问题
I want to loop over all files matching extension jpg or txt. I use:
for file in myDir/*.{jpg,txt}
do
echo "$file"
done
Problem: If the directory contains no jpg file at all, the loop will have one iteration with output myDir/*.jpg. I thought * will be replaced by an arbitrary file (and if no file exists it cannot be expanded). How can I avoid the unwanted iteration?
回答1:
Use this to avoid the unwanted iteration:
shopt -s nullglob
From man bash:
nullglob: If set, bash allows patterns which match no files (see Pathname Expansion above) to expand to a null string, rather than themselves.
See: help shopt and shopt
回答2:
This and a duplicate question both were in context of not just pathname-expansion, but also brace-expansion, and a duplicate asked for POSIX.
The compgen -G does bash --posix compatible pathname-expansion (no brace-expansion) and... you guessed it: yields nothing if there are no matches.
Therefore write a bash --posix function to do it. Brief outline: temporarily use set -f to first do brace-expansion (without pathname-expansion) via an echo, then apply compgen -G to each result for pathname-expansion. Full function left as an exercise.
来源：https://stackoverflow.com/questions/29438045/how-to-match-nothing-if-a-file-name-glob-has-no-matches