# Referensi Cepat chmod

*Izin file, mode numerik & simbolik, bit khusus, umask*

> Source: chmod(1) Manual Page · MIT

## Mode Numerik

### Digit Oktal Izin

| Command | Description |
|---------|-------------|
| `4` | Baca (r) |
| `2` | Tulis (w) |
| `1` | Eksekusi (x) |
| `0` | Tanpa izin |

### Format Tiga Digit

```
chmod 755 file    # rwxr-xr-x
chmod 644 file    # rw-r--r--
chmod 700 file    # rwx------
chmod 600 file    # rw-------
```

### Perhitungan Digit

| Command | Description |
|---------|-------------|
| `7 (4+2+1)` | rwx — baca, tulis, eksekusi |
| `6 (4+2)` | rw- — baca, tulis |
| `5 (4+1)` | r-x — baca, eksekusi |
| `4` | r-- — baca saja |
| `3 (2+1)` | -wx — tulis, eksekusi |
| `2` | -w- — tulis saja |
| `1` | --x — eksekusi saja |
| `0` | --- — tanpa izin |

## Mode Simbolik

### Sintaks: [ugoa][+-=][rwxXst]

| Command | Description |
|---------|-------------|
| `u` | User (pemilik) |
| `g` | Group |
| `o` | Others (lainnya) |
| `a` | Semua (u + g + o) |
| `+` | Tambah izin |
| `-` | Hapus izin |
| `=` | Set izin tepat |

### Contoh Simbolik

```
chmod u+x file         # owner: add execute
chmod g-w file         # group: remove write
chmod o=r file         # others: set read only
chmod a+r file         # all: add read
chmod u+x,g-w,o= file # combined operations
```

## Izin Umum

### Preset Izin File

| Command | Description |
|---------|-------------|
| `644  rw-r--r--` | File default — pemilik rw, lainnya baca |
| `755  rwxr-xr-x` | Script/binary — pemilik rwx, lainnya rx |
| `600  rw-------` | File privat — pemilik saja |
| `400  r--------` | Baca saja privat (SSH key) |
| `666  rw-rw-rw-` | File world-writable (hindari) |
| `777  rwxrwxrwx` | Akses penuh semua pihak (hindari) |

### Referensi Cepat

```
chmod 644 *.html       # web files: owner rw, world r
chmod 755 *.sh         # scripts: owner rwx, world rx
chmod 600 ~/.ssh/id_*  # SSH keys: owner only
chmod 400 secret.pem   # certificate: read-only
```

## Izin Direktori

### Arti Izin untuk Direktori

| Command | Description |
|---------|-------------|
| `r (4)` | Tampilkan isi direktori (`ls`) |
| `w (2)` | Buat/hapus file di direktori |
| `x (1)` | Masuk ke direktori (`cd`) |
| `rx (5)` | Tampilkan + akses (khas untuk baca) |
| `rwx (7)` | Kontrol penuh |

### Izin Direktori Umum

```
chmod 755 dir/     # standard: owner rwx, others rx
chmod 700 dir/     # private: owner only
chmod 750 dir/     # group access: owner rwx, group rx
chmod 1777 /tmp    # sticky bit: only owner can delete
```

## Bit Khusus

### Setuid, Setgid, Sticky

| Command | Description |
|---------|-------------|
| `Setuid (4xxx)` | Jalankan sebagai pemilik file (mis. `passwd`) |
| `Setgid (2xxx)` | Jalankan sebagai group file / warisi group direktori |
| `Sticky (1xxx)` | Hanya pemilik yang bisa hapus file (mis. `/tmp`) |

### Mengatur Bit Khusus

```
chmod 4755 program     # setuid: -rwsr-xr-x
chmod 2755 dir/        # setgid: drwxr-sr-x
chmod 1755 dir/        # sticky: drwxr-xr-t
chmod u+s program      # symbolic setuid
chmod g+s dir/         # symbolic setgid
chmod +t dir/          # symbolic sticky bit
```

## Rekursif

### Perubahan Izin Rekursif

```
chmod -R 755 dir/           # set all to 755 recursively
chmod -R u+rwX dir/         # owner rw, +x on dirs only
chmod -R go-w dir/          # remove group/other write
```

### File vs Direktori dengan find

```
# set directories to 755, files to 644
find /path -type d -exec chmod 755 {} +
find /path -type f -exec chmod 644 {} +
```

### X Besar — Eksekusi Kondisional

| Command | Description |
|---------|-------------|
| `x (huruf kecil)` | Tambah execute ke semua file dan direktori |
| `X (huruf besar)` | Tambah execute hanya ke direktori dan file yang sudah executable |

## umask

### Cara Kerja umask

| Command | Description |
|---------|-------------|
| `umask` | Tampilkan umask saat ini |
| `umask 022` | File: 644, Direktori: 755 |
| `umask 077` | File: 600, Direktori: 700 |
| `umask 002` | File: 664, Direktori: 775 |

### Perhitungan umask

```
# default permission minus umask = effective
# Files:  666 - 022 = 644 (rw-r--r--)
# Dirs:   777 - 022 = 755 (rwxr-xr-x)
umask          # display current umask
umask 022      # typical default
umask -S       # show in symbolic notation
```

## Pola Umum

### Kasus Penggunaan Sehari-hari

| Command | Description |
|---------|-------------|
| `Web root` | `chmod -R 755 /var/www/html` |
| `File config` | `chmod 600 app.conf` |
| `Direktori SSH` | `chmod 700 ~/.ssh` |
| `SSH authorized_keys` | `chmod 600 ~/.ssh/authorized_keys` |
| `Direktori bersama` | `chmod 2775 /shared` (setgid) |
| `File log` | `chmod 640 /var/log/app.log` |
| `Script cron` | `chmod 755 /etc/cron.daily/myjob` |
| `Direktori temp` | `chmod 1777 /tmp` (sticky) |

### Melihat Izin

```
ls -l file.txt       # show permissions
ls -ld dir/          # show directory permissions
stat -c '%A %a %n' * # symbolic + numeric + name
getfacl file.txt     # show ACLs (if in use)
```