Bash: loop over a list of (possibly non-existing) files using wildcards with nullglob (or failglob) option
By Marco Matarazzo
December 12, 2016
Let’s say you’re working in Bash, and you want to loop over a list of files, using wildcards.
The basic code is:
#!/bin/bash
for f in /path/to/files/*; do
echo "Found file: $f"
done
Easy as that. However, there could be a problem with this code: if the wildcard does not expand to actual files (i.e. there’s no file under /path/to/files/ directory), $f will expand to the path string itself, and the for loop will still be executed one time with $f containing “/path/to/files/*”.
How to prevent this from happening? Nullglob is what you’re looking for.
Nullglob, quoting shopts man page, “allows filename patterns which match no files to expand to a null string, rather than themselves”.
Using shopt -s you can enable BASH optional behaviors, like Nullglob. Here’s the final code:
#!/bin/bash
shopt -s nullglob
for f in /path/to/files/*; do
echo "Found file: $f"
done
Another interesting option you may want to check for, supported by Bash since version 3, is failglob.
With failglob enabled, quoting again, “patterns which fail to match filenames during filename expansion result in an expansion error”. Depending on what you need, that could even be a better behavior.
Wondering why nullglob it’s not the default behavior? Check this very good answer to the question.
Comments