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27. File permissions

Each file has a set of file mode bits that control the kinds of access that users have to that file. They can be represented either in symbolic form or as an octal number.


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27.1 Structure of File Mode Bits

The file mode bits have two parts: the file permission bits, which control ordinary access to the file, and special mode bits, which affect only some files.

There are three kinds of permissions that a user can have for a file:

  1. permission to read the file. For directories, this means permission to list the contents of the directory.
  2. permission to write to (change) the file. For directories, this means permission to create and remove files in the directory.
  3. permission to execute the file (run it as a program). For directories, this means permission to access files in the directory.

There are three categories of users who may have different permissions to perform any of the above operations on a file:

  1. the file's owner;
  2. other users who are in the file's group;
  3. everyone else.

Files are given an owner and group when they are created. Usually the owner is the current user and the group is the group of the directory the file is in, but this varies with the operating system, the file system the file is created on, and the way the file is created. You can change the owner and group of a file by using the chown and chgrp commands.

In addition to the three sets of three permissions listed above, the file mode bits have three special components, which affect only executable files (programs) and, on most systems, directories:

  1. Set the process's effective user ID to that of the file upon execution (called the set-user-ID bit, or sometimes the setuid bit). For directories on a few systems, give files created in the directory the same owner as the directory, no matter who creates them, and set the set-user-ID bit of newly-created subdirectories.
  2. Set the process's effective group ID to that of the file upon execution (called the set-group-ID bit, or sometimes the setgid bit). For directories on most systems, give files created in the directory the same group as the directory, no matter what group the user who creates them is in, and set the set-group-ID bit of newly-created subdirectories.
  3. Prevent unprivileged users from removing or renaming a file in a directory unless they own the file or the directory; this is called the restricted deletion flag for the directory, and is commonly found on world-writable directories like `/tmp'.

    For regular files on some older systems, save the program's text image on the swap device so it will load more quickly when run; this is called the sticky bit.

In addition to the file mode bits listed above, there may be file attributes specific to the file system, e.g., access control lists (ACLs), whether a file is compressed, whether a file can be modified (immutability), and whether a file can be dumped. These are usually set using programs specific to the file system. For example:

ext2

On GNU and GNU/Linux the file attributes specific to the ext2 file system are set using chattr.

FFS

On FreeBSD the file flags specific to the FFS file system are set using chflags.

Even if a file's mode bits allow an operation on that file, that operation may still fail, because:

For example, if the immutable attribute is set on a file, it cannot be modified, regardless of the fact that you may have just run chmod a+w FILE.


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27.2 Symbolic Modes

Symbolic modes represent changes to files' mode bits as operations on single-character symbols. They allow you to modify either all or selected parts of files' mode bits, optionally based on their previous values, and perhaps on the current umask as well (see section The Umask and Protection).

The format of symbolic modes is:

 
[ugoa…][+-=]perms…[,…]

where perms is either zero or more letters from the set `rwxXst', or a single letter from the set `ugo'.

The following sections describe the operators and other details of symbolic modes.


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27.2.1 Setting Permissions

The basic symbolic operations on a file's permissions are adding, removing, and setting the permission that certain users have to read, write, and execute or search the file. These operations have the following format:

 
users operation permissions

The spaces between the three parts above are shown for readability only; symbolic modes cannot contain spaces.

The users part tells which users' access to the file is changed. It consists of one or more of the following letters (or it can be empty; see section The Umask and Protection, for a description of what happens then). When more than one of these letters is given, the order that they are in does not matter.

u

the user who owns the file;

g

other users who are in the file's group;

o

all other users;

a

all users; the same as `ugo'.

The operation part tells how to change the affected users' access to the file, and is one of the following symbols:

+

to add the permissions to whatever permissions the users already have for the file;

-

to remove the permissions from whatever permissions the users already have for the file;

=

to make the permissions the only permissions that the users have for the file.

The permissions part tells what kind of access to the file should be changed; it is normally zero or more of the following letters. As with the users part, the order does not matter when more than one letter is given. Omitting the permissions part is useful only with the `=' operation, where it gives the specified users no access at all to the file.

r

the permission the users have to read the file;

w

the permission the users have to write to the file;

x

the permission the users have to execute the file, or search it if it is a directory.

