set -e and short tests

Question

When I was new to shell scripting, I used a lot of short tests instead of if statements, like false && true.

Then later I learned using set -e, and found my scripts were dying for some reason, and they would work if I replaced the short tests with full if statements. Now, the time has gone, and I still use full if statements only.

The most interesting is that if I open an interactive shell and do the following:

set -e false && true echo $? 

it returns 1 but the shell doesn't die!

I see that I have been wasting too many lines of code. Anyone could explain to me how can I use set -e with short tests safely, eg. without the script dying?

Answer 1

The Single UNIX Specification describes the effect of set -e as:

When this option is on, if a simple command fails for any of the reasons listed in Consequences of Shell Errors or returns an exit status value >0, and is not part of the compound list following a while, until, or if keyword, and is not a part of an AND or OR list, and is not a pipeline preceded by the ! reserved word, then the shell shall immediately exit.

As you see, a failing command in an AND list will not make the shell exit.

Using set -e

Starting shell scripts with set -e is considered a best practice, since it is usually safer to abort the script if some error occurs. If a command may fail harmlessly, I usually append || true to it.

Here is a simple example:

#!/bin/sh set -e  # [...] # remove old backup files; do not fail if none exist rm *~ *.bak || true