use expect to check if command output contains a word (else error)

Question

I want to check if there is an interface in the output of the command:

ip a

If there is not, then I want to stop the execution. I've tried this code:

#!/usr/bin/expect -f
spawn bash
set spi_bash $spawn_id
send "ip a\r"
expect {
    -re "docker0" {puts "docker0 is there\n"; exp_continue }
    timeout {puts "no docker0\n"; exit 1}
}
exit

but exp_continue continues to timeout case.

Answer 1

I'd suggest you don't need expect for this:

set output [exec ip a]
if {[string first docker0 $output] != -1} {
    puts "no docker0"
}

But if this is part of a larger expect program, you can write

spawn bash
set spi_bash $spawn_id

# a regex matching your prompt: a dollar sign, a space, and end of input
# adjust as required
set prompt {\$ $}

send "ip a\r"
set has_docker false
expect {
    -re "docker0" {set has_docker true; exp_continue}
    -re $prompt
}

send "exit\r"
expect eof

if {$has_docker}
    puts "docker0 is there"
} else {
    puts "no docker0"
}

exit [expr {!$has_docker}]