For example, to give everyone permission to read and write a regular file, but not to execute it, use:

 
a=rw

To remove write permission for all users other than the file's owner, use:

 
go-w

The above command does not affect the access that the owner of the file has to it, nor does it affect whether other users can read or execute the file.

To give everyone except a file's owner no permission to do anything with that file, use the mode below. Other users could still remove the file, if they have write permission on the directory it is in.

 
go=

Another way to specify the same thing is:

 
og-rwx

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27.2.2 Copying Existing Permissions

You can base a file's permissions on its existing permissions. To do this, instead of using a series of `r', `w', or `x' letters after the operator, you use the letter `u', `g', or `o'. For example, the mode

 
o+g

adds the permissions for users who are in a file's group to the permissions that other users have for the file. Thus, if the file started out as mode 664 (`rw-rw-r--'), the above mode would change it to mode 666 (`rw-rw-rw-'). If the file had started out as mode 741 (`rwxr----x'), the above mode would change it to mode 745 (`rwxr--r-x'). The `-' and `=' operations work analogously.


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27.2.3 Changing Special Mode Bits

In addition to changing a file's read, write, and execute/search permissions, you can change its special mode bits. See section Structure of File Mode Bits, for a summary of these special mode bits.

To change the file mode bits to set the user ID on execution, use `u' in the users part of the symbolic mode and `s' in the permissions part.

To change the file mode bits to set the group ID on execution, use `g' in the users part of the symbolic mode and `s' in the permissions part.

To set both user and group ID on execution, omit the users part of the symbolic mode (or use `a') and use `s' in the permissions part.

To change the file mode bits to set the restricted deletion flag or sticky bit, omit the users part of the symbolic mode (or use `a') and use `t' in the permissions part.

For example, to set the set-user-ID mode bit of a program, you can use the mode:

 
u+s

To remove both set-user-ID and set-group-ID mode bits from it, you can use the mode:

 
a-s

To set the restricted deletion flag or sticky bit, you can use the mode:

 
+t

The combination `o+s' has no effect. On GNU systems the combinations `u+t' and `g+t' have no effect, and `o+t' acts like plain `+t'.

The `=' operator is not very useful with special mode bits. For example, the mode:

 
o=t

does set the restricted deletion flag or sticky bit, but it also removes all read, write, and execute/search permissions that users not in the file's group might have had for it.

See section Directories and the Set-User-ID and Set-Group-ID Bits, for additional rules concerning set-user-ID and set-group-ID bits and directories.


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27.2.4 Conditional Executability

There is one more special type of symbolic permission: if you use `X' instead of `x', execute/search permission is affected only if the file is a directory or already had execute permission.

For example, this mode:

 
a+X

gives all users permission to search directories, or to execute files if anyone could execute them before.


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27.2.5 Making Multiple Changes

The format of symbolic modes is actually more complex than described above (see section Setting Permissions). It provides two ways to make multiple changes to files' mode bits.

The first way is to specify multiple operation and permissions parts after a users part in the symbolic mode.

For example, the mode:

 
og+rX-w

gives users other than the owner of the file read permission and, if it is a directory or if someone already had execute permission to it, gives them execute/search permission; and it also denies them write permission to the file. It does not affect the permission that the owner of the file has for it. The above mode is equivalent to the two modes:

 
og+rX
og-w

The second way to make multiple changes is to specify more than one simple symbolic mode, separated by commas. For example, the mode:

 
a+r,go-w

gives everyone permission to read the file and removes write permission on it for all users except its owner. Another example:

 
u=rwx,g=rx,o=

sets all of the permission bits for the file explicitly. (It gives users who are not in the file's group no permission at all for it.)

