command

chmod
chmod [options] mode files chmod [options] --reference=filename files

Change the access mode (permissions) of one or more files. Only the owner of a file or a privileged user may change the mode. mode can be numeric or an expression in the form of who opcode permission. who is optional (if omitted, default is a); choose only one opcode. Multiple modes are separated by commas.

Options
-c, --changes
Print information about files that are changed.

-f, --silent, --quiet
Do not notify user of files that chmod cannot change.

--help
Print help message and then exit.

-R, --recursive
Traverse subdirectories recursively, applying changes.

--reference=filename
Change permissions to match those associated with filename.

-v, --verbose
Print information about each file, whether changed or not.

--version
Print version information and then exit.

Who
u
User.

g
Group.

o
Other.

a
All (default).

Opcode
+
Add permission.

-
Remove permission.

=
Assign permission (and remove permission of the unspecified fields).

Permissions
r
Read.

w
Write.

x
Execute.

s
Set user (or group) ID.

t
Sticky bit; used on directories to prevent removal of files by non-owners.

u
User's present permission.

g
Group's present permission.

o
Other's present permission.

Alternatively, specify permissions by a three-digit octal number. The first digit designates owner permission; the second, group permission; and the third, other's permission. Permissions are calculated by adding the following octal values:

4
Read.

2
Write.

1
Execute.

Note that a fourth digit may precede this sequence. This digit assigns the following modes:

4
Set user ID on execution to grant permissions to process based on the file's owner, not on permissions of the user who created the process.

2
Set group ID on execution to grant permissions to process based on the file's group, not on permissions of the user who created the process.

1
Set sticky bit.

Examples
Add execute-by-user permission to file:

chmod u+x file

Either of the following will assign read/write/execute permission by owner (7), read/execute permission by group (5), and execute-only permission by others (1) to file:

chmod 751 file chmod u=rwx,g=rx,o=x file

Any one of the following will assign read-only permission to file for everyone:

chmod =r file chmod 444 file chmod a-wx,a+r file

The following makes the executable setuid, assigns read/write/execute permission by owner, and assigns read/execute permission by group and others:

chmod 4755 file