Why is file output different when called from a loop with a .swp file?

Question

I've been experimenting with the file command and I came across something intriguing. When I try to loop all the files of a folder and call the file command the output for .swap and .swp files exhibit a weird behaviour. All 3-letter file/folder names in the current folder are printed as valid extensions for the swap file.

file --extension .test.swp outputs .test.swp: ??? but calling it in a loop has a different output. the difference can be seen below. This seems to happen only if

• There is a for loop
• file --extension is used
• There is a .swp/.swap file which is not empty
• There are files/folders with exactly three characters.

touch a.txt b c def 123 456
for file in `find . -type f`; do 
    echo $file "->" `file --extension $file` ; 
done
echo "test" > .test.swp
echo "-------------"
for file in `find . -type f`; do 
    echo $file "->" `file --extension $file` ; 
done

This prints the output before and after the swap file is added.

./def -> ./def: ERROR: (null)
./a.txt -> ./a.txt: ERROR: (null)
./456 -> ./456: ERROR: (null)
./123 -> ./123: ERROR: (null)
./c -> ./c: ERROR: (null)
./b -> ./b: ERROR: (null)
-------------
./def -> ./def: ERROR: (null)
./.test.swp -> ./.test.swp: 123 456 def
./a.txt -> ./a.txt: ERROR: (null)
./456 -> ./456: ERROR: (null)
./123 -> ./123: ERROR: (null)
./c -> ./c: ERROR: (null)
./b -> ./b: ERROR: (null)

Can anyone explain this weird behaviour?

EDIT: My Shell is GNU bash, version 3.2.57

Answer 1

The reason is that you didn't quote

`file --extension $file`

It's returning ??? for .test.swp, which is then being processed as a filename wildcard. So your output is the same as if you'd written

echo $file "->" ???

The wildcard ??? matches all 3-character filenames, so those are shown.

Change to:

echo "$file -> `file --extension $file`"

You should always quote variables and command substitutions unless you have a good reason not to. See I just assigned a variable, but echo $variable shows something else (it talks about variables, but command substitution works the same way).