();
 }
diff --git a/user/tests/filetest.c b/user/tests/filetest.c
new file mode 100644
index 0000000..1d9e169
--- /dev/null
+++ b/user/tests/filetest.c
@@ -0,0 +1,58 @@
+/**
+ * User test program - file system operations.
+ */
+
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stddef.h>
+
+#include "../lib/debug.h"
+#include "../lib/printf.h"
+#include "../lib/syscall.h"
+
+
+void
+main(int argc, char *argv[])
+{
+    (void) argc;    // Unused.
+    (void) argv;
+    
+    char dirname[128] = "temp";
+    char filepath[128] = "temp/test.txt";
+    char filename[128] = "test.txt";
+
+    printf("[P] Created dir '%s' -> %d\n", dirname, create(dirname, CREATE_DIR));
+    printf("[P] Created file '%s' -> %d\n", filepath, create(filepath, CREATE_FILE));
+    printf("[P] Changed cwd to '%s' -> %d\n", dirname, chdir(dirname));
+
+    int8_t pid = fork(0);
+    assert(pid >= 0);
+
+    if (pid == 0) {
+        // Child.
+        char cwd[100];
+        printf("[C] Called getcwd -> %d\n", getcwd(cwd, 100));
+        printf("    cwd: %s\n", cwd);
+        int8_t fd = open(filename, OPEN_WR);
+        printf("[C] Opened file '%s' -> %d\n", filename, fd);
+        printf("[C] Written to fd %d -> %d\n", fd, write(fd, "AAAAA", 5));
+        printf("    src: %s\n", "AAAAA");
+        exit();
+
+    } else {
+        // Parent.
+        assert(wait() == pid);
+        printf("[P] Changed cwd to '%s' -> %d\n", "./..", chdir("./.."));
+        int8_t fd = open(filepath, OPEN_RD);
+        printf("[P] Opened file '%s' -> %d\n", filepath, fd);
+        char buf[6] = {0};
+        printf("[P] Read from fd %d -> %d\n", fd, read(fd, buf, 5));
+        printf("    dst: %s\n", buf);
+        printf("[P] Closing fd %d -> %d\n", fd, close(fd));
+        printf("[P] Removing file '%s' -> %d\n", filepath, remove(filepath));
+        printf("[P] Removing dir '%s' -> %d\n", dirname, remove(dirname));
+    }
+
+    exit();
+}
diff --git a/user/tests/memtest.c b/user/tests/memtest.c
new file mode 100644
index 0000000..60dbd0b
--- /dev/null
+++ b/user/tests/memtest.c
@@ -0,0 +1,40 @@
+/**
+ * User test program - file system operations.
+ */
+
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stddef.h>
+
+#include "../lib/debug.h"
+#include "../lib/printf.h"
+#include "../lib/syscall.h"
+#include "../lib/malloc.h"
+
+
+void
+main(int argc, char *argv[])
+{
+    (void) argc;    // Unused.
+    (void) argv;
+    
+    printf("On-stack buffer of size 8200...\n");
+    char buf[8200];
+    buf[0] = 'A';
+    buf[1] = '\0';
+    printf("Variable buf @ %p: %s\n", buf, buf);
+
+    printf("\nOn-heap allocations & frees...\n");
+    char *buf1 = (char *) malloc(200);
+    printf("Buf1: %p\n", buf1);
+    char *buf2 = (char *) malloc(4777);
+    printf("Buf2: %p\n", buf2);
+    mfree(buf1);
+    char *buf3 = (char *) malloc(8);
+    printf("Buf3: %p\n", buf3);
+    mfree(buf3);
+    mfree(buf2);
+
+    exit();
+}
diff --git a/user/tests/proctest.c b/user/tests/proctest.c
new file mode 100644
index 0000000..61fd3b4
--- /dev/null
+++ b/user/tests/proctest.c
@@ -0,0 +1,75 @@
+/**
+ * User test program - file system operations.
