Shell from zero: read the exit code

Every command finishes with a number. 0 means it worked. Anything else means it did not. The shell does not stop on a failure unless you ask it to. An empty drink name is a failure you can test. If you ignore the number, the next command runs on an order that has no drink.

The exit code is the result that matters#

Every command returns a small integer. 0 means success. Any other value means failure. The shell stores that integer where $? can read it, and the next command replaces it, including echo itself. Save it on the following line if you still need it. test -n "$name" succeeds when the string has characters and fails when it does not. An empty name is a blank cup. The code is 1. set -e would exit the script on that failure, which is why this lesson does not turn it on before printing $?. Printed text is not the code. A command can print a warning and return 0, or print nothing and return 1.

1. nametext(empty)
2. test -nexit1

name=""
test -n "$name"
echo $?

1

$? is the code of the last command#

name="" stores no characters. test -n "$name" asks whether the name is non-empty and returns 1 when it is not. $? expands to that code, so echo prints 1. name=latte and the same test would lead to a printed 0. false is the command that always returns 1, and true always returns 0. The value of $? changes as soon as another command runs, including the echo itself. Save it if you need it later: code=$? on the very next line.

set -e tells the shell to exit the script when a command returns non-zero. This lesson does not turn it on, because echo $? would never run. In a script that should stop on a blank drink, set -e is the right switch, and you still print or log the failure in a place that is allowed to see it.

Check the code, then use the output#

A command can print nothing and return 1, or print a warning and return 0. The text is not the code. After a command that writes a label or a link, look at $? before you tell someone it worked. 0 is the only success. The number is the result that carries the outcome, and the printed line is just text.