chmod

Change access permissions

Usage: chmod [OPTION]... MODE[,MODE]... FILE...
  or:  chmod [OPTION]... OCTAL_MODE FILE...
  or:  chmod [OPTION]... --reference=RFILE FILE...

  -c, --changes           like verbose but report only when a change is made

  -f, --silent, --quiet   suppress most error messages

  -v, --verbose           output a diagnostic for every file processed

      --reference=RFILE   use RFILE's mode instead of MODE values

  -R, --recursive         change files and directories recursively

      --help              display this help and exit

      --version           output version information and exit

  Each MODE is one or more of the letters ugoa, 
  one of the symbols +-= and
  one or more of the letters rwxXstugo.

This page documents the GNU version of chmod.

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 is `[ugoa...][[+-=][rwxXs­tugo...]...][,...]'. Multiple symbolic operations can be given, separated 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)
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.

all users (a) is effectively
user + group + others

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),
save program text on swap device (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).

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 (1) attributes.
The second digit selects permissions for the user who owns the file: read (4), write (2), and execute (1);
The third DIGIT 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. 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.

"Whether a pretty woman grants or withholds her favours, she always likes to be asked for them" - Ovid (Ars Amatoria)

Related commands:

The ls -l command will list current permissions for -- u (owner) rwx -- g (group) rwx -- O (Other) rwx --
chgrp - Change group ownership
chown - Change file owner and group

Windows equivalent commands:

ATTRIB - Change file attributes
CACLS - Change file permissions
XCACLS - Change file permissions