OSH(8) OSH(8)
16 November 1994
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.
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<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:
; & | < > NEWLINES* SPACES* TABS*
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.
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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)
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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 "$1")
then
echo "Test what file? Give me a file to test."
exit
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16 November 1994
fi
if ($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...)
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