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| 1 | + | # Filesystem | |
| 2 | + | ||
| 3 | + | The UNIX filesystem is a hierarchical collection of files and directories. Directories are containers that can contain named files or directories. A path in UNIX is separated by `/`, and the root of the filesystem is denoted by `/`, such that `/foo/bar/baz` refers to a file or directory named `baz` inside the `bar` directory that lives in the `foo` directory in the root of the filesystem. | |
| 4 | + | ||
| 5 | + | The entries in a directory are called `directory entries`, `dirents`, or `dentries`. There are minor technical differences between these terms, mostly based on what level of abstraction they typically involve. A directory entry has a name and points to an `inode`. Typically this is a 1-to-1 correlation, but it can sometimes be useful to have two different directory entries point to the same inode -- this is called a `hardlink` when it does happen, and means you have two names for the same file. | |
| 6 | + | ||
| 7 | + | An inode represents, generally, the concept of a file or directory in the filesystem. It is where all the properties of what it means to be a file live and as such, I'll use `file` and `inode` interchangeably from now on. | |
| 8 | + | ||
| 9 | + | The filesystem can be navigated by using `cd` to change your current directory, and `ls` to list the contents of a directory (or your current directory). By convention, files starting with a `.` are considered hidden, and you will need to use `ls -a` to see them. There are two special files in each directory: `.` which refers to this directory and `..` which refers to this directory's containing directory. | |
| 10 | + | ||
| 11 | + | ## File Properties | |
| 12 | + | ||
| 13 | + | A file has a `user`. This is an integer representing the user ID that owns the file. This can be changed by using the `chown` tool and viewed with `ls -l`. | |
| 14 | + | ||
| 15 | + | A file has a `group`. This is an integer representing the group ID that owns the file. This can be changed by using the `chgrp` tool, or the combined set option of the `chown` tool; and viewed with `ls -l`. | |
| 16 | + | ||
| 17 | + | A file has a `mode`. This is typically represented as an octal number, and is often called the `permissions` on the file. It is a bitfield of flags: | |
| 18 | + | ||
| 19 | + | | Mode | Description | | |
| 20 | + | | -------- | ------- | | |
| 21 | + | | `04000` | Set UID; sets the effective user ID of a process created by executing this file to the `user` of this file. | | |
| 22 | + | | `02000` | Set GID; sets the effective group ID of a process created by executing this file to the `group` of this file. | | |
| 23 | + | | `01000` | Sticky bit; when set on a directory prevents users with write access to the directory from renaming or deleting files in the directory. | | |
| 24 | + | | `00400` | User Read; allows the file's `user` to read the file. | | |
| 25 | + | | `00200` | User Write; allows the file's `user` to write the file. | | |
| 26 | + | | `00100` | User Execute; when set on a directory, allows the file's `user` to access files in the directory; otherwise allows the file's `user` to execute the file. | | |
| 27 | + | | `00040` | Group Read; allows members of the file's `group` to read the file. | | |
| 28 | + | | `00020` | Group Write; allows members of the file's `group` to write the file. | | |
| 29 | + | | `00010` | Group Execute; when set on a directory, allows members of the file's `group` to access files in the directory; otherwise allows members of the file's `group` to execute the file. | | |
| 30 | + | | `00004` | Other Read; allows other users to read the file. | | |
| 31 | + | | `00002` | Other Write; allows other users to write the file. | | |
| 32 | + | | `00001` | Other Execute; when set on a directory, allows other users to access files in the directory; otherwise allows other users to execute the file. | | |
| 33 | + | ||
| 34 | + | These can be set using the `chmod` tool and viewed with `ls -l`. | |
| 35 | + | ||
| 36 | + | A file may also have extended attributes. These can be seen by using `lsattr` and `getfattr` tools, and changed using the `chattr` and `setfattr` tools. These attributes are flags that cause the file to be treated specially, and are typically quite niche. | |
| 37 | + | ||
| 38 | + | A file may also have an access control list. This is a more fine-grained permission system that allows granting specific users and groups specific permissions to files or directories. These can be seen by using the `getfacl` tool, and changed by using the `setfacl` tool. | |
| 39 | + | ||
| 40 | + | A file has a few timestamps. `ctime` is the creation timestamp, set to the time the file was first created. `mtime` is the modified timestamp, set to the last time the file was modified. `atime` is the access timestamp, set to the last time the file was accessed, though many filesystems do not set the `atime` for performance reasons. These can be viewed with `ls -l`. | |
| 41 | + | ||
| 42 | + | # Processes | |
| 43 | + | ||
| 44 | + | A `process` on a UNIX system is the result of executing a file. | |
| 45 | + | ||
| 46 | + | Each process has: | |
| 47 | + | - an integer ID called its `pid`. | |
| 48 | + | - a parent PID called its `ppid`. | |
| 49 | + | - a `user` and `group` ID. | |
| 50 | + | - argument vector (`argv`) | |
| 51 | + | - environment (`envp`) | |
| 52 | + | - virtual memory + mapping | |
| 53 | + | - a filesystem root | |
| 54 | + | - and a ton of other things. | |
| 55 | + | ||
| 56 | + | You can inspect the process table with `ps` (or `ps auxf` for more details). | |
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