How to write better Bash than ChatGPT

A few days ago, someone commented on Hacker News that ChatGPT writes "very high quality Bash scripts" with "best practices and comments," which reminded me that I've been meaning to write about Bash, because I think that ChatGPT's Bash is somewhere between "pretty bad" and "passable." It absolutely does not conform to "best practices."

I like Bash but it's pretty hard to learn to do it well – almost everything I know about Bash I learned from pairing, and pairing on kinds of work that many people consider "not good for pairing," like setting up CI systems. So here's an attempt to write down most of what I know about Bash.

Note that I develop on a Mac, so this is mostly going to assume that you're using OS X (and brew.) It should mostly apply to Linux but remember that Darwin is based on BSD, not Linux – I've mostly encountered this in the form of small differences between the arguments for ps.

This article assume assumes a relatively high level of Bash knowledge – in particular, that you already know how pipes work, and that the command line is a "lathe for text." If that statement doesn't immediately parse but you'd like to know more let me know.

tl;dr Use Shellcheck

You can improve your Bash immediately by running all scripts you write through shellcheck, a Bash linter, either on the command line or on shellcheck.net. Many of the examples I'll give in this article would be caught by shellcheck.

If you have a lot of shell scripts, or load bearing shell scripts, it's handy to run shellcheck on all your scripts in CI. If you do this, you'll want to be familiar with the directors for ignoring errors. Occasionally you will actually want to do something that shellcheck dislikes – so it has a mechanism for using comments to ignore specific errors from specific lines.

Simplify ChatGPT's scripts with set -e

Let's take this simple example: a script that runs tests, suitable to be used as a pre-commit git hook.

Prompt

Write a git hook file that runs the tests for an Elixir application and confirms that it complies with no warnings before committing

Output

#!/bin/bash

# Run the tests
mix test
if [ $? -ne 0 ]; then
  echo "Tests failed. Aborting commit."
  exit 1
fi

# Check for compiler warnings
warnings=$(mix compile --warnings-as-errors 2>&1 | grep "warning:")

if [ -n "$warnings" ]; then
  echo "Commit aborted due to warnings:"
  echo "$warnings"
  exit 1
fi

exit 0

What's wrong with this script?

Unnecessary if statements
Unnecessary equality tests
Unnecessary sub shell
Unnecessary variables

In order to function correctly as a pre-commit hook, this script needs to exit with a non-zero exit code. It's capturing the output of mix test and mix compile respectively and then determining its exit code based on that output. (It's also doing this in an excessively complicated and fragile way for mix compile – it should just check the exit code with $? like it does for mix test.)

Overall it's about twice as long as it needs to be – and it's using a lot of confusing features of Bash. if statements in Bash are strictly a necessary evil – very few people can read Bash equality statements comfortably.

We can improve this script a lot by adding one sentence to the prompt.

Start the script with set -e

This produces a zippy 4 lines.

#!/bin/sh
set -e

# Run tests
mix test

# Check for warnings
mix compile --warnings-as-errors

# All checks passed
echo "All tests passed and no warnings detected. Proceeding with commit."

set -e causes the script to exit immediately if a command returns a non-zero status, and returns the non-zero status of the last executed command. It allows us to dispense entirely with all of that output capture – if mix test fails, the whole script just exits with an error code.

set -e should be at the top of most scripts that you write. It makes Bash behave much more like a "typical" programming language, and prevents a lot of surprising behavior.

Use set -x instead of echo

ChatGPT also has a habit of including a lot of echo statements in its scripts to announce what it's doing. This is kind of a style thing – there are (kind of obviously, given ChatGPT's habits) a lot of people who like to use a lot of echo in their scripts – but I also think it's unnecessary – and ChatGPT stops if you add another directive to the set command.

Start the script with set -ex

#!/bin/bash
set -ex

# Run Elixir tests
mix test

# Compile the Elixir application and check for warnings
mix compile --warnings-as-errors

# If any of the above commands fail, the commit will be aborted

The -x directive causes scripts to print the output of the commands running inside them. I don't usually include it in finished versions of scripts, but it's useful for debugging, and it sometimes it's handy for replacing echo commands.

Shell is not Bash

There's another subtle problem in this set of scripts we've generated so far. Note the shell directive at the top – one of them is #!/bin/bash and another is #!/bin/sh. This "shebang" directive controls what interpreter gets used to run the file. By default if you call a script as an executable, without specifying the interpreter at the command line, it will use whatever shell ran the command, but with a shebang, the specified interpreter will get used instead.

Sh and Bash are not interchangeable. They are different programs with different behavior. Most of the differences are features that Bash adds, but some of the differences are pretty subtle – string comparison, for instance, works slightly differently. Worse, on some systems /bin/sh will point to /bin/bash – but not on others. By randomly choosing /bin/sh sometimes and /bin/bash other times you're setting yourself up for confusion when you use a Bash-only feature and it gets executed in sh.

