Bash: nullglob

The nullglob shell option controls how Bash maps glob patterns that don’t match.

See below for demonstration:

#!/usr/bin/env bash

set -e

# ------------------------------------------------------------------------------
# nullglob is turned off here so $files will be set to /tmp/foo/* if no files
# match that pattern. This is almost always unwanted.
# ------------------------------------------------------------------------------

shopt -u nullglob

files=(/tmp/foo/*)

echo "========================================="
echo "shopt: ON"
echo "========================================="
echo "Matched ${#files[@]} file(s)"
echo "First file: ${files[0]}"
echo -e "=========================================\n\n"


# ------------------------------------------------------------------------------
# nullglob is turned on here so $files will be set to nothing (zero words).
# if no files match that pettern.
# ------------------------------------------------------------------------------
shopt -s nullglob

files=(/tmp/foo/*)

echo "========================================="
echo "shopt: OFF"
echo "========================================="
echo "Matched ${#files[@]} file(s)"
echo "First file: ${files[0]}"
echo -e "========================================="

Output:

=========================================
shopt: ON
=========================================
Matched 1 file(s)
First file: /tmp/foo/*
=========================================


=========================================
shopt: OFF
=========================================
Matched 0 file(s)
First file: 
=========================================