OSH(8)

NAME
osh - operator shell, a privileged restricted shell

SYNOPSIS
osh [ command ]

DESCRIPTION

osh, the Operator Shell, is a setuid root, security enhanced, restricted shell. It allows the administrator to carefully limit the access of special commands and files to the users whose duties require their use, while at the same time automatically maintaining audit records.

  Invocation
Osh can be invoked with no arguments to start a subshell, or a single command can be specified on the command line.

USAGE

  Commands

A command is a sequence of nonblank words separated by blanks. The first word specifies the name of the command to be executed. The remaining words are passed as arguments to the invoked command.

A pipeline is a sequence of one or more commands separated by `|'. The standard output of each command but the last is connected by a pipe (see pipe(2V)) to the standard input of the next command. Each command is run as a separate process; the shell normally waits for the last command to terminate before prompting for or accepting the next input line.

A list is a sequence of one or more commands or pipelines, separated by `;', and optionally terminated by `;' or `&'. A semicolon (;) sequentially executes the preceding pipeline; an ampersand (&) asynchronously executes the preceding pipeline (the shell does not wait for that pipeline to finish). An arbitrary number of NEWLINE characters may appear in a list, instead of semicolons, to delimit commands.

  Comments
A word beginning with # and all the following characters up to a NEWLINE are ignored.

  Environment Substitution
The character $ may be used to reference environment variables. Environment variables may NOT be assigned values inside osh.

  Input/Output

A command's input and output may be redirected using a special notation interpreted by the shell. The following may follow a command and are not passed on to the invoked command. Note: environment substitution occurs before word is used.

<word
    Use file word as standard input (file descriptor 0).

>word
    Use file word as standard output (file descriptor 1). If the file does not exist it is created; otherwise, it is truncated to zero length.

>>word
    Use file word as standard output. If the file exists output is appended to it (by first seeking to the EOF); otherwise, the file is created.

Redirection will be evaluated for the entire command.

If a command is followed by & the default standard input for the command is the empty file /dev/null. Otherwise, the environment for the execution of a command contains the file descriptors of the invoking shell as modified by input/output specifications.

  Filename Generation

Before a command is executed, each command word is scanned for the characters `*', `?', `{', and `['. If one of these characters appears the word is regarded as a pattern. The word is replaced with alphabetically sorted filenames that match the pattern. If no filename is found that matches the pattern, the error `No Match' will be returned, and the command will not be executed. `.' at the start of a filename or immediately following a `/', as well as the character `/' itself, must be matched explicitly.

    *   Matches any string, including the null string.
    ?   Matches any single character.
    [...]
        Matches any one of the enclosed characters.

  Quoting
The following characters have a special meaning to the shell and cause termination of a word unless quoted:

    ; & | < > NEWLINE SPACE TAB

A character may be quoted (made to stand for itself) by preceding it with a backslash (\) or inserting it between a pair of quote marks (""). During processing, the shell may quote certain characters to prevent them from taking on a special meaning. Backslashes used to quote a single character are removed from the word before the command is executed.

  Prompting
When used interactively, the shell prompts the user for commands. If a command is given on the command line, no prompting is performed.

  Signals
The INTERRUPT and QUIT signals for an invoked command are ignored if the command is followed by &; otherwise signals have the values inherited by the shell from its parent (but see also the trap command below). INTERRUPT is handled asynchronously.

  Execution
Each time a command is entered, the above substitutions are carried out. If the user invoking the shell is allowed by the administrator to execute the command, and the parameters do not violate the file access control lists specified by the administrator, the command is executed.

The following commands are usually available to the user. Of course, the system administrator can turn any of them off, but there's little reason to.

help
    Help will usually give the user the list of commands allowed to be executed by him.

logout
    Logout or ^D <eof> will always exit the user from Osh.

cd directory
    Change the current directory.

more [ filename ]
    Read a file using an internal, secure, version of more.

alias [ name command ]
    Specify an alias called name to perform command. If alias is called without arguments, print out a list of known aliases.

The following commands are only available to noninteractive users.

test [ -r|-w ] filename
    Test the file against the Operator Shell's access control lists for readability or writeability. Exit's 1 on readable/writeable, 0 otherwise.

EXIT STATUS

If the shell is being used noninteractively, the exit status is meaningless except in the case of the 'test' command, whereby osh will exit with 1 if the file access is allowed, 0 if it is not allowed.

SEE ALSO
sh(1), csh(1)

Neuman, Michael; The Operator Shell: A Means for Privilege Distribution Under Unix . (Usenix/SANS III)

WARNINGS

Words used for filenames in input/output redirection are not interpreted for filename generation (see File Name Generation, above). For example, `cat file1 > a*' will create a file named `a*'.

Because commands in pipelines are run as separate processes, variables set in a pipeline have no effect on the parent shell.

EXAMPLES

The following shows a sample interactive osh session:

    example%  /usr/local/bin/osh
    Mike Neuman (mcn)
    Operator Shell version 1.5alpha1
    example.mcn (/sec/opshell/osh/osh) #> help
    Operator Shell (osh) Version 1.5alpha1
    	by Michael Neuman <mcn@lanl.gov>
    Defines:
    NO_COMPILE_TABLE
    LOGGING to FILE
    CHECK_ACESS
    OPER_OVERRIDE
    Commands accessible:
    help     cd     more    alias
    example.mcn (/sec/opshell/osh/osh) #>

The following is an example of how to run an osh command from the command line (assuming the user has permission to read the file /etc/shadow in the osh access control list).

    example%  /usr/local/bin/osh cat /etc/shadow
    root:passwordhere:::::::
    daemon:NP:6445:::::::
    bin:NP:6445:::::::

The following is an example of a shell script calling Osh to test the accessibility of a file. This is a simple way to write a handler. You could, for example, write the following shell script, make it executable only by root, and add it to the Osh command table. The script, when executed, will correctly call osh to test the readability/writeability of certain arguments.

For example, if you wanted to write a simple `ln' handler, you would use a script similar to the one below to test the writeability of the last argument (the destination). By default, Osh automatically checks the readability of every argument, so it's not necessary to test them separately.

    #!/bin/sh -f
    OSHPATH="/usr/local/bin/osh"
    if (test -z "$1")
    	then
              echo "Test what file? Give me a file to test."
              exit
    	fi
    if ($OSHPATH test -w $1)
    	then
              echo "$1 writeable"
    #         Do something with this fact.
    	else
              if ($OSHPATH test -r $1)
                   then
                        echo "$1 readable"
    #                    Do something with this fact
                   else
                        echo "No permission."
              fi
    fi
    exit

BUGS

None. (suuure...)