Command-Line Program Patterns

Command Dispatch

Route a command word to a small action label.

command-dispatch Dispatch turns a command word into the branch of work to run. The same pattern works for small tools and larger command groups.

Command Dispatch

command
command_dispatch.pl
Replay: real traced execution (multi-file project)
use strict;
use warnings;

my $command = "run";
my $action = "show-usage";

if ($command eq "run") {
    $action = "execute";
} elsif ($command eq "check") {
    $action = "validate";
}

print "command=$command\n";
print "action=$action\n";

use strict;
use warnings;

my $command = "check";
my $action = "show-usage";

if ($command eq "run") {
    $action = "execute";
} elsif ($command eq "check") {
    $action = "validate";
}

print "command=$command\n";
print "action=$action\n";

use strict;
use warnings;

my $command = "unknown";
my $action = "show-usage";

if ($command eq "run") {
    $action = "execute";
} elsif ($command eq "check") {
    $action = "validate";
}

print "command=$command\n";
print "action=$action\n";

  1. $command ← run, $action ← show-usage

    4my $command→ run = "run"; #@command="check", "unknown"5my $action→ show-usage = "show-usage";

  2. $action ← execute

    7if ($commandrun eq "run") {8    $action→ execute = "execute";9} elsif ($command eq "check") {

  3. print "command=$command ";

    13print "command=$commandrun\n";14print "action=$actionexecute\n";
    outputcommand=run
action=execute

  1. $command ← check, $action ← show-usage

    4my $command→ check = "check";5my $action→ show-usage = "show-usage";

  2. $action ← validate

    8    $action = "execute";9} elsif ($commandcheck eq "check") {10    $action→ validate = "validate";11}

  3. print "command=$command ";

    13print "command=$commandcheck\n";14print "action=$actionvalidate\n";
    outputcommand=check
action=validate

  1. $command ← unknown, $action ← show-usage

    4my $command→ unknown = "unknown";5my $action→ show-usage = "show-usage";67if ($command eq "run") {8    $action = "execute";9} elsif ($command eq "check") {10    $action = "validate";11}1213print "command=$commandunknown\n";14print "action=$actionshow-usage\n";
    outputcommand=unknown
action=show-usage