Exploring Expect

Example—A Smarter Terminal Emulator

The previous example was very simple-minded. The characters from the output of the spawned processes were copied to their own text widget. The only attempt at formatting was to handle line endings. Most programs expect more than this. For example, tabs are usually expanded to spaces, and backspaces cause the terminal cursor to move left instead of right.

More sophisticated programs require character addressing. By sending special terminal manipulation character sequences (I will just call them sequences from now on), programs can write to arbitrary character locations on the screen. The following terminal emulator supports this. You can use it to run programs such as emacs and vi.

As before, a text widget is used for display. Its name is stored in the variable term. For simplicity, the code only supports a single emulator, assumes a fixed size display of 24 rows of 80 columns, and runs a shell. The following code starts the process and creates the text widget.

# tkterm - term emulator using Expect and Tk text widget



set rows 24          ;# number of rows in term
set cols 80          ;# number of columns in term
set term .t          ;# name of text widget used by term

log_user 0

# start a shell and text widget for its output
set stty_init "-tabs"
eval spawn $env(SHELL)
stty rows $rows columns $cols < $spawn_out(slave,name)
set term_spawn_id $spawn_id

text $term -width $cols -height $rows

Once the terminal widget has been created, it can be displayed on the screen with a pack ...