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[NAME]
chmod \- change file access permissions
[DESCRIPTION]
This manual page
documents the GNU version of
.BR chmod .
.B chmod
changes the permissions of each given file according to
.IR mode ,
which can be either a symbolic representation of changes to make, or
an octal number representing the bit pattern for the new permissions.
.PP
The format of a symbolic mode is
`[ugoa...][[+-=][rwxXstugo...]...][,...]'. Multiple symbolic
operations can be given, separated by commas.
.PP
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), or 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.
.PP
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.
.PP
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),
sticky (t), the permissions granted to the user who owns the file (u),
the permissions granted to other users who are members of the file's group (g),
and the permissions granted to users that are in neither of the two preceding
categories (o).
.PP
A numeric mode is from one to four octal digits (0-7), derived by
adding up the bits with values 4, 2, and 1. Any omitted digits are
assumed to be leading zeros. The first digit selects the set user ID
(4) and set group ID (2) and sticky (1) attributes. The second digit
selects permissions for the user who owns the file: read (4), write (2),
and execute (1); the third selects permissions for other users in the
file's group, with the same values; and the fourth for other users not
in the file's group, with the same values.
.PP
.B chmod
never changes the permissions of symbolic links; the
.B chmod
system call cannot change their permissions. This is not a problem
since the permissions of symbolic links are never used.
However, for each symbolic link listed on the command line,
.B chmod
changes the permissions of the pointed-to file.
In contrast,
.B chmod
ignores symbolic links encountered during recursive directory
traversals.
.SH STICKY FILES
On older Unix systems, the sticky bit caused executable files to be
hoarded in swap space. This feature is not useful on modern VM
systems, and the Linux kernel ignores the sticky bit on files. Other
kernels may use the sticky bit on files for system-defined purposes.
On some systems, only the superuser can set the sticky bit on files.
.SH STICKY DIRECTORIES
When the sticky bit is set on a directory, a file in that directory may
be unlinked or renamed only by the directory owner, the file owner, or root.
Without the sticky bit, anyone able to write to the
directory can delete or rename files. The sticky bit is commonly found
on directories, such as /tmp, that are world-writable.
.SH OPTIONS
[SEE ALSO]
chmod(2)
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