OSF DCE Administration Guide--Core Components

DCE Control Program Introduction

you might want to add some entry to a CDS directory only if some other speciﬁed entry

already exists in CDS. The DCE control program makes this possible by utilizing Tcl’s

built-in commands that imitate elements commonly found in numerous programming and

shell languages.

The DCE control program contains many C-like constructs that control command

execution. Some examples are if statements for conditional execution, looping

commands such as while, for, and foreach used to repeat operations under various

conditions, a case command for testing values against various patterns, and proc for

writing your own customized commands.

The DCE control program also includes other syntactic elements such as "" (quotes), { }

(braces), [ ] (brackets), and \ (backslash), which it uses to group elements together and

for controlling interpretation of special characters.

Although many features are designed for use in scripts, you’ll probably ﬁnd yourself

using some constructs and elements (particularly quotes, braces, brackets, and

backslashes) in interactive operations as well. You’ll need to decide when it makes

sense to perform operations interactively or to use a script. In general, complexity and

potential for reuse can help you decide.

Now let’s look at a couple of simple examples that illustrate some DCE control program

and Tcl basics. Some dcecp operations can be very straightforward like

dcecp> account modify N_Long -expdate 1996-06-30

This operation lets you change information in the DCE Security Service registry. Here,

we’re changing the account expiration date for the principal (N_Long) named in the

command line. While it’s relatively simple to execute this operation for one or two

principals, it’s more difﬁcult to change the account expiration date for many principals.

Imagine that your organization employs six temporary workers and the project they’re

associated with has been extended for three months. Rather than execute the account

modify operation six times, you can use a dcecp foreach command to loop (repeat) an

action for each item of a list:

dcecp> foreach i {N_Long L_Jones P_Sawyer

> D_Witt M_Dougherty S_Preska} {

> account modify $i -expdate 1996-06-30 }

dcecp>

In the example, the foreach looping command has three arguments: a variable, a list,

and the body. The variable i substitutes sequentially for each item in the list (N_Long,

L_Jones, and so on). The foreach command executes the body (account modify $i

-expdate 1996-06-30)) for each item in the list. The $i variable in the body takes on the

value of each principal name in the list, in turn, until all items in the list have been used.

See Section 2.9.2 for more detailed information about looping commands.

This example illustrates several other important syntax rules. The DCE control program

uses { } (braces) to determine where command arguments, such as the script body, begin

and end. For example, the foreach command has three arguments: a variable name, a

list, and a script body. Normally, command arguments are separated by spaces. To

prevent dcecp from incorrectly interpreting the spaces between list elements as

argument separators, we use braces to enclose the list and disable special interpretation

124243 Tandem Computers Incorporated 1−7