| 1 | 6.10 The Restricted Shell
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| 2 | =========================
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| 3 |
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| 4 | If Bash is started with the name 'rbash', or the '--restricted' or '-r'
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| 5 | option is supplied at invocation, the shell becomes restricted. A
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| 6 | restricted shell is used to set up an environment more controlled than
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| 7 | the standard shell. A restricted shell behaves identically to 'bash'
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| 8 | with the exception that the following are disallowed or not performed:
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| 9 |
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| 10 | * Changing directories with the 'cd' builtin.
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| 11 | * Setting or unsetting the values of the 'SHELL', 'PATH', 'HISTFILE',
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| 12 | 'ENV', or 'BASH_ENV' variables.
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| 13 | * Specifying command names containing slashes.
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| 14 | * Specifying a filename containing a slash as an argument to the '.'
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| 15 | builtin command.
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| 16 | * Specifying a filename containing a slash as an argument to the
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| 17 | 'history' builtin command.
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| 18 | * Specifying a filename containing a slash as an argument to the '-p'
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| 19 | option to the 'hash' builtin command.
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| 20 | * Importing function definitions from the shell environment at
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| 21 | startup.
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| 22 | * Parsing the value of 'SHELLOPTS' from the shell environment at
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| 23 | startup.
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| 24 | * Redirecting output using the '>', '>|', '<>', '>&', '&>', and '>>'
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| 25 | redirection operators.
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| 26 | * Using the 'exec' builtin to replace the shell with another command.
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| 27 | * Adding or deleting builtin commands with the '-f' and '-d' options
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| 28 | to the 'enable' builtin.
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| 29 | * Using the 'enable' builtin command to enable disabled shell
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| 30 | builtins.
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| 31 | * Specifying the '-p' option to the 'command' builtin.
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| 32 | * Turning off restricted mode with 'set +r' or 'set +o restricted'.
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| 33 |
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| 34 | These restrictions are enforced after any startup files are read.
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| 35 |
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| 36 | When a command that is found to be a shell script is executed (*note
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| 37 | Shell Scripts::), 'rbash' turns off any restrictions in the shell
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| 38 | spawned to execute the script.
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| 39 |
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| 40 | The restricted shell mode is only one component of a useful restricted
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| 41 | environment. It should be accompanied by setting 'PATH' to a value that
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| 42 | allows execution of only a few verified commands (commands that allow
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| 43 | shell escapes are particularly vulnerable), leaving the user in a
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| 44 | non-writable directory other than his home directory after login, not
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| 45 | allowing the restricted shell to execute shell scripts, and cleaning the
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| 46 | environment of variables that cause some commands to modify their
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| 47 | behavior (e.g., 'VISUAL' or 'PAGER').
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| 48 |
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| 49 | Modern systems provide more secure ways to implement a restricted
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| 50 | environment, such as 'jails', 'zones', or 'containers'.
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| 51 |
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