See [here](https://www.thegeekstuff.com/2011/01/expect-expressions-loops-conditions/) (summarised below)

## VLC example
### mute audio
This sets the volume to 0 using `volume 0`.
```expect
set timeout 5

set host "HOST"
set port "7010"
set password "mrflibble"

spawn telnet $host $port
expect "Password:"
send "$password\r"

expect "Welcome, Master"
send "volume 0\r"
sleep 0.01
```
and this is a pure script to login with my default password
```expect
#!/usr/bin/expect
set timeout 5

set host "HOST"
set tport "[lindex $argv 0]"
puts "tport '$tport'"
if { $tport == "" } {
  set tport 0
}
set port "[expr 7000 + $tport]"
set password "PASSWORD"

spawn telnet $host $port
expect "Password:"
send "$password\r"

expect "Welcome, Master"

interact
```
### vlccmd
This assumes that vlc has been started with `--telnet-port=7000 --telnet-password=mrflibble`
and uses a python script to assemble the script which is passed to `expect` via its `stdin`
```python
#!/usr/bin/env python3
import sys
import os
from subprocess import run, PIPE, Popen
import time
scr = """
set timeout 5

set host "HOST"
set port "PORT"
set password "PASSWORD"

spawn telnet $host $port
expect "Password:"
send "$password\\r"

expect "Welcome, Master"
"""
args = sys.argv[1:]
try:
  host,port,*xs = args
except ValueError:
  print(f"vlccmd <host> <port-offset> [<cmd> ...]")
  exit(1)
port = int(port)
if port < 0 or port >= 1000:
  print(f"Invalid port: {port}")
  exit(1)
port = str(port+7000)
s = scr.replace("HOST",host).replace("PORT",port).replace("PASSWORD","mrflibble")
for x in args:
  s += f'\nsend "{x}\\r"\nsleep 0.01\n'
with Popen(["expect"],stdin=PIPE) as p:
  p.communicate(input=s.encode())
```

## Geek Stuff article
From [this Geek Stuff article](https://www.thegeekstuff.com/2011/01/expect-expressions-loops-conditions/) by Balakrishnan Mariyappan on January 18, 2011

The Expect scripting language is easy to learn. It expects a specific string, 
and sends (or responds) strings accordingly.

#### Define Expect Variables – set command
In expect, you can set the variable values using set command as shown below,
```
# To set the variable with numeric values
set var1 10

# To set the variable with string literal
set name "john"

# To set the variable value which includes output of another command
set proc_id "process id : [pid]"
```
Note: For expect command line arguments, read 6 Expect Script Command Line Argument Examples.

#### Expect Expressions – expr command
To evaluate the expressions, use the expr command, which executes the given expression and returns the result. Expect expressions are similar to the C expressions. Some of the valid expressions are listed below.

```expect
# To add two simple numerical values
set sum "[expr 1 + 1]"

# To multiple the value of variables
set mul "[expr $sum * $sum]"

# To evaluate conditions and returns 1 or 0 to indicate success or failure accordingly.
set ret "[expr (1+1) == 2]"

# Conditions may contain the command return values.
set ret [expr [pid] == 0]
```
#### Expect Conditional Expressions – If command
If command is used for conditional flow of execution of statements as shown in the example below.

```expect
if { $count < 0} {
    puts "True : $count\n";
} else {
    puts "False : $count\n";
}
```
Just like any other programming language, you can use the elseif command in expect as shown below.
```expect
if { $count < 0} {
    puts "Success Condition1 : $count\n";
} elseif { $count == 0 } {
    puts "Success Condition2 : $count\n";
} else {
    puts "False : $count\n";
}
```
### Expect Looping Constructs
#### Expect For Loop Examples:
As we know, for loop is used to do repeated execution of expression until certain condition.

General for loop construct :
```expect

for {initialization} {conditions} {incrementation or decrementation} {
...
}
```
#### Expect for loop example :

```expect
for {set i 1} {$i < $no} {incr i 1} {
 set $total [expr $total * $i ]
}
puts "$total";
```
Note: You should place the loop open brace in the same line as it contains “for” keyword.

#### Expect While Loop Examples:
```expect
set count 5;
while {$count > 0 } {
puts "count : $count\n";
set count [expr $count-1];
}
```