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# Filesystem 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. 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. 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. 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. ## File Properties 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`. 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`. 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: | Mode | Description | | -------- | ------- | | `04000` | Set UID; sets the effective user ID of a process created by executing this file to the `user` of this file. | | `02000` | Set GID; sets the effective group ID of a process created by executing this file to the `group` of this file. | | `01000` | Sticky bit; when set on a directory prevents users with write access to the directory from renaming or deleting files in the directory. | | `00400` | User Read; allows the file's `user` to read the file. | | `00200` | User Write; allows the file's `user` to write the file. | | `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. | | `00040` | Group Read; allows members of the file's `group` to read the file. | | `00020` | Group Write; allows members of the file's `group` to write the file. | | `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. | | `00004` | Other Read; allows other users to read the file. | | `00002` | Other Write; allows other users to write the file. | | `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. | These can be set using the `chmod` tool and viewed with `ls -l`. 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. 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. 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`. # Processes A `process` on a UNIX system is the result of executing a file. Each process has: - an integer ID called its `pid`. - a parent PID called its `ppid`. - a `user` and `group` ID. - argument vector (`argv`) - environment (`envp`) - virtual memory + mapping - a filesystem root - and a ton of other things. You can inspect the process table with `ps` (or `ps auxf` for more details).

Filesystem

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.

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.

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.

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.

File Properties

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.

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.

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:

Mode	Description
`04000`	Set UID; sets the effective user ID of a process created by executing this file to the `user` of this file.
`02000`	Set GID; sets the effective group ID of a process created by executing this file to the `group` of this file.
`01000`	Sticky bit; when set on a directory prevents users with write access to the directory from renaming or deleting files in the directory.
`00400`	User Read; allows the file's `user` to read the file.
`00200`	User Write; allows the file's `user` to write the file.
`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.
`00040`	Group Read; allows members of the file's `group` to read the file.
`00020`	Group Write; allows members of the file's `group` to write the file.
`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.
`00004`	Other Read; allows other users to read the file.
`00002`	Other Write; allows other users to write the file.
`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.

These can be set using the chmod tool and viewed with ls -l.

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.

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.

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.

Processes

A process on a UNIX system is the result of executing a file.

Each process has:

an integer ID called its pid.
a parent PID called its ppid.
a user and group ID.
argument vector (argv)
environment (envp)
virtual memory + mapping
a filesystem root
and a ton of other things.

You can inspect the process table with ps (or ps auxf for more details).