Exploring Expect

Comparing send To puts

Both the send command and the puts command cause characters to be output. However, they have differences which should already be apparent. But I will compare and contrast them here just to make things clear. Note that all of the variations on send follow the style of the send command itself. For example, it is possible to call send_user with the -h flag or send_log with the -- flag.

The primary difference between send and puts is that send works with processes started by spawn, whereas puts works with files opened by Tcl’s open command. For example, to write an AT-style reset command (the characters "ATZ\r“) to a modem with a serial interface, puts could be used as follows:

set file [open "/dev/modem" "w"]
puts -nonewline $file "ATZ\r"

Doing the same thing with send requires a spawned process instead of an open file.

spawn tip modem
send −i $spawn_id "ATZ\r"

In this example, the "−i $spawn_id" is not needed. It is just here to contrast it with the $file argument in the puts. Without the −i specification, send writes to the tip process anyway because spawn automatically sets spawn_id. In contrast, with no file specification, puts writes to the standard output. The previous open has no effect on the default destination of puts.

puts "ATZ\r"    ;# write to standard output
send "ATZ\r"    ;# write to currently spawned process

In UNIX, processes can be viewed as files (to a degree). The puts command, thus, can write to processes as well—but only those opened by open. The ...