Adding upstream version 5.1.upstream/5.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+6.10 The Restricted Shell
+=========================
+
+If Bash is started with the name 'rbash', or the '--restricted' or '-r'
+option is supplied at invocation, the shell becomes restricted.  A
+restricted shell is used to set up an environment more controlled than
+the standard shell.  A restricted shell behaves identically to 'bash'
+with the exception that the following are disallowed or not performed:
+
+   * Changing directories with the 'cd' builtin.
+   * Setting or unsetting the values of the 'SHELL', 'PATH', 'HISTFILE',
+     'ENV', or 'BASH_ENV' variables.
+   * Specifying command names containing slashes.
+   * Specifying a filename containing a slash as an argument to the '.'
+     builtin command.
+   * Specifying a filename containing a slash as an argument to the
+     'history' builtin command.
+   * Specifying a filename containing a slash as an argument to the '-p'
+     option to the 'hash' builtin command.
+   * Importing function definitions from the shell environment at
+     startup.
+   * Parsing the value of 'SHELLOPTS' from the shell environment at
+     startup.
+   * Redirecting output using the '>', '>|', '<>', '>&', '&>', and '>>'
+     redirection operators.
+   * Using the 'exec' builtin to replace the shell with another command.
+   * Adding or deleting builtin commands with the '-f' and '-d' options
+     to the 'enable' builtin.
+   * Using the 'enable' builtin command to enable disabled shell
+     builtins.
+   * Specifying the '-p' option to the 'command' builtin.
+   * Turning off restricted mode with 'set +r' or 'set +o restricted'.
+
+These restrictions are enforced after any startup files are read.
+
+When a command that is found to be a shell script is executed (*note
+Shell Scripts::), 'rbash' turns off any restrictions in the shell
+spawned to execute the script.
+
+The restricted shell mode is only one component of a useful restricted
+environment.  It should be accompanied by setting 'PATH' to a value that
+allows execution of only a few verified commands (commands that allow
+shell escapes are particularly vulnerable), leaving the user in a
+non-writable directory other than his home directory after login, not
+allowing the restricted shell to execute shell scripts, and cleaning the
+environment of variables that cause some commands to modify their
+behavior (e.g., 'VISUAL' or 'PAGER').
+
+Modern systems provide more secure ways to implement a restricted
+environment, such as 'jails', 'zones', or 'containers'.
+