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Permissions

Parts of the Permission String

There are 4 parts in the permission string.
It may look something like this: 

-rwxr-xr-x
This represents a basic file.

The permission string can be broken down into these 4 parts:

  1. The first character indicates the type of file. (-)
    The next nine characters represent the permissions for users, broken up into 3 characters for each permission.

  2. user (owner) - (rwx)

  3. group - (r-x)
  4. others - (r-x)

File Type Indicator Bit (first bit)

-rw------
^
# Filetype indicator bit

The first character in the permission string indicates the type of file:

  • -: Regular file.
  • d: Directory.
  • l: Symbolic link.
  • c: Character device file (special file that represents a device).
  • b: Block device file (special file that represents a device such as a hard disk).
  • s: Socket (used for IPC - inter-process communication).
  • p: Named pipe (FIFO).

Permission Bits

The next nine characters are in three sets of three characters, each set representing the permissions for the user (owner), group, and others:

  • r: Read permission.

    • For a file, this means the contents of the file can be read.
    • For a directory, this means the contents of the directory can be listed.

  • w: Write permission.

    • For a file, this means the contents of the file can be modified.
    • For a directory, this means files can be created, deleted, or renamed within the directory.

  • x: Execute permission.

    • For a file, this means the file can be executed (if it's a program or a script).
    • For a directory, this means the ability to access the directory's contents.

The three sets are:

  1. User (Owner) Permissions: The first set of three characters after the file type.
  2. Group Permissions: The second set of three characters.
  3. Others Permissions: The third set of three characters.

Special Permission Bits

In addition to r, w, and x, there are a few special permissions:

  • s: Setuid/Setgid.

    • Appears in the user or group permission field instead of the x.
    • If set on a file, the file will execute with the permissions of the file owner or group.

  • t: Sticky bit.

    • Appears in the others permission field.
    • Often used on directories, like /tmp.
    • Indicates that only the file owner (or root) can delete or rename files in the directory.

  • -: Means the absence of a permission.

    • The - in a field means that the corresponding set of users does not have that permission.