+ */
+
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stddef.h>
+
+#include "../lib/debug.h"
+#include "../lib/printf.h"
+#include "../lib/syscall.h"
+
+
+void
+main(int argc, char *argv[])
+{
+    (void) argc;    // Unused.
+    (void) argv;
+    
+    int32_t mypid = getpid();
+    printf(" Parent: parent gets pid - %d\n", mypid);
+    sleep(2000);
+
+    cprintf(VGA_COLOR_LIGHT_GREEN, "\n Round 1 --\n");
+    printf("  Parent: forking child 1\n");
+    int32_t pid1 = fork(0);
+    if (pid1 < 0) {
+        cprintf(VGA_COLOR_RED, "  Parent: fork failed\n");
+        exit();
+    }
+    if (pid1 == 0) {
+        // Child.
+        printf("  Child1: entering an infinite loop\n");
+        while (true)
+            sleep(5000);
+    } else {
+        // Parent.
+        printf("  Parent: child 1 has pid - %d\n", pid1);
+        sleep(1500);
+        printf("  Parent: slept 1.5 secs, going to kill child 1\n");
+        kill(pid1);
+        wait();
+        printf("  Parent: killed child 1\n");
+    }
+
+    cprintf(VGA_COLOR_LIGHT_GREEN, "\n Round 2 --\n");
+    printf("  Current uptime: %d ms\n", uptime());
+    printf("  Going to sleep for 2000 ms...\n");
+    sleep(2000);
+    printf("  Current uptime: %d ms\n", uptime());
+
+    cprintf(VGA_COLOR_LIGHT_GREEN, "\n Round 3 --\n");
+    printf("  Parent: forking child 2\n");
+    int32_t pid2 = fork(0);
+    if (pid2 < 0) {
+        cprintf(VGA_COLOR_RED, "  Parent: fork failed\n");
+        exit();
+    }
+    if (pid2 == 0) {
+        // Child.
+        printf("  Child2: going to sleep 2 secs\n");
+        sleep(2000);
+        exit();
+    } else {
+        // Parent.
+        printf("  Parent: child 2 has pid - %d\n", pid2);
+        wait();
+        printf("  Parent: waited child 2\n");
+    }
+
+    cprintf(VGA_COLOR_GREEN, "\n Cases done!\n");
+
+    exit();
+}
diff --git a/user/tests/schedtest.c b/user/tests/schedtest.c
new file mode 100644
index 0000000..d6aea62
--- /dev/null
+++ b/user/tests/schedtest.c
@@ -0,0 +1,54 @@
+/**
+ * User test program - weighted scheduling.
+ */
+
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stddef.h>
+
+#include "../lib/debug.h"
+#include "../lib/printf.h"
+#include "../lib/syscall.h"
+
+
+static void
+_test_child_workload(void)
+{
+    int32_t res = 0;
+    for (int32_t i = 12345; i < 57896; ++i)
+        for (int32_t j = i; j > 12345; --j)
+            res += (i * j) % 567;
+    printf("res %d: %d\n", getpid(), res);
+}
+
+
+void
+main(int argc, char *argv[])
+{
+    (void) argc;    // Unused.
+    (void) argv;
+
+    int8_t i;
+
+    printf("parent: forking...\n");
+    for (i = 1; i <= 3; ++i) {
+        int8_t pid = fork(i*4);
+        if (pid < 0)
+            error("parent: forking child i=%d failed", i);
+        if (pid == 0) {
+            // Child.
+            _test_child_workload();
+            exit();
+        } else
+            printf("parent: forked child pid=%d, timeslice=%d\n", pid, i*4);
+    }
+
+    printf("parent: waiting...\n");
+    for (i = 1; i <= 3; ++i) {
+        int8_t pid = wait();
+        printf("parent: waited child pid=%d\n", pid);
+    }
+
+    exit();
+}
diff --git a/wiki b/wiki
index 2b48c13..a1c932e 160000
--- a/wiki
+++ b/wiki
@@ -1 +1 @@
-Subproject commit 2b48c13242a74af964b2a23186ff4a18698f02c6
+Subproject commit a1c932ec867e9cc10abf74851ccadf6b19476dd2