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Bash by default is forgiving in ways that hide bugs. Always start scripts with strict mode:

#!/bin/bash
set -euo pipefail

• -e — exit on any non-zero command (instead of plowing ahead)
• -u — error on unset variables (catches typos)
• -o pipefail — pipeline fails if any stage fails (not just the last)

With these, cat missing.txt | grep foo actually fails the script. Without, you'd silently continue.

ALWAYS quote your variables unless you have a specific reason not to:

# WRONG — breaks on filenames with spaces
for f in $files; do
    rm $f
done

# RIGHT — quoted, preserves spaces
for f in "${files[@]}"; do
    rm "$f"
done

Word splitting and glob expansion happen on UNQUOTED variables. Quoting is rarely wrong; not quoting is usually wrong.

Test commands — [[ ... ]] is the modern Bash conditional (vs the older [ ... ]):

[[ "$x" == "hello" ]]      # string equality
[[ "$x" =~ ^[0-9]+$ ]]      # regex match
[[ -f "$path" ]]            # file exists
[[ -z "$s" ]]               # string empty
[[ "$a" -gt "$b" ]]         # numeric (or use (( ))  )

[[ ... ]] doesn't word-split, doesn't glob — safer than [ ... ].