1 Introduction

The name pies (pronounced ‘p-yes’) stands for ‘Program Invocation and Execution Supervisor’. This utility starts and controls execution of external programs. In this document these programs will be referred to as components. Each component is a stand-alone program, which is executed in the foreground.

Upon startup, pies reads the list of components from its configuration file, starts them, and remains in the background, controlling their execution. Each component is defined by the name of the external program to be run and its arguments (command line). The program is normally run directly (via exec), but you can instruct pies to run it via sh -c as well.

The standard output and standard error streams of a component can be redirected to a file or to an arbitrary syslog channel.

The way of handling each component, and in particular the action to be taken upon its termination is determined by the component’s mode.

A respawn component is restarted each time it terminates. If it terminates too often, pies puts it to sleep for certain time and logs that fact. This prevents badly configured components from taking too much resources and allows administrator to take measures in order to fix the situation. More specific action can be configured, depending on the exit code of the component.

An inetd-style components is not started. Instead, pies opens a socket associated with it and listens for connections on that socket. When a connection arrives, pies runs this component to handle it. The connection is bound to the component’s ‘stdin’ and ‘stdout’ streams. The ‘stderr’ stream can be redirected to a file or to syslog, as described above. This mode of operation is similar to that of the inetd utility.

Yet another type of components supported by pies are pass-style or meta1-style components. As the name suggests, this type is designed expressly as a support for MeTA1¹ components, namely smtps. This type can be regarded as a mixture of the above two. For each meta1-style component pies opens a socket and starts the component executable program. Once the program is running, pies passes it the file descriptor of that socket, through another preconfigured UNIX-style socket. Further handling of the socket is the responsibility of the program itself.

An accept component is basically handled as ‘inetd’, except that after binding to the socket pies immediately starts the program, without waiting for actual connections.

Finally, two special component modes are available. Startup components are run right after pies startup, prior to running any other components. Their counterpart, shutdown components are run before program termination, after all other components have finished.

Any number of components of all types can be handled simultaneously.

Components are started in the order of their appearance in the configuration file and terminated in reverse order. This order can be modified by declaring component prerequisites or dependents. This is described in the following chapter.

As an exception, this order is reversed for the components read from MeTA1 configuration files, either included by include-meta1 statement (see include-meta1) or expressly supplied in the command line (see config syntax).