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Is this a Bash bug?

$ mkdir test && cd test && echo "a" > "some.file"
test$ echo '*'
*
test$ TEST=$(echo '*')
test$ echo $TEST
some.file

Why is the second output the resolution of * into (all) filenames instead of just a literal * output? Looks like a bug in Bash?

Tried on Ubuntu 18.04, bash version 4.4.19(1)-release. Expect it will be the same on other OS'es.

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    General procedure when finding bugs in Unix: 1) Check out Single Unix Specification online for relevant definitions and rationales. 2) Submit a bugreport to austingroupbugs.net 3) Submit bugreport to the developer (with a patch if possible). Commented Jun 3, 2019 at 1:10
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    @DannyNiu: Your comment is not very practical.  Documents like POSIX and the Single Unix Specification do not consist of millions and millions of examples; they contain explanations (with, maybe, a few examples).  Those documents are of little help to somebody who doesn’t understand what is happening and why; reading them is like learning to drive by reading a roadmap. And this question isn’t asking what to do.  If you want to post a question, “What should I do if I’ve found a bug in a widely used software product?”, and post an answer, go ahead. Commented Jun 3, 2019 at 4:26
  • @G-Man I did find DannyNiu's answer helpful in terms of the information provided, "just in case". Commented Jun 3, 2019 at 4:28
  • Try the set -f (and set +f) options before/after the final echo to disable globbing/wildcard expansion if this behavior is unwanted. Commented Mar 12, 2024 at 9:52

1 Answer 1

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No, it is not a bug. You have shown that

echo '*'

will produce a literal *. Hence when you substitute this output, as per the following command

TEST=$(echo '*')

it will put * into the variable $TEST. Then when you

echo $TEST

the glob will expand here. You can verify this by running this last command, changing directories, then running it again.

You will get the * output if you say

echo "$TEST"

as explained here, the double quotes allow the variable to be expanded but prevent the glob from expanding.

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  • Aha, I see. Thank you. I missed the fact that the echo * does not have quotes and thus resolves. It's harder to see when you have it inside a complex script :) Makes sense now. Commented Jun 3, 2019 at 1:36
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    @Roel FWIW, after a few years of experience you will start to treat $TEST instead of "$TEST" as a code smell and it becomes a bit easier to see. It's still not much of a smell though so it will still trip you up sometimes Commented Jun 3, 2019 at 9:24
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    And this is why my general approach is, if it starts with a dollar, quote it :). Yes, you CAN learn the specific cases where you don't have to - but it's almost always harmless to quote them in cases where you don't have to, so you may as well just quote your variables everywhere! Commented Jun 3, 2019 at 10:15
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    @slebetman: And for the rest of us without this instinct, shellcheck will do the complaining ( shellcheck.net ). Commented Jun 3, 2019 at 12:42
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    @Roel many decades ago, I learned -- the hard way -- that 99.99999% of the time it's your fault, not a bug in the compiler or interpreter... :) Commented Jun 3, 2019 at 14:56

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