24 January 2021

Safer Bash: no unset

Last week, I talked about set -e; while it does add a layer of security on many potential Bash errors, it is unfortunately not quite enough by itself. In the next few posts, I'll explore a few other things you can do to improve the safety of your Bash scripting.

Note that I am not advocating writing complex Bash scripts. There is a point (around 10 lines, maybe?) where you really need to be asking yourself whether Bash is the right language, and whether you wouldn't be better served by a more modern programming language. Python, Ruby, Haskell (via stack), Clojure (via lein exec) etc. all have facilities to replace Bash scripts by providing both a nice API to run subprocesses and an easy way to turn source files into executables. It's likely your favourite programming language has similar facilities. Still, in some environments Bash remains the best option.

The additional safety features I am suggesting in this series of posts are not things you should add when your Bash scripts get complicated; they are things you should just always add to all your Bash scripts, no matter how small they seem.

With this caveat out of the way, let's move on to our topic of the week. By default, Bash, being a dynamic language, essentially behaves as if unknown variables were set to the empty string. So, for example:

$ cat hello.sh
set -e

YOU="world"

echo Hello, $you
$ bash hello.sh
Hello,
$ echo $?
0
$

(By the way, in case you're wondering, echo $? prints the exit code of the last command, which is an integer between 0 and 255 with 0 meaning success and all the other values being error codes.)

As you can see, this succeeds despite the typo (lack of capitalization). Just like last week, this may not seem like a big deal. However, let's consider this silly example that would never happen in the real world:

set -e

MY_APP_FILES=/tmp/my-app

rm -rf $My_APP_FILES/

Guess what happens? Since there is a typo in My_APP_FILES, Bash will happily substitute an empty string for it. This means the command becomes rm -rf /, a.k.a. "please delete all the files on my hard drive". If this script is run as root, your machine is likely to become unable to boot, which can be annoying for, say, a remote server with no physical access.

However, don't go thinking you're safe as long as you're not running scripts as root. All things considered, breaking your OS files is a best-case scenario these days, as they are super easy to replace. What's a lot worse about the above is that rm itself does not stop on error (and, at least on my system, does not have an option to). So if you run this as a normal user, the command will fail to delete system files, but will keep going until it finds files it can delete, for example all your baby pictures you have no backup of.

The solution to this is another flag we can set to tell Bash to stop on encountering any unset variable. The flag is -u, and just like the -e flag, it can be set both when starting the executable and from within a script:

$ cat unset.sh
set -e

MY_VAR="some value"

echo my_var: $MYVAR

set -u

echo my_var again: $MYVAR

echo Done
$ bash unset.sh
my_var:
unset.sh: line 9: MYVAR: unbound variable
$ bash -u unset.sh
unset.sh: line 5: MYVAR: unbound variable
$ echo $?
1
$

Just like -e, it would be annoying to require all users of our script to add the flag to all Bash invocations, and the flag only affects whatever happens after the set line. So you should really start all your Bash scripts with this line:

set -eu

Like many Unix utilities, Bash allows you to combine single-letter flags, such that set -u -e is the same as set -eu (and the same as set -e; set -u).

Tags: bash unix