Expect - Use default variable if no argument is specified - variables

Question

I've searched on stackoverflow and haven't really found an answer to this.
I'm pretty new to scripting and I have created a fully functional Expect script but I would like to improve it a bit. Currently I have created 3 lindex values of argv 0, 1 and 2 for hostaddress, username and password.
I would like to create a default username and password if argv 1 and 2 is NOT specified. I tried solving this through some if statements but after searching through stackoverflow it seems that TCL/Expect does not support NULL or empty values. Instead you have to make a query for it. Currently my code looks like this:
#!/usr/bin/expect
#Variables
set HOSTADDRESS [lindex $argv 0]
set USER [lindex $argv 1]
set PASSWORD [lindex $argv 2]
spawn ssh $USER#$HOSTADDRESS
set timeout 100
expect {
"(yes/no)?" {send "yes\n"; exp_continue}
"assword:" {send "$PASSWORD\n"}
}
expect {
"%" {send "cli\r"; exp_continue}
">" {sleep 1}
}
send "show interfaces st0 terse | match st0. | count \r"
expect "Count:???"
puts [open $HOSTADDRESS.op5.vpn.results w] $expect_out(0,string)
expect ">"
send "exit\r"
expect {
"%" {send "exit\r"; exp_continue}
"closed." {exit}
}
exit
Can you guys please help me create a default variable for $USER and $PASSWORD if it's not specified in the argv 1 or argv2?

Answer 1

Tcl doesn't support NULL at all. Or rather, it actually maps it to the variable being unset (that's exactly what happens with local variables under the hood; global variables are different). To query whether a variable exists, you use info exists (and yes, that's actually a NULL check in its implementation).
However, for handling defaulting of values from users on the command line, it is better to do it like this:
proc parseArgv {hostAddress {user "TheDefaultUser"} {pass "TheDefaultPassword"}} {
variable ::HOSTADDRESS $hostAddress
variable ::USER $user
variable ::PASSWORD $pass
}
parseArgv {*}$argv
If you're using 8.4 (upgrade, man!) then replace that last line with:
eval parseArgv $argv
You could also do it by looking at the llength of $argv (or the value in $argc) and doing conditional stuff based on that, but leveraging Tcl's proc default argument value stuff is easier (and you even get a reasonable error when someone gives too few or too many arguments).