Exploring Expect

Ptys And Processes

The Expect library provides three functions to start new interactive processes. Each of them creates a new process so that its standard input, standard output, and standard error can be read and written by the current process.

exp_spawnl is useful when the number of arguments is known at compile time. exp_spawnv is useful when the number of arguments is not known at compile time. (The third function exp_popen will be described later.) In both cases, the arguments are passed literally—no shell pattern matching is done and no redirection occurs. The shell is simply not involved. I occasionally will refer to these functions generically as the spawn functions.

exp_spawnl and exp_spawnv parallel those of the UNIX functions execlp and execvp respectively. The calling sequences are as follows:

int
exp_spawnl(file, arg0 [, arg1, ..., argn] (char *)0);
char *file;
char *arg0, *arg1, ... *argn;

int
exp_spawnv(file,argv);
char *file, *argv[ ];

In both functions, the file argument is a relative or absolute file specification. No special character processing occurs (such as ~ or * expansion). exp_spawnl and exp_spawnv duplicate the shell’s actions in searching for an executable file from the list of directories associated with the PATH environment variable.

The argv parameter in exp_spawnv is made available to the new process as the argv parameter in main. exp_spawnl collects its remaining arguments and then massages them so that they also appear as the argv parameter in main ...