chmod(1) USER COMMANDS chmod(1)
NAME
chmod - change file mode
SYNOPSIS
chmod [ -R ] mode file ...
chmod [ugoa ]{+ |- |=}[ rwxlstugo ] file ...
DESCRIPTION
chmod changes or assigns the mode of a file. The mode of a
file specifies its permissions and other attributes. The
mode may be absolute or symbolic. An absolute mode is
specified using octal numbers:
chmod nnnn file ...
where n is a number from 0 to 7. An absolute mode is con-
structed from the OR of any of the following modes:
4000 Set user ID on execution.
20#0 Set group ID on execution if # is 7, 5, 3, or
1.
Enable mandatory locking if # is 6, 4, 2, or
0.
This bit is ignored if the file is a direc-
tory; it may be set or cleared only using the
symbolic mode.
1000 Turn on sticky bit [(see chmod(2)].
0400 Allow read by owner.
0200 Allow write by owner.
0100 Allow execute (search in directory) by owner.
0070 Allow read, write, and execute (search) by
group.
0007 Allow read, write, and execute (search) by
others.
A symbolic mode is specified in the following format:
chmod [ who ] operator [ permission(s) ] file ...
who is zero or more of the characters u, g, o, and a speci-
fying whose permissions are to be changed or assigned:
u user's permissions
g group's permissions
o others' permissions
a all permissions (user, group, and other)
If who is omitted, it defaults to a. operator is one of +,
-, or =, signifying how permissions are to be changed:
+ Add permissions.
- Take away permissions.
= Assign permissions absolutely.
Unlike other symbolic operations, = has an absolute effect
in that it resets all other bits. Omitting permission(s) is
useful only with = to take away all permissions.
permission(s) is any compatible combination of the following
letters:
r read permission
1
chmod(1) USER COMMANDS chmod(1)
w write permission
x execute permission
s user or group set-ID
t sticky bit
l mandatory locking
u, g, o indicate that permission is to be taken from
the current user, group or other mode respec-
tively.
Permissions to a file may vary depending on your user iden-
tification number (UID) or group identification number
(GID). Permissions are described in three sequences each
having three characters: center ; c c c cf4 cf4 cf4 .
User Group Other
rwx rwx rwx This example (user, group, and others all have
permission to read, write, and execute a given file) demon-
strates two categories for granting permissions: the access
class and the permissions themselves.
Multiple symbolic modes separated by commas may be given,
though no spaces may intervene between these modes. Opera-
tions are performed in the order given. Multiple symbolic
letters following a single operator cause the corresponding
operations to be performed simultaneously. The letter s is
only meaningful with u or g, and t only works with u.
Mandatory file and record locking (l) refers to a file's
ability to have its reading or writing permissions locked
while a program is accessing that file. It is not possible
to permit group execution and enable a file to be locked on
execution at the same time. In addition, it is not possible
to turn on the set-group-ID bit and enable a file to be
locked on execution at the same time. The following exam-
ples, therefore, are invalid and elicit error messages:
chmod g+x,+l file
chmod g+s,+l file
Only the owner of a file or directory (or the super-user)
may change that file's or directory's mode. Only the
super-user may set the sticky bit on a non-directory file.
If you are not super-user, chmod will mask the sticky-bit
but will not return an error. In order to turn on a file's
set-group-ID bit, your own group ID must correspond to the
file's and group execution must be set. The -R option
recursively descends through directory arguments, setting
the mode for each file as described above.
EXAMPLES
Deny execute permission to everyone:
chmod a-x file
Allow read permission to everyone:
chmod 444 file
2
chmod(1) USER COMMANDS chmod(1)
Make a file readable and writable by the group and others:
chmod go+rw file
chmod 066 file
Cause a file to be locked during access:
chmod +l file
Allow everyone to read, write, and execute the file and turn
on the set group-ID.
chmod =rwx,g+s file
chmod 2777 file
Absolute changes don't work for the set-group-ID bit of a
directory. You must use g+s or g-s.
SEE ALSO
ls(1).
chmod(2) in the Programmer's Reference Manual
NOTES
chmod permits you to produce useless modes so long as they
are not illegal (e.g., making a text file executable).
3