The two methods can be combined. The mode:

 
a+r,g+x-w

gives all users permission to read the file, and gives users who are in the file's group permission to execute/search it as well, but not permission to write to it. The above mode could be written in several different ways; another is:

 
u+r,g+rx,o+r,g-w

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27.2.6 The Umask and Protection

If the users part of a symbolic mode is omitted, it defaults to `a' (affect all users), except that any permissions that are set in the system variable umask are not affected. The value of umask can be set using the umask command. Its default value varies from system to system.

Omitting the users part of a symbolic mode is generally not useful with operations other than `+'. It is useful with `+' because it allows you to use umask as an easily customizable protection against giving away more permission to files than you intended to.

As an example, if umask has the value 2, which removes write permission for users who are not in the file's group, then the mode:

 
+w

adds permission to write to the file to its owner and to other users who are in the file's group, but not to other users. In contrast, the mode:

 
a+w

ignores umask, and does give write permission for the file to all users.


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27.3 Numeric Modes

As an alternative to giving a symbolic mode, you can give an octal (base 8) number that represents the mode. This number is always interpreted in octal; you do not have to add a leading `0', as you do in C. Mode `0055' is the same as mode `55'.

A numeric mode is usually shorter than the corresponding symbolic mode, but it is limited in that normally it cannot take into account the previous file mode bits; it can only set them absolutely. (As discussed in the next section, the set-user-ID and set-group-ID bits of directories are an exception to this general limitation.)

The permissions granted to the user, to other users in the file's group, and to other users not in the file's group each require three bits, which are represented as one octal digit. The three special mode bits also require one bit each, and they are as a group represented as another octal digit. Here is how the bits are arranged, starting with the lowest valued bit:

 
Value in  Corresponding
Mode      Mode Bit

          Other users not in the file's group:
   1      Execute/search
   2      Write
   4      Read

          Other users in the file's group:
  10      Execute/search
  20      Write
  40      Read

          The file's owner:
 100      Execute/search
 200      Write
 400      Read

          Special mode bits:
1000      Restricted deletion flag or sticky bit
2000      Set group ID on execution
4000      Set user ID on execution

For example, numeric mode `4755' corresponds to symbolic mode `u=rwxs,go=rx', and numeric mode `664' corresponds to symbolic mode `ug=rw,o=r'. Numeric mode `0' corresponds to symbolic mode `a='.


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27.4 Directories and the Set-User-ID and Set-Group-ID Bits

On most systems, if a directory's set-group-ID bit is set, newly created subfiles inherit the same group as the directory, and newly created subdirectories inherit the set-group-ID bit of the parent directory. On a few systems, a directory's set-user-ID bit has a similar effect on the ownership of new subfiles and the set-user-ID bits of new subdirectories. These mechanisms let users share files more easily, by lessening the need to use chmod or chown to share new files.

These convenience mechanisms rely on the set-user-ID and set-group-ID bits of directories. If commands like chmod and mkdir routinely cleared these bits on directories, the mechanisms would be less convenient and it would be harder to share files. Therefore, a command like chmod does not affect the set-user-ID or set-group-ID bits of a directory unless the user specifically mentions them in a symbolic mode, or sets them in a numeric mode. For example, on systems that support set-group-ID inheritance:

 
# These commands leave the set-user-ID and
# set-group-ID bits of the subdirectories alone,
# so that they retain their default values.
mkdir A B C
chmod 755 A
chmod 0755 B
chmod u=rwx,go=rx C
mkdir -m 755 D
mkdir -m 0755 E
mkdir -m u=rwx,go=rx F

If you want to try to set these bits, you must mention them explicitly in the symbolic or numeric modes, e.g.:

 
# These commands try to set the set-user-ID
# and set-group-ID bits of the subdirectories.
mkdir G H
chmod 6755 G
chmod u=rwx,go=rx,a+s H
mkdir -m 6755 I
mkdir -m u=rwx,go=rx,a+s J

If you want to try to clear these bits, you must mention them explicitly in a symbolic mode, e.g.:

 
# This command tries to clear the set-user-ID
# and set-group-ID bits of the directory D.
chmod a-s D

This behavior is a GNU extension. Portable scripts should not rely on requests to set or clear these bits on directories, as POSIX allows implementations to ignore these requests.


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