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How to use the chmod command

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

UsageEdit

chmod [options] [mode] files

ModeEdit

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

symbolic modeEdit

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 modeEdit

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

  • set-user-ID (4)
  • set-group-ID (2)
  • save text image [`sticky'] (1) attributes.

The second digit selects permissions for the user who owns the file:

  • 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

OptionsEdit

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.

CreditsEdit

  • derived from man page GNU fileutils 4.0, 2004-06-17, CHMOD(1)


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