Restricted Shells

Advanced Bash-Scripting Guide:
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# Chapter 22. Restricted Shells

Disabled commands in restricted shells

  . Running a script or portion of a script in restricted mode disables certain commands that would otherwise be available. This is a security measure intended to limit the privileges of the script user and to minimize possible damage from running the script.

The following commands and actions are disabled:

- Using `cd` to change the working directory.

- Changing the values of the `$PATH`, `$SHELL`, `$BASH_ENV`, or `$ENV` environmental variables.

- Reading or changing the `$SHELLOPTS`, shell environmental options.

- Output redirection.

- Invoking commands containing one or more /'s.

- Invoking exec to substitute a different process for the shell.

- Various other commands that would enable monkeying with or attempting to subvert the script for an unintended purpose.

- Getting out of restricted mode within the script.

Example 22-1. Running a script in restricted mode

    #!/bin/bash

    #  Starting the script with "#!/bin/bash -r"
    #+ runs entire script in restricted mode.

    echo

    echo "Changing directory."
    cd /usr/local
    echo "Now in `pwd`"
    echo "Coming back home."
    cd
    echo "Now in `pwd`"
    echo

    # Everything up to here in normal, unrestricted mode.

    set -r
    # set --restricted    has same effect.
    echo "==> Now in restricted mode. <=="

    echo
    echo

    echo "Attempting directory change in restricted mode."
    cd ..
    echo "Still in `pwd`"

    echo
    echo

    echo "\$SHELL = $SHELL"
    echo "Attempting to change shell in restricted mode."
    SHELL="/bin/ash"
    echo
    echo "\$SHELL= $SHELL"

    echo
    echo

    echo "Attempting to redirect output in restricted mode."
    ls -l /usr/bin > bin.files
    ls -l bin.files    # Try to list attempted file creation effort.

    echo

    exit 0

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