chmod [OPTION]... MODE[,MODE]... FILE...
This manual page documents the GNU version of chmod. chmod changes the
file mode bits 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 mode bits.
The format of a symbolic mode is [ugoa...][[+-=][perms...]...], where
perms is either zero or more letters from the set rwxXst, or a single
letter from the set ugo. Multiple symbolic modes can be given, sepa-
rated 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), 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.
The operator + causes the selected file mode bits to be added to the
existing file mode bits of each file; - causes them to be removed; and
= causes them to be added and causes unmentioned bits to be removed
except that a directory’s unmentioned set user and group ID bits are
not affected.
The letters rwxXst select file mode bits for the affected users: read
(r), write (w), execute (or search for directories) (x), execute/search
only if the file is a directory or already has execute permission for
some user (X), set user or group ID on execution (s), restricted dele-
tion flag or sticky bit (t). Instead of one or more of these letters,
you can specify exactly one of the letters ugo: 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).
A numeric mode is from one to four octal digits (0-7), derived by
adding up the bits with values 4, 2, and 1. Omitted digits are assumed
to be leading zeros. The first digit selects the set user ID (4) and
set group ID (2) and restricted deletion or 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; 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 sym-
bolic link listed on the command line, chmod changes the permissions of
the pointed-to file. In contrast, chmod ignores symbolic links encoun-
tered during recursive directory traversals.