spawn

spawn {command} [args]

command
    The executable program to run. This can be a full path to the executable or a command name that can be found in the system's PATH.

[args]
    Optional arguments to pass to the command. These are passed to the spawned process exactly as specified.

The spawn command is a utility primarily used within Expect scripts to initiate and manage interactive processes.
It's designed to create a new process running a specified command and connects it to a pseudo-terminal (pty), allowing Expect to interact with it programmatically as if it were a human user. This interaction includes sending commands, receiving output, and reacting to prompts or other patterns in the process's output.

spawn is not typically used directly on the command line in the same way as commands like ls or grep; its power is realized within the context of an Expect script where its return value is fundamental for using send and expect procedures.

spawn is part of the Expect package and won't be available unless Expect is installed.
It is not usually run directly from the shell, it is designed to be called from an expect script.
The use of spawn without proper error handling can lead to scripts that hang indefinitely if the spawned process does not behave as expected.

After the spawn command is executed it associates the spawned process with a file descriptor which is stored in the variable spawn_id. This spawn_id is what is used by other expect commands such as send and expect to interact with the spawned process.

If the command specified in spawn cannot be found or executed, Expect will typically raise an error. It is crucial to implement appropriate error handling within Expect scripts to gracefully handle such situations and prevent unexpected script termination.

spawn is a core component of the Expect scripting language, which was created by Don Libes in the early 1990s.
Expect was designed to automate interactive applications, where direct command-line control is not possible.
The spawn command specifically facilitates the launching of these interactive processes under the control of Expect scripts. Its functionality is deeply tied to the history and evolution of Expect as a tool for automating terminal interactions.

expect(1), send(1), interact(1)