Stalling problem with wildcard in bash script

Question

I am very new to writing bash scripts. This question is likely very basic, but I have not been able to find a clear answer yet. I'm working on an Ubuntu subsystem installation on Windows 10.

I'm running a script that contains the following conditional:

if [ -z "$date1" ]; then
    date1=$(head -n 1 "$dir"/*.txt | sed "s/^[^0-9]*//g" | date +%Y%m%d -f - 2>/dev/null)
fi

It runs into issues when it encounters a directory (the dir variable) that has no .txt file, but I don't quite understand the nature of the problem. I do know the issue is in the head command, at least partially. I don't get an error, the script just stalls when it reaches a directory without a .txt file. I want the script to simply move on. If I run the line on its own (without the conditional) in the terminal, I get a No such file or directory error, which makes sense. What really confuses me is that if I place quotes (single or double) around the wildcard portion (i.e. '*.txt'), then the script spits out the head error and moves on. My limited and perhaps incorrect understanding is that the quotes in this case mean the program no longer treats the * as a wildcard and simply looks for a file by the literal name *.txt. But I thought that when the * was interpreted by bash that it first looks for any possible expansion and then tries the literal interpretation if it finds none. So why does the script stall in one case and not the other. Shouldn't both simply give me the same No such file or directory error, as they do when run outside the script?

I'll also mention that the script includes preceding conditionals that first look for .docx files and only moves on to .txt files when there are no .docx files. It handles the cases where there are no .docx files perfectly well, although the first command in that pipe is unzip rather than head. This question seems relevant, but since the script is able to move on when there are quotes around the wildcard, and since it moves on in the similar scenario where there are no .docx files, I wanted to understand what the issue is here and the best way to fix it.

I appreciate your help.

Answer 1

In quotes, * will not expand and will be a literal * character.

On the other hand, when * tries to expand and fails, one of three things happens:

• it is interpreted literally as the string *.txt (plus whatever $dir/ expands to)
  • You can enforce this behavior with shopt -u nullglob, which should be the default.
• it expands to nothing, making the string $dir/*.txt equal to the empty string
  • You can enforce this behavior with shopt -s nullglob.
• it raises an error
  • You can enforce this behavior with shopt -s failglob (or turn it off with shopt -u failglob).

Examples:

bash-5.0# shopt -s | grep glob
globasciiranges on
bash-5.0# echo *.asdf
*.asdf
bash-5.0# shopt -s nullglob
bash-5.0# echo *.asdf

bash-5.0# shopt -u nullglob
bash-5.0# echo *.asdf
*.asdf
bash-5.0# shopt -s failglob
bash-5.0# echo *.asdf
bash: no match: *.asdf
bash-5.0# shopt -s | grep glob
failglob        on
globasciiranges on

When the glob expands to the empty string, head will hang forever unless you enter stdin (head $(echo '') | cat will never complete unless you type)