So you should tell ChatGPT which one you want to use explicitly.

Start the script with
#!/bin/bash
set -ex

This is also an area where shellcheck gets really handy. Occasionally you will need to write a script that must use only use shell capabilities. Shellcheck will pick up the interpreter from the shebang and let you know if you use something that shell doesn't have.

Avoid surprises by quoting strings and variables

One thing that ChatGPT does do well pretty consistently is quote strings. This prevents shell expansion surprises, like Bash treating a string with a space in it as two separate arguments. For example, if you have the following directory

➜  test_directory ll
total 0
drwxr-xr-x@  5 nat  staff  160 Jul 22 13:43 .
drwxr-xr-x@ 21 nat  staff  672 Jul 22 13:42 ..
drwxr-xr-x@  2 nat  staff   64 Jul 22 13:43 directory
drwxr-xr-x@  2 nat  staff   64 Jul 22 13:42 some
drwxr-xr-x@  2 nat  staff   64 Jul 22 13:42 some directory

And then you run

rm -rf some directory

You will get a totally different result than if you had typed

rm -rf "some directory"

The first directive will pass "some" and "directory" as separate arguments and delete "some" and "directory" respectively, leaving "some directory" alone.

So, always quote your strings.

It's also a good habit to put braces around variables, like this

echo "${prefix}_${suffix}"

Especially when you're doing string concatenation, without the {} you can end up accidentally specifying different variable names than you think you did.

prefix="horse"
suffix="sense"

echo "${prefix}_${suffix}"
echo "$prefix_$suffix"

produces this output

horse_sense
nonsense

because without the {}, Bash reads the variable name in the second echo as $prefix_

You can make this kind of thing easier to catch with another set directive.

set -u

prefix="horse"
suffix="sense"

echo "${prefix}_${suffix}"
echo "$prefix_$suffix"

Now this script outputs

horse_sense
./prefix: line 7: prefix_: unbound variable

-u makes Bash stop and error when it encounters an unbound variable – instead of treating it as ""

Pipefail

Another good standard set directive is

set -o pipefail

I don't have this one burned into my brain the way that I do the others, but it also prevents hidden errors from producing unexpected nonsense. It's sort of like set -e but for directives in pipelines. Without it, commands that fail will continue into the next step of the pipeline, and you'll get the result of the final command in the pipeline.

The full prompt

My custom ChatGPT prompt includes this section

When I ask for a script, produce a Bash or a Mix script.

Start Bash scripts with

#!/bin/bash
set -euo pipefail

Always quote Bash strings, and surround variables with {} like this "${variable}"

Write Bash that can pass shellcheck

GPT still writes Bash with unnecessary variables and uses more comments than I would but this improves its output.

A few more Bash tricks

A few things that aren't directly related to scripting but are handy when working on the command line.

pbcopy and pbpaste are Mac commands that move text in and out of the clipboard. You pipe content into pbcopy and out of pbpaste.

"Up arrow" to get the most recent command in your history. Ctrl-r to search history.

Command-A to move the cursor to the beginning of the line. Command-E to move it to the end. Command-K to clear the line after your cursor.

What does this tell us about ChatGPT?

I do like using ChatGPT for Bash. I use it a lot for data manipulation-type things that I know are technically possible with e.g. sed but that I don't remember exactly how to do. The nice thing about Bash is that it's very fast to check whether a script does what I meant, which means that it's very fast to check for ChatGPT errors.

Some of the problems in ChatGPT's Bash aren't functional errors, though – they're just verbose, obscure, or could cause problems under rare circumstances.

I think it's striking that there's such a gap between my impression of ChatGPT's Bash ("mediocre at best") and the assessment of someone who doesn't seem to be as experienced writing Bash ("excellent, best practices.") I notice there's also a big gap between my assessment of it's writing (also something like "very good at writing extremely medium-quality paragraphs") and the reactions I've seen from people who struggle with writing. I'm not sure that I'm willing to call myself an expert at either of these things but I suspect this pattern holds across a lot of domains with LLMs – beginners think its output is excellent, while experts see it as mediocre.

This makes sense to me given what the model is – a distillation of a very large dataset, much of which must, by the nature of large datasets, be mediocre. ChatGPT's Bash is bad in the ways that I expect people who haven't delved deep into the bash-lore to be bad at Bash, and that likely describes most of the people who wrote most of the Bash that its drawing from. It can't be better than its dataset.

The thing that GPT is very good at, though, is convincing people that it's good at things – at generating plausible output. This combination – deeply mediocre output that can have a surface appearance of excellence – has been making me very cautious about using it for professional work – even to the point that I've been considering cancelling my ChatGPT and Copilot subscriptions. I do find it useful for quickly generating text that I more or less already know how to generate, but I'm increasingly skeptical that this is worth the risk of being profoundly misled in areas that are outside my direct expertise.