DESCRIPTION

This program is part of the DACS suite.

The dacsexpr utility evaluates DACS expressions (see dacs.exprs(5)). It is often a useful aid when composing or testing expressions to be used in access control rules, or when debugging ACLs and configuration directives. While they continue to be referred to as "expressions" for historical reasons, it has become possible to write small programs, and the language can also be useful as a simple scripting language independent of the rest of DACS.

If an expression is provided, it is evaluated and the result is printed to the standard output. At most one expression can be specified. If the -q flag is given (one of the dacsoptions), nothing is printed, expression evaluation errors are suppressed, and the program terminates with an appropriate exit status; otherwise the result is written to the standard output. If neither a -q flag is given nor any flag that controls the logging level, then the logging level is set to warn overriding any configuration file logging level directive; this behaviour is usually convenient.

If no expression is provided, the program reads its standard input. If the input is not coming from a terminal type device, the program runs in "batch mode" and prompting is suppressed; otherwise, the program runs in "interactive mode". When prompted in interactive mode, enter help for assistance. If the readline(3) functionality was configured when the program was built, command line editing and history are available in interactive mode.

To write to stderr, use the eprint() or eprintf() functions.

A script can be executed through the system's "#!" mechanism:

#!/usr/local/dacs/bin/dacsexpr

printf("%s\n", "Hello, world.")

Such programs always use the script file as input, therefore no expression or other file can be specified on the "#!" line.

Command line arguments can be passed to a script. If dig-it contains:

#!/usr/local/dacs/bin/dacsexpr

if (${Argv::#} != 3) {
  eprintf("Usage: dig-it digest-name msg\n");
  exit(1);
}

printf("%s\n", digest(${Argv::2}, 0, ${Argv::1}));

then running the script produces:

% ./dig-it sha3-224 mymsg
88fdaa49d0371a79efa2d6e8a55d27d62cd39ad393309d9a18665763

The Env namespace is initialized from the program's environment. For example, if the value of the environment variable LOGNAME is bobo, then ${Env::LOGNAME} will be instantiated with that value. Syntactically invalid variable names are silently ignored.

An empty Expr namespace is created. In interactive use, the prompt string can be changed by assigning a string to ${Expr::prompt} and the continuation prompt can be changed by setting ${Expr::prompt2}.

OPTIONS

If an expression or file has not already been specified, a filename may appear as the last argument. If filename is "-", the standard input is read.

EXAMPLES

The following command evaluates the expression argument (note that it is a single argument to the command) and outputs the result to stdout:

% dacsexpr -e "1+1"
2
% dacsexpr -e '${Env::USER}'
"bobo"
% dacsexpr -u example.com -e '"FEDERATION_NAME=" . ${Conf::FEDERATION_NAME}'
"FEDERATION_NAME=EXAMPLE"
% dacsexpr - a b c <<HERE
? print("First arg is \"\${Argv::1}\"")
? HERE
First arg is "a"
% cat ex
#!/usr/local/dacs/bin/dacsexpr

print("Argv[2] is ${Argv::2}");
% chmod 0755 ex
% ./ex foo bar baz
Argv[2] is bar

DIAGNOSTICS

If an error occurs, a message may be written to stderr, depending on the logging level. In general, the program exits 0 if and only if everything was fine. If a command line expression is evaluated, the program exits 0 if the expression evaluates to True, 1 if it evaluates to False, and 2 if an error occurs. If an explicit call to exit() is made and no true error condition occurred, then the program will exit with the argument value.

SEE ALSO

dacs.exprs(5)

BUGS

Nothing beyond simple I/O is provided by dacs.exprs(5) and that is unlikely to change.

New and little-used features should be used with care. This advice applies to all software.

AUTHOR

Distributed Systems Software (www.dss.ca)

COPYING

Copyright © 2003-2024 Distributed Systems Software. See the LICENSE file that accompanies the distribution for licensing information.