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In Unix and Unix-like operating systems, chmod is the command and system call used to change the access permissions and the special mode flags (the setuid, setgid, and sticky flags) of file system objects (files and directories). Collectively these were originally called its modes, [1] and the name chmod was chosen as an abbreviation of change ...
Distinct permissions apply to the owner. Files and directories are assigned a group, which define the file's group class. Distinct permissions apply to members of the file's group. The owner may be a member of the file's group. Users who are not the owner, nor a member of the group, comprise a file's others class. Distinct permissions apply to ...
The command chown / ˈ tʃ oʊ n /, an abbreviation of change owner, is used on Unix and Unix-like operating systems to change the owner of file system files and directories. Unprivileged (regular) users who wish to change the group membership of a file that they own may use chgrp. The ownership of any file in the system may only be altered by ...
The most common modern use of the sticky bit is on directories residing within filesystems for Unix-like operating systems. When a directory's sticky bit is set, the filesystem treats the files in such directories in a special way so only the file's owner, the directory's owner, or root can rename or delete the file.
The setuid and setgid bits are normally represented as the values 4 for setuid and 2 for setgid in the high-order octal digit of the file mode. For example, 6711 has both the setuid and setgid bits (4 + 2 = 6) set, and also the file read/write/executable for the owner (7), and executable by the group (first 1) and others (second 1).
In Unix and Unix-like systems, including POSIX-conforming systems, each file has a 'mode' containing 9 bit flags controlling read, write and execute permission for each of the file's owner, group and all other users (see File-system permissions §Traditional Unix permissions for more details) plus the setuid and setgid bit flags and a 'sticky' bit flag.
Generally, the mask only affects file permissions during the creation of new files and has no effect when file permissions are changed in existing files; however, the chmod command will check the mask when the mode options are specified using symbolic mode and a reference to a class of users is not specified. The mask is stored as a group of bits.
Permission 644, usually shown as rw-r--r--, gives the file owner full permission to read and write the file, while other users have read-only access. In some shells, the {} must be quoted. The trailing " ; " is customarily quoted with a leading " \ ", but could just as effectively be enclosed in single quotes.