chmod changes the permissions of each given
file according to
mode, which can be either a symbolic representation of changes to make, or
an octal number representing the bit pattern for the new permissions.
The format of a symbolic mode change argument is
‘[ugoa...][[+-=][rwxXstugo...]...][,...]’.
Such an argument is a list of symbolic mode change commands,
separated by commas.
Each symbolic mode change command starts with zero or more
of the letters ‘ugoa’; these control 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). Thus, ‘a’ is here equivalent to ‘ugo’.
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.
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),
sticky bit (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), and the permissions that
other users not in the file’s group have for it (o).
(Thus, ‘chmod g-s file’ removes the set-group-ID bit,
‘chmod ug+s file’ sets both the set-user-ID and set-group-ID bits, while
‘chmod o+s file’ does nothing.)
The name of the ‘sticky bit’ derives from the original meaning:
keep program text on swap device.
These days, when set for a directory, it means that
only the owner of the file and the owner of that directory
may remove the file from that directory.
(This is commonly used on directories like /tmp that have
general write permission.)
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 save text image [‘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.
chmod never changes the permissions of symbolic links, since the
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,
chmod changes the permissions of the pointed-to file. In contrast,
chmod ignores symbolic links encountered during recursive directory traversals.
