Exploring Expect

Sending Strings That Look Like Flags

Because the send command looks for special arguments such as -s and -h, you cannot send strings like these without taking some special action. An argument of "--" (two hyphens) forces send to interpret the following argument literally.^[44] So if you want to send -h, you can say:

send -- "-h"

The "--" does not affect strings that do not otherwise have a special meaning to send. Thus you can preface any string with it. For example, the following two commands both do exactly the same thing—they send the string foo.

send -- "foo"
send "foo"

This behavior is particularly useful if you are sending the contents of a variable that can have any value.

send $unknown                            ;# DANGEROUS
send -- $unknown                            ;# SAFE

Without the "--,”, it is possible that the unknown variable might contain something that looks like one of send’s special flags. Using the "--" guarantees that the command is interpreted correctly.

You might also use the "--" with literals when you are generating Expect commands from another program rather than writing them by hand. For example, imagine that you are writing a C program which in turn spits out Expect commands. Your code might look like this:

printf("send \"%s\"",unknown);      /* DANGEROUS */
printf("send -- \"%s\"",unknown);   /* SAFE */

While the resulting Expect script supplies the send command with literal values, the same problem arises. The C program is not checking to make sure that the value it is supplying to the send command does not look ...