syntax: breaks some substring expansions

[CS-Account]

Summary of issue

In the case of bash shell shell parameter expansion, substring expansion containing a parameter, offset and length is incorrectly formatted, losing syntactically important whitespace in the process, and making bash incorrectly interpret it as ‘:-’ expansion. Further information regarding this process can be found in the official Bash manual under the "3.5.3 Shell Parameter Expansion (linked here)" heading.

You can ctrl-f to [‘:-’ expansion.] (brackets excluded) to find the relevant section of the manual.

Relevant Information

• This issue is being reported for substring expansion where length is present and is negative, with or without double quotes.
• Language mode set to bash explicitly.
• I am using version shfmt_v3.6.0, which at the time of writing is the latest.
• My effective options are as follows from shell-format: "Effective shfmt flags: -i=4,-ci,-s,--language-dialect=bash"
• I have no EditorConfig files outside of what vscode shell-formatter may provide
• Just to be a good, diligent bug reporter, I also tried with shfmt directly with the following command: shfmt_v3.6.0_linux_amd64 -i=4 -ci -s --language-dialect=bash test_file

Examples

Below are some reproducible examples with a few variations tested.

Variations where formatting does not work correctly

Before:

test_var="${test_var:0: -2}"
test_var=${test_var:0: -2}
test_var="${test_var: -2: -1}"
test_var=${test_var: -2: -1}
test_var="${test_var: -2: 1}"
test_var=${test_var: -2: 1}
test_var="${test_var: -2}"
test_var=${test_var: -2}

After:

test_var="${test_var:0:-2}"
test_var=${test_var:0:-2}
test_var="${test_var: -2:-1}"
test_var=${test_var: -2:-1}
test_var="${test_var: -2:1}"
test_var=${test_var: -2:1}

---

Variations where formatting does work correctly

Before:

test_var="${test_var: -2}"
test_var=${test_var

After:

test_var="${test_var: -2}"
test_var=${test_var: -2}

If there is more information you need to reproduce / get context about this issue please ask.

Sitenote

Unless I am missing it, I don't notice any functionality to just ignore specific lines. This would be a really useful feature for times like this where a bug is currently out and there isn't another way to deal with it.

Edit: After searching through the codebase, issues and discussions for a while, I believe there is some hesitancy towards adding this feature, so I just wanted to be another person to voice support for such a feature. No matter how strict, opinionated or well tuned a formatter is, languages change, bugs show up, etc. Not including a method to offer temporary relief for such an issue leaves very few solutions as the only solutions I can think of are to either temporarily go without formatting, change your code to the whims of a formatter even if syntactically and logically correct, or alternatively, wrapping shfmt in a script, with that script implementing a self built line ignore feature which is more complexity than if it was natively built in.

[mvdan]

Perhaps I'm missing something, but I don't think shfmt is currently breaking parameter expansions. We avoid turning the first : - into :-, because that would change the meaning, as you point out. In the case of the second colon, we can remove the whitespace as that doesn't change anything.

For example:

$ cat f.bash
foo=bar
echo ${foo: 1: -1}

foo=bar
echo ${foo: -1: 1}
$ bash f.bash
a
r
$ shfmt f.bash > f2.bash
$ cat f2.bash
foo=bar
echo ${foo:1:-1}

foo=bar
echo ${foo: -1:1}
$ bash f2.bash
a
r

So, as far as I can tell, there isn't a bug here.

[mvdan]

Closing per the above for now.