How to execute command if previous command fails with return code 0

Question

I have a script which logs in to a server. Regardless of the success (i.e. whether the password is valid or not) the script will exit with return code 0, and the text:

Error login failed.

I'd like it to retry login if this happens by running another command, but because of the return code I can't.

If I pipe the output to grep for 'Error' so I can execute a command based on that then the login dialog doesn't show up in the terminal.

Is there anyway around this?

Pseudo code as this is for work:

loginCmd && echo "$?"
username:
password:

Error login failed. 
0

Update

I tried using an idea from a commentator, but I can't capture the text with 'Error' as that line occurs after the command completes...

Here's what it actually looks like, with incorrect password:

agent registration
username: xyz
password:

Error login failed.

I tried this:

result=$(loginCmd | tee /dev/tty)

grep -q 'Error' <<< "$result" && 
    loginCmd

The error string outputs after the command completes, so result doesn't capture it:

result=agent registration

So I guess there is no way to capture this string?

Error login failed.

Any ideas?

Ideally on login failure the script would return 1, or have some sort of retry mechanism, but it doesn't and I can't change that, it's a company script and this only affects the terminal, not the GUI which is what customers use.

Answer 1

An expect solution might look like this

#!/usr/bin/env expect -f
set looping true
while {$looping} {
  spawn loginCmd
  interact {
    -o
    "Error Login failed." { puts "Try again"; return }
    "login successful"    { set looping false }
  }
}

---

Mixing shell and expect is basically a matter of invoking expect just like any other command. The tricky part is ensuring that the quoting is correct, and that expect variables are not inadvertently expanded by the shell

#!/usr/bin/env bash
echo "do some bash stuff first ..."

# a shell variable you want to use in expect, put it in the environment
export username="john doe"

# the expect code gets sent to expect on its stdin.
# a quoted heredoc quotes the whole document.
# shell variables are accessed through the expect `env` array.
expect << 'END_EXPECT'
    puts "hello $env(username)"

    set looping true
    while {$looping} {
      spawn loginCmd
      interact {
        -o
        "Error Login failed." { puts "Try again"; return }
        "login successful"    { set looping false }
      }
    }
END_EXPECT

Answer 2

This is a weird case and many thanks to everyone that helped sincerely. Many would've written it off just because of how strange or hard to communicate it was.

Props to Ikkachu (always a good contributer to my stuff) and G man.

Here's what I done:

while true; do
    result=$(loginCmd 2>&1 | tee /dev/tty)

    if grep -q 'Error' <<< "$result"; then
        loginCmd
    else
        break
    fi
done

Output:

agent registration
username: xyz
password: 
Error Login failed.
agent registration
username:xyz
password: 
login successful

It also didn't break getting the login right the first time.

I added the loop so it keeps prompting no matter how many times the password is entered wrong.

If anyone has any suggestions to make this better I'm open to it, I learned a lot here.