Check of folder's content never succeeds

Question

The bash script snippet

folder="some folder's name"
a="$folder/"*".pdf"
if [ "$a" != "$folder"'/*.pdf' ]
then
    echo check
fi

never prints check.

I'd expect it to print check iff there is at least 1 file whose name ends in .pdf in folder $folder.

echo $a and echo "$folder"'/*.pdf' print exactly what I'd expect them to print: The former prints the matched paths iff there is at least 1 *.pdf file in the specified folder and otherwise prints what the latter prints. The latter always prints the name of the specified older concatenated with /*.pdf.

Why does bash behave like that?
How do I make it work?

The proper way to do it is described here: Test if there are files matching a pattern in order to execute a script — don_crissti, Oct 23 '17 at 19:20
With bash, that condition ([ "$#" -gt 0 ]) always passes. It think this is because bash just doesn't replace the asterisk if there is no match, so there always is at least 1 result. — UTF-8, Oct 23 '17 at 22:41
No. If you bother reading the answer there you'll see there's a shopt -s nullglob before that condition. — don_crissti, Oct 23 '17 at 22:43
Oh, you're right. I'm sorry. With that option set, it works. Thank you. — UTF-8, Oct 23 '17 at 22:53

RomanPerekhrest · Answer 1 · 2017-10-23T20:47:47.800

2

This expression "$folder/"*".pdf" will expand to all possible filenames that end with .pdf.
Why? The * sign retains its special meaning i.e. globbing.

While this string "$folder"'/*.pdf' is just a concatenation of 2 separate strings "$folder" and '/*.pdf'.
As * sign enclosed in single quotes - it loses its special meaning.

According to your main goal check if the current folder has at least one .pdf file and printing some "pdf file found!" phrase on success:

if [ ! -z `find foldername -type f -name "*.pdf" -print -quit` ]; then 
    echo "pdf file found!"
fi

edited Oct 23 '17 at 20:47

answered Oct 23 '17 at 19:19

RomanPerekhrest

30,212

That's the plan. So the condition should be true if there are PDFs in the specified folder, right? (Note the exclamation point.) – UTF-8 Oct 23 '17 at 20:02
@UTF-8, your final goal is to echo "check" if the current folder has at least one *.pdf file, right? – RomanPerekhrest Oct 23 '17 at 20:08
Yes, that's what I'm trying to do. – UTF-8 Oct 23 '17 at 20:41
@UTF-8, see my update – RomanPerekhrest Oct 23 '17 at 20:47
This fails on a number of occasions. I'm not entirely sure what occasions, though: https://pastebin.com/nasQePMx – UTF-8 Oct 23 '17 at 22:47
@UTF-8, enclose the condition in double brackets [[ ! -z find foldername -type f -name "*.pdf" -print -quit ]]` – RomanPerekhrest Oct 24 '17 at 06:59

sborsky · Answer 2 · 2017-10-23T20:20:18.167

1

Your if statement doesn't work, because $a enclosed in double quotes is just a string containing a * (compare echo "$a" with echo $a). If you remove the double quotes, your if statements fails as soon as there are more than one files matching.

don_crissti's link has good solutions.

edited Oct 23 '17 at 20:20

answered Oct 23 '17 at 19:38

sborsky

1,028

We both agree that the right expression doesn't depend on the folder's contents, right? If there is no file in the folder, bash doesn't replace the wild card * in the variable assignment. So my plan is to check whether that replacement happened. – UTF-8 Oct 23 '17 at 20:06
Yes, Bash does not replace * when there is no match (corrected my answer). I still don't think you can handle the case of >1 matches (many pdf files in the folder) this way. If you are not enclosing $a in ", the if statement is going to fail with too many arguments. If you are enclosing $a in ", * is not going to be resolved/replaced by file names. Thus, I don't think if in combination with file name expansion is a good idea. I have been using ls' exit status before, but there other options. – sborsky Oct 23 '17 at 20:26

Check of folder's content never succeeds

2 Answers2