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Essential System Administration Pocket Reference by Æleen Frisch

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File Ownership and Protection

chmod

chmod [-R] access-string[,access-string...] files

Change the file mode of the specified files as directed in the access-string. -R operates recursively on any directories in the file list.

Access-string can be a numeric file mode or one or more symbolic permission strings; symbolic permission strings have the general syntax:

[class]+|-|=[type]

where class is one or more access class code letters: u for user, g for group, o for other, a for all. a is the default (although on some systems, omitting the access class causes the specified permissions to be limited by the current umask).

The second item is an operator: + to add permissions, - to remove them, and = to set permissions to the specified value.

Type is one or more access type code letters: r (read), x (execute), w (write), or one of the special types listed in the following section.

Special access types

X

Set execute access only if it has already been set for some access class.

t (save text mode/sticky bit) [numeric 1000]

When used for other access on a directory, restricts user file deletions to his own files.

s (SetUID and SetGID) [numeric 4000 and 2000]

When used with user and group access, causes a program to execute in the user/group context of the files’s owner(s), rather than the user who ran it. Both require the corresponding execute access to be on.

SetGID access on a directory causes new file group ownership to follow the directory’s group owner rather than the primary ...

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