Posted On 2007-04-27 by FortyPoundHead
Keywords: Command Reference
Tags: Linux Commandline Linux
Views: 1767

Change access permissions, change mode.

Usage: chmod [Options]... MODE [,MODE]... File...

chmod [Options]... NUMERIC_MODE File...

chmod [Options]... --reference=RFILE File...

-f, --silent, --quiet suppress most error messages

-v, --verbose output a diagnostic for every file processed
-c, --changes like verbose but report only when a change is made

--reference=RFILE use RFILE''s mode instead of MODE values

-R, --recursive change files and directories recursively

--help display help and exit

--version output version information and exitchmod changes the permissions of each given file according to MODE, which can be either an octal number representing the bit pattern for the new permissions or a symbolic representation of changes to make, (+-= rwxXstugoa)

Numeric mode:

From one to four octal digits
Any omitted digits are assumed to be leading zeros.

The first digit = selects attributes for the set user ID (4) and set group ID (2) and save text image (1)S
The second digit = permissions for the user who owns the file: read (4), write (2), and execute (1)
The third digit = permissions for other users in the file''s group: read (4), write (2), and execute (1)
The fourth digit = permissions for other users NOT in the file''s group: read (4), write (2), and execute (1)

The octal (0-7) value is calculated by adding up the values for each digit
User (rwx) = 4+2+1 = 7
Group(rx) = 4+1 = 5
World (rx) = 4+1 = 5
chmode mode = 0755


chmod 400 file - Read by owner
chmod 040 file - Read by group
chmod 004 file - Read by world
chmod 200 file - Write by owner
chmod 020 file - Write by group
chmod 002 file - Write by world
chmod 100 file - execute by owner
chmod 010 file - execute by group
chmod 001 file - execute by world

To combine these just add the numbers together:
chmod 444 file - Allow read permission to owner and group and world
chmod 777 file - Allow everyone to read, write, and execute file

Symbolic Mode

The format of a symbolic mode is `[ugoa...][[+-=][rwxXstugo...]...][,...]''.

Multiple symbolic operations can be given, separated by commas.

A combination of the letters `ugoa'' controls which users'' access to the file will be changed:

The user who owns it (u)
Other users in the file''s group (g)
Other users not in the file''s group (o)
All users (a)

If none of these are given, the effect is as if `a'' were given, but bits that are set in the umask are not affected.

all users (a) is effectively
user + group + others

The operator ''+'' causes the permissions selected to be added to the existing permissions of each file; ''-'' causes them to be removed; and ''='' causes them to be the only permissions that the file has.

The letters ''rwxXstugo'' select the new permissions for the affected users:

Read (r),
Write (w),
Execute (or access for directories) (x),
Execute only if the file is a directory or already has execute permission for some user (X),
Set user or group ID on execution (s),
Save program text on swap device (t),
The permissions that the user who owns the file currently has for it (u),
The permissions that other users in the file''s group have for it (g),
Permissions that other users not in the file''s group have for it (o).

Deny execute permission to everyone:
chmod a-x file

Allow read permission to everyone:
chmod a+r file

Make a file readable and writable by the group and others:
chmod go+rw file

Allow everyone to read, write, and execute the file and turn on the set group-ID:
chmod =rwx,g+s file

When chmod is applied to a directory:
read = list files in the directory
write = add new files to the directory
execute = access files in the directory

chmod never changes the permissions of symbolic links. This is not a problem since the permissions of symbolic links are never used. However, for each symbolic link listed on the command line, chmod changes the permissions of the pointed-to file. In contrast, chmod ignores symbolic links encountered during recursive directory traversals.

This page documents the GNU version of chmod.

About the Author

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