How to use the chmod command

The chmod command changes file permissions on unix type operating systems.

Usage
chmod [options] [mode] files

Mode
Either symbolic or numerical modes can be used to represent the desired permissions.

symbolic mode
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)
 * 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),
 * 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.)

Numerical mode
A numeric  mode  is  from  one  to four octal digits (0-7). Each digit is a sum of all the settings. Any omitted  digits  are assumed  to  be leading zeros. For example 755 is equivalent to 0755.

The first digit selects the The second  digit  selects permissions for the user who owns the file:
 * set-user-ID (4)
 * set-group-ID (2)
 * save text image [`sticky'] (1) attributes.
 * read (4)
 * write (2)
 * execute (1)

The third selects permissions for other users  in the file's group, with the same values as above

The fourth for other users not in the file's group, with the same values as above.

Example:
 * chmod 755 filename
 * changes the permissions of the filename to have
 * owner of the file: read, write, and executable access
 * group of the file: read, and executable access
 * any user: read, and executable access

Options
ADDITIONAL GNU DESCRIPTION A GNU extension (new in fileutils 4.0)  allows  one  to  use
 * --reference=rfile as a mode description: the same mode as that of rfile.

GNU OPTIONS
 * -c, --changes Verbosely describe  the  action for each file whose permissions actually changes.


 * -f, --silent, --quiet Do not print error messages about files whose permissions cannot be changed.


 * -v, --verbose Verbosely describe  the  action  or  non-action taken for every file.


 * -R, --recursive Recursively change permissions of directories  and  their  contents.

GNU STANDARD OPTIONS
 * --help Print a usage message on standard output and exit successfully.


 * --version Print version information on standard output, then exit successfully.

--    Terminate option list.

Credits

 * derived from man page GNU fileutils 4.0, 2004-06-17, CHMOD(1)
 * From HowTo Wiki, a Wikia wiki.