# Demonstration: Watching Exit Codes in Real Time

## OBSERVE

Run a command you know will succeed, then immediately check its exit code:

```
$ true
$ echo $?
0
```

## HYPOTHESIS

Now predict: what will the exit code be if the command fails instead?

## INSPECT

Run a command that fails, and check immediately:

```
$ ls /this-does-not-exist
ls: cannot access '/this-does-not-exist': No such file or directory
$ echo $?
2
```

## CHANGE

Try a few other failing commands and note their codes: `grep nomatch /etc/hostname` (exit 1 —
"no matches found," a different kind of failure than "file not found").

## VERIFY

Notice: exit codes aren't just "0 or 1" — different failures often get different non-zero
codes. `0` always means success; the exact non-zero value's MEANING varies by command, but ANY
non-zero value reliably means "something went wrong," which is the part every script actually
relies on (`if command; then ... else ...`).

## Instructor Notes

**Demo goal:** Make the abstract "exit code" concept concrete by actually watching `$?` change
in real time, immediately after each command — this is far more convincing live than described
in prose.

**Approximate timing:** 8 minutes live.

**Common mistake to watch for:** running `echo $?` after ANY other command in between (even
`ls`) silently checks the WRONG command's exit code — emphasize "check it immediately."