chmod

  chmod [options] mode [, mode] file1 [file2 ...]

Options:

• -R recursive change of permissions for directories and their contents
• -f do not issue error messages for files whose permissions cannot be changed.
• -v detail the action or the lack of action for each file .

chmod never changes permissions on symlinks. However, for each symbolic link specified on the command line, chmod changes the permissions of the associated file. In doing so, chmod ignores symbolic references occurring during recursive processing of directories.

The chmod argument specifying permissions can be written in two formats: numeric and character.

Rights are written in one line at once for three types of users:

• file owner (u);
• other users in the owner group (g);
• all other users (o);

In a numerical form, absolute rights are set to a file or directory, at the same time, separate rights for different types of users can be set in a symbolic form.

Example: in numerical form, set the rights rwxr-xr-x:

chmod 755 filename

Thus, the rights "755" are written in symbolic form as "rwxr-xr-x". At the same time to understand the essence of the task of rights in Unix-systems it is useful to know the representation of numbers in the binary number system .

Some permissions only make sense when combined with others. Of the first four items (not giving the right to read the file), only “---” is usually used for files, that is, a complete ban on access to the file for this type of users. For directories from the entire list, only 0, 5 and 7 are usually applied - ban, read and execute, and full access.

Summarizing these codes for the three types of users, you can get a numeric or character entry. For example, chmod 444 {filename} : 400 + 40 + 4 = 444 - all have read-only rights (identical to “rrrrr--”).

In addition to the standard 'rwx' permissions, the chmod command also manages the SGID, SUID, and T bits. The set SUID or SGID attributes allow you to run the file with the rights of the file or group owner, respectively.

For SUID, the weight is 4000, and for SGID it is 2000. These attributes make sense when the corresponding performance bit is set and are designated when the symbol is written with the letter “s”: “rw s rwxrwx” and “rwxrw s rwx”, respectively.

Example: chmod 4555 {filename} - everyone has the right to read and execute, but the file will be launched for execution with the rights of the owner.

Setting the SGID for a directory will result in setting the membership of each new file being created to the same group to which the directory itself belongs, and not to the owner’s main group, as is the default. SUID for the directory does not make sense ^[1] .

sticky bit or restricted deletion flag (t-bit) is used only with directories. When the t-bit for a directory is not set, a file in this directory can be deleted (renamed) by any user who has write access to this directory. By installing the t-bit on the directory, we change this rule in such a way that only the owner of this file can delete (rename) the file. Following the above encoding, the t-bit has a weight of 1000.

Note: The right to write (w) allows the user to write or change a file, and the right to write to the directory - the ability to create new files or delete files from this directory. If the directory has the ability to write (w), then the file inside this directory can be deleted even if the right to write to it is not set . (In accordance with the concept of the POSIX file system).

Popular Values

400 (-r --------)

The owner has the right to read; no one else has the right to do anything

644 (-rw-r - r--)

All users have read permission; owner can edit

660 (-rw-rw ----)

Owner and group can read and edit; the rest have no right to perform any actions

664 (-rw-rw-r--)

All users have read permission; owner and group can edit

666 (-rw-rw-rw-)

All users can read and edit.

700 (-rwx ------)

The owner can read, write and run; no one else has the right to do anything

744 (-rwxr - r--)

Each user can read, the owner has the right to edit and run.

755 (-rwxr-xr-x)

Each user has the right to read and run; owner can edit

777 (-rwxrwxrwx)

Each user can read, edit and run.

1555 (-r-xr-xr-t)

Each user has the right to read and run; Only the owner of this file can delete the file.

2555 (-r-xr-sr-x)

Each user has the right to read and run with the rights of the group (user group) of the file owner.

0440 (-r - r -----)

The owner and the group has the right to read no one else has the right to perform any actions.

4555 (-r-sr-xr-x)

Each user has the right to read and run with the rights of the owner of the file

In symbolic form, using the chmod command allows more flexibility to add, set, or remove permissions on file (s) or directories.

  $ chmod [references] [operator] [modes] file ...

References define users who will change rights. References are defined by one or several letters:

Operator defines the operation that chmod will perform:

Modes determines which rights will be set, added or deleted:

Examples of using the command in symbolic form

Set permissions of the file "rwxr-xr-x" (0755):

chmod u=rwx,g=rx,o=rx filename

Set execute rights for the file owner, delete execute rights from the group, delete write and execute rights for other users:

chmod u+x,gx,o-wx filename

Set recursive read permissions for all users:

chmod -R a+r directory

Recursively remove the SUID and SGID attributes:

chmod -R us,gs directory

The default values ​​are:

• for files: 644 (-rw-r - r--)
• for catalogs: 755 (drwxr-xr-x)

The meanings are different for files and directories because the "execute" flag affects files and directories differently. For ordinary files, “execution” means opening files, for executable files — launching them, and for directories — viewing content.

For example, you can make from the command line:

The following command recursively applies rules for all files in the / home / test directory, as well as for all files in all subdirectories:

  # find / home / test -type f -exec chmod 644 {} \;

The following command will recursively apply the rules for all directories in the “/ home / test” directory, as well as for all directories in all subdirectories:

  # find / home / test -type d -exec chmod 755 {} \;

The same result can be achieved without using find (note the capital X):

  # chmod -R go = rX, u = rwX / home / test

• umask - mask for creating user files
• chown
• Programs UNIX-like operating systems

• chmod(1) - man help page for GNU / Linux user commands (eng.) (eng.)
• FreeBSD chmod Help
• chmod.ru - chmod in brief, 2008
• The basics of managing access to files.
• chmod - interactive chmod command assistant