Debugging shell scripts

If you haven’t already figured it out, shell scripting isn’t… it isn’t a great language for programming. I’m not judging shell scripts, but. No. Yes, I am judging shell scripts. And my judgment is that they are perfectly fine the way they are, but they are not a good general purpose programming language.

Shell scripting languages do not come with an IDE and they do not come with a debugger. You can, however, do something to debug shell scripts.

Log-based debugging

Re-read (or read) log-based debugging.

You can print values of variables in shell scripts using the commands echo or printf.

NAME="Franklin"
echo "Hi, $NAME"

PI=3 # close enough
printf "%d\n" $PI

echo is pretty straightforward to use; printf can be as complex as printf(3) in the C programming language, but you can refer to the manual pages to find out more about how to use printf by running man 1 printf.

Changing Bash Behaviour

Bash is not a good general purpose programming language. For many reasons. But two reasons that are relevant right now are:

1. By default, Bash does not stop running a script if a variable that’s being used does not have a value or has not been initialized.
2. By default, Bash does not stop running a script if a program in the script exits with an error.

In other words, if Bash runs into something that could be a possible error, it just keeps going without stopping.

#!/usr/bin/env bash
echo "Your name is $YOUR_NAME"
echo "Hi $YOUR_NAME!"

You can change how Bash behaves in these two (obviously!) error situations by changing Bash’s behaviour using the set command.

The two options we’re going to change are the ones that correspond to -e and -x. The -e option says “exit on any error”, and the -x option says to print a “trace” of commands (it prints out the commands that have run, this helps give you a sense of the flow of the script).

#!/usr/bin/env bash
set -e    # exit on any error
set -x    # print out all commands that are run

Further reading

Shell scripting isn’t… it isn’t a great language for general purpose programming, but maybe that’s because nobody has built up the kind of tooling we have for debugging like in other programming languages.

Here are some links where you can read a bit more about debugging shell scripts:

• On Julia Evans’ “wizard zines” there’s a comic specifically on “bash debugging”. It’s short, visual, but gives some additional tips like using the trap builtin for even more debugging goodness.
• An actual Bash debugging tool exists as “BASH Debugger”, but it’s not usually installed anywhere by default (it’s not on Aviary, for example), and I’m not entirely sure how it works in the sense that commands have side-effects outside of the language. This is something you will have to build and install on your own if you want to use it.
• Greg’s Wiki entry on Bash Pitfalls explains why some of the options that we’re listing in this book are still not exactly great ways to debug a shell script (but it’s kinda all we’ve got).