Exploring Expect

Using File I/O Commands On Spawned Processes

You cannot directly read from or write to spawned processes with puts and gets. In general, there is little need for it because you can emulate the behavior with suitable send and expect commands. Nonetheless, it may be convenient to do so at times.

Earlier, I showed how the -open flag of the spawn command converts a file identifier to a spawn id. The exp_open command does the opposite. It converts a spawn id to a file identifier that may be used with gets and puts. The file identifier will be open for both reading and writing. If exp_open is called with no arguments, it converts the spawn id of the currently spawned process. If called with a −i argument, exp_open converts the given spawn id.

By default, after calling exp_open, the spawn id can no longer be accessed using send and expect. It becomes owned entirely by Tcl and should eventually be closed in the Tcl style and without doing a wait. On some systems, processes return spurious error indications during a close operation. Expect knows to ignore these errors; however, you may have to explicitly catch them from Tcl.

spawn /bin/csh
set file [exp_open]
catch {close $file}

You may have to call flush explicitly after I/O operations because the file commands normally buffer internally. Process output that does not terminate with a newline may be impossible to read unless you disable buffering or read it explicitly with read. In the following example, the first output from telnet is read ...